Gulls and Terns  — Family Laridae

South Polar Skua Stercorarius maccormicki

Of the ocean birds that migrate from the southern hemisphere to the North Pacific for the austral winter, the South Polar Skua comes the farthest, for it nests around the coasts of Antarctica.  Off San Diego County it is a rare migrant, recorded in both spring and fall.

Migration: The South Polar Skua has been recorded off San Diego most frequently in spring, from 5 May (1987, one at Tanner Bank 100 miles off shore, R. R. Veit, AB 41:488, 1987) to 4 June (1983, R. E. Webster, AB 37:1027, 1983).  In most cases just a single individual was seen in a full day’s boat trip, but four were counted 20 May 1978 (AB 32:1055, 1978).  Fall records extend from 12 August (1961, AFN 15:493, 1961) to 14 October (1971, Cortez Bank, Jehl 1973), except for the one from shore, the only specimen, picked up at Silver Strand State Beach (T9) 23 November 1975 (AB 31:376, 1977, CSULB 4785).  The only fall record of more than one individual was of two off San Diego 7 September 1991 (G. McCaskie, AB 46:149, 1992).

Pomarine Jaeger Stercorarius pomarinus

The Pomarine Jaeger is primarily a migrant over the ocean off San Diego County and the commonest jaeger here in both spring and fall.  It is regular if uncommon in winter as well.  Occasional Pomarine Jaegers remain through the summer.  The species is rarely seen on shore; perhaps any that alight on beaches are in poor health.

Winter: Most Pomarine Jaegers winter in the tropics; California is at the northern edge of the species’ normal winter range.  Of San Diego County’s Christmas bird counts, the San Diego and Oceanside counts regularly deploy a boat on the ocean, thanks to the consistent dedication of D. W. Povey.  The San Diego count has returned the Pomarine Jaeger in 15 of 34 years, 1969–2002, with a maximum of three individuals.  The Oceanside count has returned it in 17 of 28 years, 1976–2002, with a maximum of four, except for 25 on 28 December 1980.  The birds can occasionally be seen from shore; our maximum 1997–2002 was of four from Torrey Pines State Reserve (N7) 22 December 2000 (S. Walens). 

Migration: The Pomarine Jaeger is most numerous in fall migration (up to 140 between San Diego and San Clemente Island 11 September 1971, G. McCaskie), and common also in spring migration.  It is seldom seen in June or July, but eight were off San Diego 17 June 1982 (D. W. Povey, AB 36:1016, 1982).  One was in the south San Diego Bay salt works (U10) 9 June 1999 (found dead 13 June, SDNHM 50438); another was there 23 June 1999 (R. T. Patton).

Parasitic Jaeger Stercorarius parasiticus

A migrant and winter visitor along San Diego County’s coastline, the Parasitic less numerous here than the Pomarine but seen uncommonly, from shore as well as at sea.  The Parasitic Jaeger enters San Diego Bay rarely.  There are two summer records.

Winter: The Parasitic Jaeger, like other ocean birds, is most likely to be seen from shore, especially at La Jolla (P7), during the strong northwest winds that follow the passage of a cold front.  Maximum daily counts from shore are of 18 from La Jolla 13 February 2001 (B. L. Sullivan) and 17 from Oceanside (H5) 26 December 1998 (S. Walens).  The San Diego Christmas bird count has recorded the Parasitic Jaeger on 25 of 34 counts 1969–2002, with a maximum of eight on 20 December 1986.  The Oceanside count has recorded it on 19 of 28 counts 1976–2002, with a maximum of 19 on 26 December 1998 (including 18 from shore).  There were no reports within San Diego Bay during the atlas period, but weekly surveys of the bay 1993–94 yielded four sightings totaling five individuals (Mock et al. 1994).

Migration: The Parasitic Jaeger’s seasonal distribution in San Diego County remains that reported by Unitt (1984): 27 August–23 May, except for sightings of one at the Santa Margarita River mouth (G4) 12 June–10 July 1971 (AB 35:906, 1971) and of two 1 mile off La Jolla 14 June 1978 (AB 32:1208, 1978).

Long-tailed Jaeger Stercorarius longicaudus

The smallest of the jaegers is also the scarcest in San Diego County, where it occurs as a rare fall migrant.  It typically remains far out to sea, and there are only two reports from shore.  The only jaeger identified inland in San Diego County, however, was a Long-tailed grounded by Tropical Storm Doreen in 1977.

Migration: Off San Diego, the Long-tailed Jaeger is usually seen singly, if at all.  The largest concentration reported near San Diego was nine that were 20 miles east of Cortez Bank (and thus about 90 miles west of San Diego) 13 October 1971 (Jehl 1973).  Onshore, H. W. Marsden collected an immature at Pacific Beach 19 September 1904 (Bishop 1905, FMNH 135805). 

All three jaegers undertake extensive overland migrations but seldom stop en route.  The only inland record for San Diego County remains the adult found exhausted at Pala (D11) after Tropical Storm Doreen 18 August 1977 (SDNHM 40390).  An unidentified jaeger, however, was at Lake Hodges 9 September 1982 (D. Hayward, AB 37:224, 1983).

The seasonal spread of fall records remains the 18 August–18 October reported by Unitt (1984).  In spring, one was at the south end of San Diego Bay 11 May 1962 (G. McCaskie, AFN 16:447, 1962), one was off San Clemente Island 1 May 1987 (R. R. Veit, AB 41:488, 1987) and one was at Cortez Bank 22 April 2000 (T. McGrath, NAB 54:327, 2000).

Laughing Gull Larus atricilla

Though a common postbreeding visitor and even a sporadic breeding species at the Salton Sea, the Laughing Gull is only a rare vagrant to the Pacific coast of California.  In contrast to its status as an almost exclusively summer visitor just to the east, in San Diego County the Laughing Gull has no strong seasonal pattern; the approximately 30 records are scattered throughout the year.

Winter: The atlas study from 1997 to 2002 was a slow period for the Laughing Gull in San Diego County, yielding just one record, of two first-winter birds at Buena Vista Lagoon (H6) 16 January–28 March 1998 (T. Hathaway, FN 52:257, 1998).  The winter of 2002–03, however, was unusual in having three.  Winter records of the Laughing Gull in San Diego County are coastal or up to 13 miles inland, as far as Santee Lakes (P12; 17 January–12 February 1993, C. G. Edwards, AB 47:301, 1993) and Lake Murray (Q11; 8–29 December 2002, M. Sadowski, NAB 57:257, 2003).

Migration: There were no summer records of the Laughing Gull in San Diego County before 1983 (Unitt 1984).  Since then five have accumulated, including one of an immature at Lake Henshaw (G17) 30 September 1983 (R. Higson, AB 38:247, 1984), which presumably arrived over the mountains from the southeast.  Two adults were together at San Diego 4 June 1983 (D. M. Parker, AB 37:913, 1983).

Franklin’s Gull Larus pipixcan

On its commute between a breeding range in the intermountain basins and the Great Plains and a winter range off the Pacific coast of Central and South America, Franklin’s Gull passes largely to the east of San Diego County.  The species is a rare migrant here, with a few immature stragglers recorded in winter.

Migration: In spring, Franklin’s Gull has been recorded in San Diego County only about 12 times, on dates from 11 April (1971, Oceanside, H5, AB 25:801, 1971) to 29 June (1998, Santee Lakes, P12, M. B. Mulrooney).  The latter was unusual in also being the only inland record in spring.  The only other observations during the atlas period were of one in the south San Diego Bay salt works (U10) 19 May 1999 (R. T. Patton) and three at the mouth of Las Flores Creek (E3) 30 May 1998 (R. and S. L. Breisch), the latter being the only spring sighting of more than a single bird.

            In fall Franklin’s Gull is more frequent than in spring, though its frequency has decreased since the 1970s.  Fall records extend from 6 September (1963, one at the Santa Margarita River mouth, G4, McCaskie and Cardiff 1965), exceptionally 29 July (1951, same locality, AFN 5:308, 1951), to 8 December (1966, Lake Henshaw, G17, C. G. Edwards, NASFN 51:802, 1997).  Fall records are mainly coastal but include at least two from Lake Henshaw and one from Lake Hodges (K10; G. McCaskie).

Winter: Franklin’s Gull has been noted in San Diego County seven times in winter.  Unitt (1984) listed four records, including the only one inland, at Lake Hodges 10 November 1979–5 January 1980 (AB 34:306, 1980).  Subsequently one was at the San Diego River mouth (R7) 29 December 1987–20 January 1988 (E. Lodge, AB 42:321, 1988), one was at  Chula Vista (U10) 16 January–11 February 1990 (E. R. Lichtwardt, AB 44:329, 1990), and one was at Coronado (S9/T9) 18 February 1997 (E. Copper, NASFN 51:802, 1997).

Little Gull Larus minutus

From the first sighting near the north end of the Salton Sea in 1968, the Little Gull has become ever more frequent in California, as this originally Old World species colonizes North America from the northeast.  Of the 79 well-supported records for California through 2002, however, only two are from San Diego County.

Winter: An immature at the Oceanside harbor (H5) 27 December 1981 became entangled in fishing line and was taken to Sea World, where it died 27 March 1982 (SDNHM 41883).  Another was seen at La Jolla (P7) 18 November 1994 (S. Walens, G. McCaskie, NASFN 49:101, 1995; Howell and Pyle 1997).

Bonaparte’s Gull Larus philadelphia

The only small gull regular in San Diego County, Bonaparte’s Gull is most abundant on the ocean, where, in winter, it can often be seen in flocks of hundreds a short distance offshore.  Along the coast, it is much less numerous, though still often common in San Diego Bay or the north county lagoons.  Inland, Bonaparte’s Gull is quite irregular, both from lake to lake and from year to year.  Even along the coast Bonaparte’s Gull appears to be becoming patchier in its distribution, perhaps as a symptom of the faltering productivity of the ocean off southern California.

  

Winter: Offshore, the flocks of Bonaparte’s Gulls undoubtedly shift widely with the shifting of fish and plankton.  Onshore, however, the species has a few favored concentration points, especially the sewage ponds near the Santa Margarita River along Stuart Mesa Road (G5; up to 70 on 6 December 1998, R. E. Fischer), Buena Vista Lagoon (H5; 150 on 7 February 1999, J. Determan), Los Peñasquitos Lagoon (N7; 100 on 3 January 1999, D. K. Adams), and the San Diego River mouth (R7; 200 on 22 February 2000, V. P. Johnson).  Bonaparte’s Gull is scarce in Mission Bay but fairly common in San Diego Bay.  On weekly surveys of central and south San Diego Bay 1993–94, Manning (1995) noted a maximum of 138; on weekly surveys in and near the salt works through the same period, Stadtlander and Konecny (1994) noted a maximum of 108, though usually numbers were much lower.

            Atlas observers also found Bonaparte’s Gulls on several inland lakes, especially Henshaw (G17; up to 185 on 7 February 1999, W. E. Haas), Sutherland (J16; 100 on 22 February 2000, M. B. Stowe), and Hodges (K10/K11; 100 on 14 December 1997, B. Schram).  At other lakes our counts were of 15 or fewer.  Inland, however, the species’ occurrence is quite irregular, as exemplified by the history of Lake Henshaw Christmas bird counts: Bonaparte’s Gull has been recorded on 17 of 22 counts 1981–2002, with a median of 25, a mean of 59, and a maximum of 418 on 23 December 1996.

            The only winter records from the Anza–Borrego Desert are of birds lost far from water, along the Montezuma Grade of Highway S22 (G23) 2 January 1993 (E. Post) and at Vallecito (M25) 12 December 2001 (S. Martin).

Migration: Bonaparte’s Gull does not arrive in any significant numbers until November.  On their weekly surveys of north San Diego Bay 1993–94, Mock et al. (1994), recorded an earliest date of 19 October, the earliest among any of the regular surveys of San Diego Bay.  One fall migrant has been noted in the Anza–Borrego Desert, on a golf-course pond at the Roadrunner Club (F24) 29 October–6 November 1991 (A. G. Morley).

            In spring migration Bonaparte’s Gulls make use of the migration corridor over San Diego County’s relatively low mountains, following the same route as the Brant, Surf Scoter, and Common Loon.  During the atlas period our most striking example of this was 300 at Lake Cuyamaca (M20) 2 April 2001 (P. D. Jorgensen).  Spring counts at Lake Henshaw range as high as 490 on 31 March 1979 (R. Higson, AB 33:806, 1979).  Another notable sighting was of eight flying west down Hauser Canyon (T21) 25 April 1998 (R. and S. L. Breisch).  Dates for such migrants range from 27 March (one at the Borrego sewage ponds, H25, A. G. Morley) to perhaps 5 June (1998, nine at Lake Cuyamaca, A. P. and T. E. Keenan).  When spring migration ends is difficult to say because of occasional summering birds in nonbreeding plumage (Unitt 1984).  We recorded no clearly summering Bonaparte’s Gulls 1997–2001; one at San Elijo Lagoon (L7) as late as 13 June 1998 (B. C. Moore) was still in breeding plumage.

Conservation: Numbers of Bonaparte’s Gulls on San Diego County’s three coastal Christmas bird counts fell noticeably in the 1990s.  The San Diego count, with a 50-year average of 734, reached its all-time lows of four in 1999, one in 2000, and three in 2002.  The Rancho Santa Fe count averaged 317 from 1980 to 1990 but 64 from 1991 to 2003.  The Oceanside count hit its post-1976 lows of three in 2000 and 53 in 2002, even though it hit its high of 1400 in 2001.  Similarly, the Escondido count found Bonaparte’s Gull regularly at Lake Wohlford 1987–93 but only sporadically since.  Such irregularity characterizes birds that feed on scarce, clumped resources.

Heermann’s Gull Larus heermanni

This most distinctive and attractive of North America’s gulls is common along San Diego County’s coast.  Seasonally, the abundance of Heermann’s Gull varies in tandem with that of the Brown Pelican, which the gull follows, stealing its fish.  That is, the gull’s numbers are lowest in spring, increase in summer as the birds return from their nesting colonies in the Gulf of California, then decrease again through the winter.  Most Heermann’s Gulls probably circumnavigate the peninsula, but a few cross overland, as there are scattered records from San Diego County’s inland lakes.

  

Winter: Heermann’s Gull occurs the length of San Diego County’s coast and well out to sea.  Areas of concentration are Torrey Pines State Reserve (N7; up to 560 on 23 December 2001, S. Walens), north San Diego Bay (444 on 14 December 1993, Mock et al. 1994), and the bight from Point Loma to Imperial Beach (535 on 15 December 2001, D. W. Povey).  Few Heermann’s Gulls, however, enter northern San Diego County’s lagoons (maximum 10 in the east basin of Batiquitos Lagoon, J7, 22 December 2001, R. and A. Campbell) or even the inner reaches of Mission Bay (Q8; maximum 13 on 26 January 1999, J. C. Worley).  Despite its abundance in north San Diego Bay, Heermann’s Gull is uncommon in the south bay.  On their regular surveys there, neither Macdonald et al. (1990) nor Stadtlander and Konecny (1994) found more than nine per day.

            The only winter record inland is of one at Lake Henshaw (G17) 18 January 1980 (AB 34:306, 1980).

Migration: Numbers of Heermann’s Gulls begin increasing in June, to reach a peak in September and October.  On the basis of weekly counts of San Diego Bay, Mock et al. (1994) recorded their daily maximum of 1033 on 8 September 1993.  On surveys of central San Diego Bay, both Mock et al. (1994) and Manning (1995) found numbers greatest in October.  After this, Heermann’s Gull decreases in abundance, to reach its nadir from March to May.  Even during this interval, however, nonbreeding immatures are fairly common (up to 119 in north San Diego Bay 19 April 1995, P. J. Mock; 30 at La Jolla, P7, 1 April 2001, L. and M. Polinsky).

            Most of San Diego County’s inland Heermann’s Gulls, like those at the Salton Sea, occurred at the time of postbreeding dispersal and likely arrived overland from the southeast.  Up to two were at Lake Henshaw 5–7 July 1985, four were there 6 July 1986 (R. Higson, AB 39:962, 1985, 40:1255, 1986), and one was there 24 June 1989 (G. McCaskie, AB 43:1368, 1989).  One was flying northwest over Scissors Crossing (J22) 7 July 2002 (J. R. Barth, not 3 July as in NAB 56:486, 2002), two were on Barrett Lake (S19) 21 June 1997 (J. Hannan), and one was at Jacumba (U28) 14 July 1968 (AFN 22:649, 1968).  Also at this season but closer to the coast, so possibly originating from that direction, were one at O’Neill Lake (E6) 19 June 1998 (P. A. Ginsburg) and one on Otay Mesa (V13) 16–17 October 1979 (E. Copper).

Conservation: Heermann’s Gull nests in just a few colonies around Baja California, and at least 90% of the population nests in a single massive colony on 150-acre Isla Rasa in the Gulf of California, making it vulnerable to catastrophes.  Fortunately, Isla Rasa has been a wildlife refuge since 1964, putting an end to egging and enabling the eradication of rats.  Careful management has allowed the population to increase from 55,000 pairs in 1975 to 150,000–200,000 pairs by the end of the 20^th century (E. Velarde).  A corresponding increase in numbers in San Diego County, however, is not evident.  In Christmas bird counts, annual variability overwhelms any trend.

Belcher’s or Band-tailed Gull Larus belcheri

With little doubt the least expected vagrant reaching San Diego County during the five-year atlas period from 1997 to 2001 was the Band-tailed or Belcher’s Gull, normally confined to the coasts of Peru and Chile washed by the Humboldt Current.  Vagrants have occurred north to Panama, exceptionally to Florida, but the bird amply photographed at the Tijuana River mouth represents the only well-supported record for California.

Winter: San Diego County’s single Band-tailed Gull remained more or less continuously at the Tijuana River mouth (V10) from 3 August  1997 to 2 January 1998 (D. G. Shaw; Rottenborn and Morlan 2000).  Perhaps its occurrence so far from its normal range was related to El Niño, that climatic variation striking in 1997–98 and felt most strongly in the gull’s range on the west coast of South America.

Black-tailed Gull Larus crassirostris

Proper interpretation of occurrences of rare birds requires context, and in the case of very rare birds accumulating that context may take decades.  This is the story of San Diego County’s—and California’s—only Black-tailed Gull.  Collected in 1954, this Asian bird was long dismissed as so unlikely to reach San Diego unaided that it was presumed to have been brought across the Pacific on a ship.  More recently, additional individuals have shown up elsewhere in North America, suggesting that the Black-tailed Gull occasionally does reach this continent as a vagrant on its own (Lethaby and Bangma 1998).

Migration: Monroe (1955) collected California’s only Black-tailed Gull at the former Naval Training Center on north San Diego Bay (S8) 28 November 1954, after finding it two days earlier (UMMZ 136176).  The next nearest record is of one photographed at El Golfo de Santa Clara, Sonora, 7 June 1997 (Garrett and Molina 1998).  Heindel and Patten (1996) outlined the basis for the California Bird Records Committee’s reversing course and adding the Black-tailed Gull to the state’s bird list.

Mew Gull Larus canus

San Diego County lies near the southern limit of the Mew Gull’s winter range, so the species occurs here as an uncommon winter visitor.  It is almost exclusively coastal in this area, being rare just a few miles inland.  For no clear reason, in San Diego County Mew Gulls are clumped in just two areas.

Winter: From 1997 to 2002, Mew Gulls concentrated in San Diego County at just two sites.  One is the basin and breakwater at Camp Del Mar, within Camp Pendleton (G4), where counts averaged 20–25 and reached a maximum of 69 on 10 January 2001 (P. A. Ginsburg).  Often some of these birds use the nearby sewage ponds at Stuart Mesa Road and Vandegrift Boulevard (G5), where our counts ranged up to 14 on 26 December 1998 (T. A. Burr).  The other site is the beach at North Island Naval Air Station (S8), where counts in the 20s are usual and 100 on 5 January 1998 (B. C. Moore) was exceptional.  Sometimes the birds flock near the Hotel del Coronado (T9), but we did not record any such concentration there during the atlas period.

            Elsewhere along San Diego County’s coast the Mew Gull is widely scattered but uncommon.  During the atlas period we had no count away from the two main sites greater than five at Encinitas (K6) 29 December 1998 (S. Schultz).  The only inland locations where we recorded Mew Gulls 1997–2002 were Lake San Marcos (J8), with up to two on 27 December 1997 and 2 February 1999 (J. O. Zimmer), and Kit Carson Park, Escondido (K11), with a first-year bird found after a storm 4 February 1998 (E. C. Hall).  Previous inland records are all within 7 miles of the coast (Unitt 1984).

Migration: The Mew Gull occurs primarily from late November to late March, though it does not reach peak numbers until early January (Devillers et al. 1971).  Extreme dates are 15 October (1955, one at Oceanside, H5, AFN 10:57, 1956) and 4 May (1962, one at the San Diego River mouth, R7, G. McCaskie).  There are also two summer records, of one collected on Mission Bay (Q8) 31 July 1922 (SDNHM 2351) and one seen on San Diego Bay at Chula Vista (U10) 6–30 July 1977 (AB 31:1190, 1977).

Conservation: The sites where Mew Gulls flock shift over time.  In the 1970s the San Luis Rey River mouth (H5), San Diego River mouth, and Tijuana River mouth (V10) were the main sites (Devillers et al. 1971, Unitt 1984), but by the beginning of the 21^st century the gulls were not using these regularly.  Nevertheless, the numbers reaching the county have changed little in 30 years: Devillers et al. (1971) wrote “the total population probably does not exceed 100 birds.”  Christmas bird counts suggest considerable annual variation; the San Diego count has ranged from highs of 227 in 1982 and 183 in 1983 (El Niño?) to lows of one in 1973 and 1995.

Taxonomy: Only the New World subspecies of the Mew Gull, L. c. brachyrhynchus Richardson, 1831, has been found in California.

Ring-billed Gull Larus delawarensis

The Ring-billed is San Diego County’s most widespread gull, the common gull scavenging in parking lots as well as on lakes, estuaries, and beaches.  The ocean is the one aquatic habitat the species typically avoids—the Ring-billed is not a “sea” gull.  Though the Ring-billed Gull is primarily a winter visitor to San Diego County, substantial numbers of nonbreeders remain through the summer, far more than those of the other migratory gulls.

Winter: The Ring-billed Gull is common near water throughout San Diego County’s coastal lowland.  Its abundance is greatest along the coast: up to 500 at the mouth of Las Pulgas Creek (E3) 24 December 1999 (R. and S. L. Breisch) and 700 at Mission Bay (Q8) 26 January 1999 (J. C. Worley).  But numbers inland can be high as well, with up to 328 at the Wild Animal Park (J12) 29 December 2001 (K. L. Weaver) and 815 at Santee Lakes (P12) 13 February 1999 (I. S. Quon).  Between 1500 and 3100 feet elevation the gull’s numbers are variable but sometimes high, with up to 325 at Lake Henshaw (G17) 7 February 1999 (W. E. Haas), 200 at Lake Sutherland 19 January 2000 (E. C. Hall), and 200 in Santa Teresa Valley (K16) 14 January 1999 (G. L. Rogers).  Figures on Lake Henshaw Christmas bird counts range from 2 to 329 with a mean of 57 and a median of 19.  We found no wintering Ring-billed Gulls at elevations higher than 3100 feet at Morena Village (T22; 35 on 19 December 1999, R. and S. L. Breisch).  In the Borrego Valley the Ring-billed Gull is rare.  Thirty-four at Club Circle (G24) 20 December 1998 (P. D. Ache), were the only ones found in 19 years of Anza–Borrego Christmas bird counts, and three in Borrego Springs (F24) 12 December 2001 (P. D. Jorgensen) were the only others seen there in winter during the atlas period.

Migration: Ring-billed Gull migration has not been well studied in San Diego County.  At the Salton Sea, juveniles begin arriving by 16 July (Patten et al. 2003), and the species’ numbers in San Diego begin increasing in this month.  In their weekly surveys of the San Diego Bay salt works 1993–94, Stadtlander and Konecny (1994) found the gull’s numbers spiking sharply in November and dropping to summer lows in April.  Dates for the Anza–Borrego Desert range from 1 November (1988, three at the Borrego sewage ponds, H25, A. G. Morley) to 9 May (2001, five at Club Circle, P. D. Ache), except for a sick bird at Bow Willow (P27) 8 June 1973 (ABDSP file) and an anomalous four at Agua Caliente Springs (M26) 4 June 1998 (E. C. Hall).

By mid May the Ring-billed Gull is generally uncommon.  Our highest summer counts were of 31 at the mouth of Las Pulgas Creek 17 May 1998 (R. and S. L. Breisch), 30 at Agua Hedionda Lagoon (I6) 20 June 1999 (J. Ciarletta), and 40 at La Jolla (P7) 27 May 1999 (L. Polinsky).  Almost all summering Ring-billed Gulls are along or near the coast; the only lakes well inland where we noted them 19 May–15 July were Henshaw (eight on 18 June 2000, P. Unitt), San Vicente (N13; one on 16 June 2000, R. and S. L. Breisch), and Morena (S21/S22; up to three on 1 July 2000, R. and S. L. Breisch).

Conservation: With its food supply hugely augmented by human waste, the Ring-billed Gull enjoyed an increase in number and spread of its breeding range through the 20^th century (Ryder 1993).  But numbers in San Diego County have declined since the 1960s, as suggested by results of San Diego Christmas bird counts: Ring-billed Gulls averaged 5400 from 1963 to 1970 but only 1131 from 1997 to 2002.  Changes in agriculture, more packaging of waste, the reduction in the number of landfills, and the assiduous exclusion of gulls from landfills that remain likely contributed to this decrease.

California Gull Larus californicus

Though the California Gull, like the Ring-billed, arrives in southern California from the interior of North America, in San Diego County the California Gull is much more concentrated along the coast.  Hundreds, sometimes thousands, can be seen on the ocean within a few miles of the coast.  On beaches loafing California Gulls commonly flock with other species of gulls.  A few nonbreeding California Gulls—far fewer than Ring-billed Gulls—remain in San Diego County through the summer.

Winter: The large numbers of California Gulls seen foraging at sea are likely the same birds seen resting on beaches: up to 700 at Oceanside (H5) 26 December 1998, 1430 at Torrey Pines State Reserve (N7) 23 December 2001 (S. Walens), and an exceptional 3600 in the San Diego River flood-control channel (R8) 2 February 2001 (J. C. Worley).  The species’ abundance drops off rapidly with distance inland.  From 1997 to 2001, the only place more than 5 miles from the coast where we found more than 50 was Sweetwater Reservoir (S12), where large numbers are regular, with up to 200 on 16 December 2000 (P. Famolaro).  But the species is fairly common on lakes throughout the coastal lowland.  Above an elevation of 1500 feet, we found wintering California Gulls at just two lakes.  At Henshaw (G17), our highest count during the atlas period was 40 on 12 December 2000 (J. R. Barth), but Lake Henshaw Christmas bird counts have found up to 161 (23 December 1996), noting the species on 16 of 22 counts 1981–2002.  At Morena (S21), S. E. Smith found a single individual in January of three successive years.  In the Anza–Borrego Desert, the only records from December to February are of one at Club Circle, Borrego Springs (G24) 21 February 1999 (P. D. Ache) and three at Ram’s Hill (G25) 16 December 1998 (R. Halford).

Migration: No study has addressed variation in California Gull numbers in San Diego County by the birds’ ages, and without such study, knowledge of the species’ migration schedule will remain inadequate.  At the Salton Sea, Patten et al. (2003) reported juveniles returning as early as 3 July and the numbers of the California Gull in general increasing steadily from July through October.  On the basis of weekly counts at the south San Diego Bay salt works 1993–94, however, Stadtlander and Konecny (1994) found California Gull numbers remaining at their low summer level until November.  They found another peak in April and a maximum of 500 on 21 April 1993.  At Lake Hodges (K10/K11) a peak count of 200 on 24 April 1982 (K. L. Weaver) probably reflects spring migration.  Numbers decrease through May, with migrants remaining possibly as late as 21 May (2001; 15 at the Santa Margarita River mouth, G4, P. A. Ginsburg).  The few records for the Anza–Borrego Desert range from 10 October (1990, five at the Borrego sewage ponds, H25) to 27 March (1990, two at the same site, A. G. Morley).

            In San Diego County, summering California Gulls are uncommon to rare.  From 1997 to 2001, all our records between 21 May and 10 July were of seven or fewer individuals, except for 20 at Lake Henshaw 18 June 2000 (P. Unitt).  All summer records were coastal except for this and one at El Capitan Reservoir (O16) 18 June 1998 (S. Kingswood).

Conservation: With colonization of the San Francisco Bay area and the Salton Sea, the California Gull’s breeding range has been expanding to the west and south (Molina 2000, Shuford and Ryan 2000).  The total population increased considerably through at least the second half of the 20^th century (Conover 1983, Shuford and Ryan 2000).  No trend in numbers wintering in San Diego County is obvious; the shifting of flocks makes monitoring in winter difficult.  California Gulls formerly concentrated in huge numbers at garbage dumps (6500 at the Otay dump, U12, G. McCaskie).  But since the early 1990s they have been excluded from the county’s landfills.

Taxonomy: Of the California Gull’s two subspecies, the predominant one in San Diego County is nominate L. c. californicus Lawrence, 1854, which nests in the southwestern part of the species’ breeding range.  Of eight San Diego County specimens of adults or three-year-old birds, seven are californicus, with a value for darkness of the back, as measured by a Minolta CR300 electronic colorimeter, of L = 45.2 to 49.4 (compare values in table 12 of Patten et al. 2003).  One of these, picked up at San Elijo Lagoon (L7) 5 January 1975, had been banded in a nesting colony in Weld County, Colorado. 

One specimen from San Diego County, however, is L. c. albertaensis Jehl, 1987, distinguished by its paler back and larger bill and breeding in Canada and the Dakotas.  A female collected at the Otay dump 8 January 1975, it has an exposed culmen of 46.4 mm, bill depth at gonys of 15.5 mm, and a value for L on the back of 51.6 (higher values correspond to paler colors).

Herring Gull Larus argentatus

Of all of San Diego County’s gulls, the Herring has been affected the most by the exclusion of gulls from the county’s landfills.  Formerly an abundant winter visitor at the dumps, the Herring Gull is now uncommon to at best fairly common anywhere in San Diego County.  In habitat it has the versatility of a wide-ranging scavenger, using ocean, beaches, coastal wetlands, and inland ponds and lakes.

Winter: Though widespread, the Herring Gull is more frequent along the coast than inland.  Usually it constitutes a small minority of mixed flocks dominated by the Western, California, and Ring-billed Gulls.  Concentrations as large as 100 at Point Loma (S7) 18 December 1999 (M. W. Klein), 55 at a pond on Otay Mesa (V13) 10 February 2001 (P. Unitt), and 136 at Border Field State Park 18 December 1999 (S. Walens) are now exceptional.  In the north county our largest numbers 1997–2002 were 27 at Oceanside (H5) 27 December 1997 (D. Rorick) and 14 at the Santa Margarita River mouth (G4) 29 December 1999 (P. A. Ginsburg).  During the atlas period our winter Herring Gulls were all in the coastal lowland, as far inland as Lake Wohlford (H12; one on 23 February 2002, P. Unitt) and El Capitan Reservoir (O16; one on 3 January 1998, S. Kingswood).  The Herring Gull has also been noted at Lake Henshaw on two of 22 Christmas bird counts there 1981–2002, with a maximum of four on 21 December 1987.

Migration: The Herring Gull occurs primarily late October to early April, with extreme dates of 23 September and 17 May (Unitt 1984).  Unusual records of migrants are of one at Lake Cuyamaca (M20) 25 April 1978 (AB 22:1055, 1978), and the only one for the Anza–Borrego Desert, an adult at the Borrego Country Club (G24) 3 April–9 May 1999 (P. D. Ache).  Summer stragglers were one near Imperial Beach (V10) 22 June 1980 (AB 34:930, 1980) and up to five in the San Diego Bay salt works (U10/V10) through summer 1993 (Stadtlander and Konecny 1994).

Conservation: Stephens (1919a) called the Herring Gull “rather common”; Sams and Stott (1959) called it “fairly common.”  An increase likely corresponded with the rapid increase of the county’s human population and the establishment of garbage dumps.  The number tallied on San Diego County Christmas bird counts peaked at 7323 on 2 January 1966.  Since 1985, however, the count has not returned more than 350, and it reached its all-time low of 18 on 16 December 2000.  The decrease may be not be restricted just to dumps: from 1973 to 1983 King et al. (1987) found an average of 1.3 Herring Gulls per monthly census at San Elijo Lagoon (L7), December to April.  From 1997 to 2002 not one Herring Gull was reported from this site.

Taxonomy: The only subspecies of Herring Gull nesting in North America is the pale-backed L. a. smithsonianus Coues, 1862, and only this subspecies has been recorded with certainty in California.

Thayer’s Gull Larus thayeri

Thayer’s Gull nests in the Canadian Arctic and winters mainly along the Pacific coast north of San Diego.  In San Diego County it is annual but rare—now far less numerous than before the exclusion of gulls from landfills.

Winter: Currently, Thayer’s Gull is almost exclusively coastal in San Diego County, and considerably more frequent in the southern half of the county than in the north.  From 1997 to 2002 we had only two reports of as many as three individuals, at Los Peñasquitos Lagoon (N7) 26 December 1999 (B. C. Moore) and at Border Field State Park (W10) 18 December 1999 (S. Walens).  Though the species was formerly regular at landfills in the coastal lowland, the only inland sightings during the atlas period were of single birds at Lindo Lake (O14/P14) 20–25 January 2002 (R. T. Patton, M. B. Stowe), Santee Lakes (P12) 21 February 1998 (J. L. Coatsworth), Lower Otay Lake (U13/U14) 20 January and 13 December 2001 (S. Buchanan, P. Unitt), and the Dairy Mart pond, Tijuana River valley (V11) 16 December 2000 (G. McCaskie).

Migration: Thayer’s Gulls arrive in late October or early November; their numbers increase during November and December, peak in January and February, and decline during March (Devillers et al. 1971).  The extreme dates listed by Unitt (1984), 17 October and 10 April, apparently still stand.

Conservation: The estimate reported by Devillers et al. (1971), of 100 to 150 wintering in the San Diego area, is obsolete.  The driving of gulls from the county’s garbage dumps decimated Thayer’s.  The closing of a former favored site, a pig farm near the Otay dump, did not help either.

Taxonomy: Thayer’s Gull is evidently conspecific with the Iceland Gull (Snell 2002).  Under the broader species concept, the name for Thayer’s Gull depends on the disposition of the name applied to the intermediate populations: kumlieni Brewster, 1883, antedates thayeri Brooks, 1915.

Iceland Gull Larus glaucoides

Identification of the Iceland Gull far from its normal range in the North Atlantic is fraught with uncertainty, because of extensive intergradation with Thayer’s Gull.  Indeed, evidence suggests these two are at best subspecies (Snell 2002).  The California Bird Records Committee has accepted just two identifications of the Iceland Gull from California, one from San Diego County.

Winter: San Diego County’s sole Iceland Gull was a first-winter bird photographed at the Otay dump (U12) 17–25 January 1986 (D. Delaney, Erickson and Hamilton 2001).

Taxonomy: See Snell (2002) for an analysis of variation in the Iceland and Thayer’s Gulls.  With its primaries almost uniformly white, the bird at the Otay dump resembled nominate L. g. glaucoides Meyer, 1822, which breeds mainly in Greenland, more than L. g. kumlieni Brewster, 1883, the intermediate between glaucoides and thayeri that breeds in the southeastern Canadian Arctic.

Lesser Black-backed Gull Larus fuscus

Through the 20^th century, the Lesser Black-backed Gull enjoyed a great expansion of its European breeding range, from the 1930s bringing an increasing number of vagrants to North America.  By 1978 the first of these crossed the continent to reach California, and in 1996 the species reached another milestone, the southwesternmost county in the continental United States.

Winter: San Diego County’s first and so far only Lesser Black-backed Gull was an adult photographed on the beach at the mouth of the San Luis Rey River, Oceanside (H5), 22 February–2 March 1996 (P. A. Ginsburg, NASFN 50:222, 1996; McCaskie and San Miguel 1999).  With the species’ continuing range expansion (Post and Lewis 1995), more occurrences are to be expected.

 

Taxonomy: Lesser Black-backed Gulls reaching California, like almost all others in North America, are L. f. graellsii Brehm, 1857, whose breeding range centers on the British Isles but has spread northwest to Iceland and Greenland.  It is distinguished by its back paler than that of other subspecies of the Lesser Black-backed, a shade close to that of subspecies wymani of the Western Gull.

Yellow-footed Gull Larus livens

Replacing the Western Gull in the Gulf of California, the Yellow-footed Gull is regular and increasing at the Salton Sea but just a rare vagrant to the Pacific coast.  Though at the Salton Sea the Yellow-footed Gull is primarily a postbreeding summer visitor, the seven records for San Diego County form no distinct seasonal pattern.

Migration: In San Diego County, the Yellow-footed Gull has been noted at La Jolla (P7; one on 21 May 1985, J. Nysteun, AB 39:350, 1985), Mission Bay (Q8; one on 23 June 1966, SDNHM 36001), the Otay dump (U12; one on 19 January 1979, AB 33:314, 1979; two photographed 13–28 February 1981, AB 35:336, 1981; one 25–26 Feb 1984, G. McCaskie, AB 38:357, 1984), and the Tijuana River mouth (V10; one on 7 December 1978, AB 33:314, 1979; one on 24 August 1992, D. W. Aguillard, AB 47:149, 1992).  All birds were adults, but juveniles have been found on the Pacific coast of Baja California north at least to Guerrero Negro so should be expected in San Diego County.

Western Gull Larus occidentalis

Of the gulls so ubiquitous along San Diego County’s coast, it is the rowdy Western Gull that often dominates in both numbers and size.  The Western is the only gull that nests in the county; the number of breeding pairs is modest but growing.  A large colony is within sight of San Diego, however, on Los Coronados Islands off Tijuana.  Typically coastal, the Western Gull seldom penetrates more than 15 miles inland in southern San Diego County, more than 5 miles in the north county.

Breeding distribution: The first site where the Western Gull was reported nesting in San Diego County was the cliffs at La Jolla, in 1935 (Miller 1936, Unitt 1984).   Some birds continue to nest on these cliffs (at least two pairs in 1999), but many more have begun using the tops of nearby buildings (at least seven on 12 June 1999, L. and M. Polinsky).  Other described sites are on artificial structures scattered around Mission and north San Diego bays, including the tram station at Sea World (R8), towers in the channel leading from San Diego Bay to the Marine Corps Recruit Depot (R8), and the roof of the Kona Kai Hotel, Shelter Island (S8).  At North Island Naval Air Station (S8), the birds continue to nest on the cement pilings in San Diego Bay they have used since the 1970s; on 27 May 2000 one pair was nesting on one of the runways.  It is also likely that a few Western Gulls nest on San Onofre Bluff, near which pairs were seen copulating 30 May 1998 (E3; R. and S. L. Breisch) and 15 May 2001 (D2; P. D. Jorgensen).

            Away from nesting sites the Western Gull is still common in the middle of the breeding season, with up to 100 at Encinitas (K6) 27 April and 22 May 1997 (J. M. Dart) and 85 at Los Peñasquitos Lagoon (N7) 7 June 1999 (D. K. Adams).  Inland numbers in the breeding season range up to 50 at Chollas Reservoir (R11) 15 April 1998 (P. Unitt) and 50 at Sweetwater Reservoir (S12) 8–9 June 1998 (P. Famolaro).

Nesting: Cliff ledges and predator-free islands were the Western Gull’s primitive nesting sites.  Increasingly, the gulls are now augmenting these with man-made structures, on which they build a rough nest of debris.  Before the atlas study, few data were available on the species’ nesting season in San Diego County.  Our observations suggest the Western Gull lays mainly in April in May, with young fledgling mainly in June and July.

Migration: At San Elijo Lagoon, King et al. (1987) found the Western Gull to be most numerous from June to August, least numerous in April and May.  At the south San Diego Bay salt works, however, Stadtlander and Konecny (1994) found it most numerous in September and October, least numerous in June.  Variation in the number of Western Gulls in San Diego County is more related to the birds’ concentration in breeding colonies than to long-distance migration.  Regular commuting between Los Coronados Islands and the mainland is attested by the chicken bones littering the gull colonies on the islands.

            During the atlas period, we twice noted Western Gulls much farther inland than they had been recorded in San Diego County previously.  One in second-year plumage was at Lake Morena (S21) 7 June 1997 (S. E. Smith); one was flying over Previtt Canyon (D18) 3 August 2001 (M. B. Stowe).  With the Western Gull becoming increasingly frequent at the Salton Sea (Patten et al. 2003), more sightings well inland in San Diego County are to be expected.

Winter: Even if the Western Gull is most numerous in summer or fall, it is still abundant in winter.  From 1997 to 2002 our winter counts ranged up to 482 at Oceanside (H5) 28 December 1999 (S. Walens), 775 at Torrey Pines State Reserve (N7) 26 December 1999 (B. C. Moore), and 1212 on the ocean from the mouth of San Diego Bay to Imperial Beach 18 December 1999 (D. W. Povey).  One curious feature of the Western Gull’s distribution in San Diego County is that the birds disperse inland farther and in greater numbers through metropolitan San Diego than in the north county.  Now, with extensive urbanization and many ponds and reservoirs in the north county, the difference between these two areas lacks a clear explanation.  Lakes Hodges and Lower Otay are the same distance inland, yet the Western Gull is absent from Hodges (K10) but regular at Lower Otay (U13), where counts ranged up to 35 (6 January 2001, P. Unitt).  It is especially common at Sweetwater Reservoir (S12), with up to 200 on 15 December 2001 (P. Famolaro).  Winter records extend up to 19 miles inland at San Vicente Reservoir (N14; one on 21 January 2001, N. Osborn).

Conservation: The Western Gull’s colonization of San Diego County may be part of a trend of population increase in southern California as a whole.  The banning of DDT and reduced contamination of the ocean off southern California favors the gull, while reduced upwelling and lowered productivity of the same waters disfavor it (Pierotti and Annett 1995).

Taxonomy: The subspecies of Western Gull resident in southern California is L. o. wymani Dickey and van Rossem, 1925.  The paler-backed nominate subspecies, L. o. occidentalis Audubon, 1839, breeds in northern California, Oregon, and Washington and reaches San Diego County as a rare winter visitor (Devillers et al. 1971).  Two juveniles banded in June on the Farallon Islands off San Francisco were recovered at San Diego the following October.

Glaucous-winged Gull Larus glaucescens

A winter visitor from the north, the Glaucous-winged Gull is uncommon in San Diego County, usually seen on beaches and bays in mixed flocks of the more common gulls.  With gulls now driven from the county’s landfills, there is no longer any place where the species can be seen consistently.

Winter: The Glaucous-winged Gull ranges all along San Diego County’s coast with no consistent site of concentration.  Any large mixed flock of gulls is likely to yield one or two.  From 1997 to 2002 our highest count was of nine at Oceanside (H5) 28 December 1999 (S. Walens).  When gulls were allowed to scavenge at garbage dumps these were the Glaucous-winged’s primary haunts.  During the atlas period, however, we found few away from the coast.  The site farthest inland was 12.5 miles from Mission Bay at Santee Lakes (P12; one on 7 January 1999, J. L. Coatsworth; one on 7 December 1999, M. B. Mulrooney); the only other inland sites were Lower Otay Lake (U13; up to four on 13 December 2001) and the pond at Siempre Viva and La Media roads, Otay Mesa (V13; up to five on 10 February 2001, P. Unitt).

Migration: Glaucous-winged Gulls typically begin arriving in early November, peak in late January and February, and depart through March and April.  One at La Jolla (P7) 4 October 2001 (S. Walens) was exceptionally early.  Any left after mid May are probably not migrating and may be in poor health, failing to molt on schedule.  During the atlas period we noted two such stragglers, one at the San Diego River mouth (R7) 31 May 2001 (N. Perretta) and one at the south end of the Silver Strand (U10) 27 June 1998 (C. G. Edwards).  Unitt (1984) listed six previous summer records.

Conservation: The breeding population of the Glaucous-winged Gull has increased through the 20^th century (Verbeek 1993), and winter visitors inland are becoming ever more frequent (Binford and Johnson 1995).  But the numbers seen in San Diego County have declined since the 1970s.  The San Diego Christmas bird count recorded its maximum of 270 in 1968; since 1988 the count has not yielded more than seven.  Devillers et al. (1971) estimated that numbers in the San Diego area varied from 100 to 300 annually, but from 1997 to 2002 they were probably no more than 50.  The exclusion of gulls from garbage dumps and changes in agriculture near the coast are presumably responsible for the decrease. 

Taxonomy: The Glaucous-winged Gull has always been regarded as a distinct species, but it hybridizes with the Western Gull over a long strip of coastline in Washington and Oregon, and assortative mating is weak (Hoffman et al. 1978, Bell 1996, 1997, Good et al. 2000). A second-winter bird collected at the former Balboa Park dump (R8) 23 December 1969 (SDNHM 37626) appears to be a hybrid between the Glaucous-winged and the northern subspecies of the Western Gull, L. o. occidentalis.

Glaucous Gull Larus hyperboreus

The Glaucous Gull is the northernmost of the large gulls and only a rare winter visitor as far south as San Diego County.  It is reported here at a rate of about one per year.

Winter: Most of records of the Glaucous Gull in San Diego County are coastal, ranging from a few miles offshore to 9 miles inland at Miramar Lake (N10; 25 January 1992, T. L. Williams, AB 46:315, 1992).  The Otay dump (U12) is the site with the most records, but gulls are now excluded there.  The species is typically found with other gulls, but only twice have as many as two Glaucous Gulls been found together in San Diego County—at the Otay dump.  During the atlas period from 1997 to 2002 six were noted, at Oceanside (H5) 4 May 1997 (R. T. Patton, FN 51:928, 1997), Quivira Basin, Mission Bay (R7), 4 February 2002 (D. K. Adams, NAB 56:224, 2002), San Diego Bay near Seaport Village (S9) 19 December 1998 (R. and S. L. Breisch, NAB 53:209, 1999), and Imperial Beach (V10) 13–15 March 1998 (P. A. Ginsburg, FN 52:257, 1998), 4 March 2000 (T. R. Clawson, NAB 54:221, 2000), and 4–11 March 2001 (H. L. Young, NAB 55:228, 2001).

Migration: Occurrences of the Glaucous Gull in San Diego County are concentrated late in the winter, from late January to mid March.  Extreme dates are 22 November (1988, off San Diego, D. W. Povey, AB 43:168, 1989) and 31 March (1968, Otay dump, AFN 22:478, 1968), except for three stragglers in May: 4 May 1997 (cited above), 5 May 1994 (Cardiff, L7, M. B. Stowe, NASFN 48:341, 1994), and 20 May 1973 (Point Loma, S7, AFN 27:820, 1973).

 

Taxonomy: The only specimen for San Diego County, from a former dump on the Silver Strand (T9) 22 January 1966 (SDNHM 36019), is L. h. barrovianus Ridgway, 1886.  Banks (1986a) identified all Glaucous Gulls from the Pacific coast of North America as this relatively dark, small-billed subspecies.

Sabine’s Gull Xema sabini

Sabine’s Gull crosses North America on its long migration between the Arctic and the Southern Hemisphere, but it seldom stops inland.  Off San Diego County, it is uncommon to fairly common on the ocean out of sight of land and is rarely seen from shore.

Migration:  In spring, Sabine’s Gull is recorded in San Diego County from 23 April to 3 June, with up to 35 between San Diego and San Clemente Island 8 May 1976 (AB 30:891, 1976).  The only records onshore in spring are of one on San Diego Bay 23 April 1908 (CAS 11542), one at Silver Strand State Beach (T9) 29 May 1962 (AFN 16:447, 1962), and one at Del Mar (M7) 2 June 1983 (D. Delaney, AB 37:1027, 1983).

            In fall, Sabine’s Gull occurs mainly from 11 July to October, with up to 100 between San Diego and San Clemente Island 4 September 1965 and 3 September 1967 (G. McCaskie, AFN 22:90, 1968).  Exceptionally late were one off San Diego 22 November 1969 (J. L. Dunn) and one at La Jolla (P7) 6 December 1999 (G. McCaskie, NAB 53:209, 1999).  Though still rare, the species is seen from shore more frequently in fall than in spring.  San Diego County’s two inland records are in fall, of a juvenile “about 32 miles east of the Pacific” 10 October 1920 (Lee 1921) and one at Lake Morena (T21) 2 October 1988 (B. McCausland, AB 43:168, 1989).

Black-legged Kittiwake Rissa tridactyla

This primarily pelagic gull is a highly irregular winter visitor to San Diego County.  In some years there are none; in most, there are few; occasionally, the species is abundant.  It is best looked for during the strong northwest winds following winter storms, when the birds are driven to shore and may rest on the beach.

Winter: Because of the orientation of the coastline, La Jolla (P7) is the site where the kittiwake is most easily seen in San Diego County, as for other pelagic birds.  During the only major invasion that hit shore during the 1997–2002 atlas period, February–April 2001, seawatching yielded up to 142 in 3.5 hours there 13 February 2001 (B. L. Sullivan, NAB 55:228, 2001).  On 11 February 1999 at least 60 were 30–40 miles west San Diego (P. Lonsdale, NAB 53:209, 1999), but none were seen from shore that winter.  The largest known invasion was in 1975–76, when up to 350 were on the ocean within 5 miles of shore between San Diego and Del Mar 28 March 1976 (P. Unitt).  There are three inland records, of single birds at Lake Henshaw (G17) 15 January 1981 (R. Higson, AB 35:336, 1981), Lake Murray (Q11) 3 February 1976 (SDNHM 39849), and the east end of Sweetwater Reservoir (S13) 21 April 1995 (SDNHM 49172).

Migration: The earliest date known for the Black-legged Kittiwake is 16 November (1962, one on San Diego Bay, AFN 17:69, 1963), but not until late December or January does it become abundant.  In spring, most depart in April and early May; in 2001, the latest bird was at La Jolla 21 April (M. Wilson).  After some invasions kittiwakes stay into summer.  In 1976, 60 remained at La Jolla on 17 May (J. L. Dunn), six were still there 11 July (G. McCaskie), and one was at the Oceanside harbor (H5) 8 October (A. Fries). 

Taxonomy: Rissa t. pollicaris Ridgway, 1884, is the subspecies of Black-legged Kittiwake in the Pacific Ocean.

Gull-billed Tern Sterna nilotica

San Diego County’s first Gull-billed Tern showed up in south San Diego Bay in 1985, and the species began nesting in the salt works two years later.  It has nested there annually ever since, the population growing to 32–37 pairs by 2003.  The Gull-billed Tern’s only other site in the western United States is the Salton Sea, from 1992 to 2001 home to 72–155 pairs (Molina 2004).  Only seven colonies are known in western Mexico (Palacios and Mellink 2003).  Thus, even though the San Diego population is small, it has an important role in the species’ conservation in western North America.

Breeding distribution: The south San Diego Bay salt works (U10/V10) are the Gull-billed Tern’s only nesting site in San Diego County.  From three pairs in 1987, the population grew to 27–30 pairs in 1991 and 1992 (Terp and Pavelka 1999, E. Copper, AB 45:1162, 1991, 46:1178, 1992).  From 1993 to 1998 it stabilized around 8 to 12 pairs, then increased to 11–20 in 1999, 20–27 in 2000, 30 in 2001, 32–36 in 2002, and 32–37 in  2003.  The number of young fledged reached 31–41 in 2003 (R. T. Patton).

            For foraging, the birds range from the salt works throughout south San Diego Bay, along the beach at Imperial Beach (V10; 5 to 12 seen daily through spring 2001, T. Stands), and to the Tijuana River estuary (two or three at Border Field State Park, W10, 19 July 2001, R. T. Patton).  In 2002, studying the foraging behavior of the Gull-billed Tern, K. C. Molina noted 77% of foraging over beach strand and the tidal zone of the estuary, 23% over dunes and upland scrub.  During weekly surveys of north San Diego Bay through 1993, Mock et al. (1994) encountered the Gull-billed Tern only once, two on 11 May.  One flying over the intersection of Palomar Street and Broadway in Chula Vista (U11) 11 July 2001 was 0.8 mile inland (R. T. Patton).

Nesting: Like the other terns, the Gull-billed nests on the bare dirt atop the dikes of the salt works.  Unlike the larger terns and skimmers, the Gull-billed lines the rim of its nest scrape with materials at hand—pebbles, bits of vegetation, shells, and other debris (Parnell et al. 1995).  Egg dates range from 19 April to 24 July, suggesting laying of replacement clutches until 1 July.  From 1999 to 2003, dates of first hatching varied from 17 to 27 May and dates of first fledging varied from 19 June to 4 July (R. T. Patton). 

            In 2002 the Gull-billed Terns nesting in the salt works fed their young primarily on small invertebrates (43% of all deliveries observed) and fish (25%) (Molina and Marschalek 2003).

 

Migration: Dates for the Gull-billed Tern in San Diego County extend from 1 March (1993, Stadtlander and Konecny 1994) and 10 March (1995, G. McCaskie, NASFN 49:308, 1995) to 19 September (2002, R. T. Patton).  The first sighting more than a few minutes’ flight away from the salt works was of one at Los Peñasquitos Lagoon (N7) 14 April 2001 (K. Estey).  Sightings elsewhere along the coast increased 2002–04 with up to six at the Santa Margarita River mouth (G4) 13 July 2003 (C. M. Manning, D. M. Parker).  Nearly 10 miles inland were 10–15 at the north end of Lower Otay Lake (T13/U13) 24 April 2003 (A. Grassi) and single individuals there 26 April 2003 (R. T. Patton) and 14 June 2003 (A. Grassi).

Conservation: The survey of Mexican colonies by Palacios and Mellink (2003) yielded only 376 pairs, so the population of the entire subspecies vanrossemi of the Gull-billed Tern is less than 600 pairs (K. C. Molina).   The California population is under 200 pairs, and the future of the colony at the Salton Sea is murky, given the decreasing water levels and wholesale environmental change in the offing there.  Thus the colony at San Diego Bay represents a critical hedge against the species’ extirpation from California.  The California Department of Fish and Game has recognized the precarious position of the Gull-billed Tern by designating it a species of special concern—the position is more precarious than that of some species listed as endangered.

            Ironically, around San Diego Bay the Gull-billed Tern preys regularly on the chicks of two endangered birds, the Snowy Plover and Least Tern.  As a result, several Gull-billed Terns were killed as part of the predator control undertaken to sustain these smaller species (SDNHM 48544–5, 48943, 49343–4).  Though the predation continues, after considerable debate among biologists and wildlife-management agencies, in 1999 the Gull-billed Tern was excused from the control program—at least temporarily.  One proposal is that the eggs of the Gull-billed be removed, possibly for incubation and release of the young at the Salton Sea or Gulf of California.  The conundrum is a prime example of the conflicts that arise when wildlife is confined to ever dwindling habitat, requiring ever more intensive management.

Taxonomy: On the basis of specimens from the Salton Sea, Bancroft (1929) described the Gull-billed Terns of the Pacific side of North America as S. n. vanrossemi.  His measurements show their bills as substantially larger than those of the east coast, S. n. aranea Wilson, 1814, with no overlap in length or depth at angle of gonys.  But the measurements tabulated by Parnell et al. (1995) show great overlap and suggest the difference may be inadequate for recognition of subspecies.

Caspian Tern Sterna caspia

The salt works of South San Diego Bay have been the site of a major colony of the Caspian Tern since at least the 1940s.  For foraging the birds range widely along San Diego County’s coast and on its inland lakes.  The species is primarily a summer visitor, being common from April to September, generally uncommon from October to March.  Over much of North America the Caspian Tern population is on the increase—the lack of suitable sites elsewhere is probably the reason why the salt works remain the site of the county’s only viable colony.

Breeding distribution: Caspian Terns had begun nesting in the salt works by 1941, when E. E. Sechrist collected a set of eggs on 23 April and recorded a “colony of 78 pair” (WFVZ 28472).  Over the next 40 years, the colony increased, reaching an estimated 412 pairs in 1981 (F. C. Schaffner).  Subsequently, the population has hovered around 200 to 300 pairs, with up to 320 in 1994, at least 198 in 1998 (Terp and Pavelka 1999), at least 261 in 1999, and at least 249 in 2003 (R. T. Patton).  In 1998, most of the nests were in a single cluster in the north half of the salt works (U10); in other years, they have been more dispersed.

            The only nesting known in San Diego County outside the salt works took place in 1998, when six pairs attempted nesting at Zuñiga Point at the mouth of San Diego Bay (S8).  All the nests suffered predation, however; only one chick hatched (R. T. Patton, B. Foster).

            Even during the breeding season the Caspian Tern occurs fairly commonly outside the salt works all along San Diego County’s coast (up to 30 at the Santa Margarita River mouth, G4, 21 May 2001, P. A. Ginsburg; 45 at Los Peñasquitos Lagoon, N7, 3 May 1998, K. Estey; 46 around North Island, S8, 26 May 2000, R. T. Patton).  Inland, summering Caspian Terns are generally uncommon.  Twenty at Barrett Lake (S19) 29 April 2001 (R. and S. L. Breisch) were exceptional; otherwise our highest inland count before postbreeding dispersal was of eight at Lake Hodges (K10) 26 April 1999 (R. L. Barber).  Nonbreeding Caspian Terns visit lakes as high as Cuyamaca (M20), where our counts ranged up to four on 5 June 1998 and 5 June 1999 (A. P. and T. E. Keenan).

Nesting: In the salt works, Caspian Terns nest in scrapes in the dirt on top of the dikes.  The Elegant and Royal Terns cluster around the Caspians, taking advantage of the larger species’ aggressive defense of the colony.  Cuthbert and Wires (1999) reported that egg laying at San Diego Bay begins in the first week of April, but in 1997, the first eggs were found 15 April (M. R. Smith), in 1998 on 28 April (Terp and Pavelka 1999), in 1999 on 12 May, and in 2003 on 23 April (R. T. Patton).  In 1998, most females laid in the first three weeks of May, but some laid as late as 24 July and hatched as late as about 15 August.  Late clutches are presumably replacements for ones lost to predators or accidents; the Caspian Tern raises only one brood per year.

Migration: In their 10 years of monthly censuses at San Elijo Lagoon (L7), King et al. (1987) observed peaks in Caspian Tern numbers in April and July–August.  The latter months correspond to postbreeding dispersal from the salt works; the shrill call of young still begging from their parents can be heard widely both along the coast and at inland lakes at this season.  During the atlas period counts away from the colony ranged up to 200 in northeastern Mission Bay (Q8) 27 June 1998 (E. Wallace).  Inland, they ranged up to 14 at San Vicente Reservoir (N13) 27 July 1997 (C. G. Edwards) and at Sweetwater Reservoir (S12) 7 August 1998 (P. Famolaro), though counts before the atlas period were of up to 100 at Lake Hodges 24 April 1982 and 47 there 23 July 1982 (K. L. Weaver).  Throughout the county, numbers drop sharply in October, then begin increasing again in March.  There is still only one record for the Anza–Borrego Desert, of six flying over Borrego Springs (G24) 2 April 1977 (G. McCaskie).

Caspian Terns banded at San Diego Bay have been recovered as far south as Chiapas in southernmost Mexico and as far north as Montana and Gray’s Harbor, Washington (Gill and Mewaldt 1983).  These authors noted that the San Diego Bay colony enjoyed good success but remained stable at around 400 pairs, so they concluded that it has been a source population for the species’ colonization elsewhere along the Pacific coast.

Winter: From December through February atlas observers reported only four sightings of more than 12 Caspian Terns, and the highest winter count, of 34 along the Silver Strand (T9) 27 February 2000 (Y. Ikegaya), may have been of early spring migrants.  Other high recent winter counts are of 28 at Batiquitos Lagoon (J7) 26 December 1998 (R. Stone) and 21 on the Silver Strand 24 January 2000 (J. L. Coatsworth).  Inland, our Caspian Tern sightings were of one or two individuals only, and confined to the coastal lowland, east to Lake Hodges (one on 27 December 1998, R. L. Barber), San Vicente Reservoir (one on 4 January 2002, N. Osborn), and Lower Otay Lake (U14; one on 4 December 1999, S. Buchanan).

Conservation: The Caspian Tern’s colonization of San Diego Bay was part of a range expansion covering the entire Pacific coast of North America (Gill and Mewaldt 1983).  Until 1932, there was only one record for San Diego County, in December (Saunders 1896).  Not only at the salt works but elsewhere along the Pacific coast the expansion has been facilitated by the birds’ nesting in man-made habitats, especially salt works.  In spite of the positive trend regionally, the trend locally is flat.  In spite of the Caspian Tern’s defending its nest so aggressively, the colony is vulnerable to disturbance.

            The Caspian Tern’s abundance in winter appears to have declined somewhat since the early 1970s.  Since 1972, San Diego County’s Christmas bird counts have not returned more than 30; before that year, the San Diego count yielded up to 77 (in 1967).

Royal Tern Sterna maxima

 

The Royal Tern is found along San Diego County’s coast year round, commonly in fall and winter, fairly commonly in spring and summer.  It forages mainly on the ocean near shore, then loafs in flocks on beaches and in estuaries.  A few pairs nest with other terns.  Irregular from 1959 from 1998, nesting of the Royal Tern in the salt works of south San Diego Bay became annual at the beginning of the 21^st century, with up to 35 pairs.  In the first third of the 20^th century, the Royal was the county’s commonest large tern; today, it is vastly outnumbered by the Caspian and Elegant.  In the middle of the 20^th century, numbers of the Royal apparently decreased in tandem with those of the Pacific sardine.

 

Breeding distribution: Gallup and Bailey (1960) reported the first nesting of the Royal Tern in California in the salt works in 1959, the year the Elegant Tern first colonized there.  Schaffner (1985) located the collected egg in the Western Foundation of Vertebrate Zoology and confirmed the identification.  In 1960, over 30 nests were reported in late May (AFN 14:447, 1960); there is a possibility these were misidentified Elegant Terns.  The next known nestings were in 1980, when a pair laid an egg subsequently broken by a Ruddy Turnstone, and 1982, when two pairs fledged one chick each (Schaffner 1985).  In 1984, one or two pairs nested; in 1985, one pair had a large chick 12–15 June (R. E. Webster, AB 38:1062, 1984; 39:962, 1985).  In 1997, at least one, possibly as many as four pairs nested (M. R. Smith, E. Copper, FN 51:1054, 1997).  In 1999, R. T. Patton noted 35 nests on 27 May; in 2003, about 27 nests on 7 May.

            Even during the breeding season the Royal Tern is widespread along San Diego County’s coast.  Spring flocks are usually rather small, of 15 or fewer birds, but range up to 34 at North Island (S8) 26 May 2000 (R. T. Patton).

 

Nesting: In the salt works, Royal Terns nest in close association with Caspian and Elegant Terns.  The two smaller species rely on the aggressive Caspian for defense of the colony (Schaffner 1985).  The Royal’s nesting is thus synchronized with that of the other species.  In 1982, the birds laid on or just before 8 May and the chicks hatched between 8 and 15 June (Schaffner 1985).  In 1999, adults were incubating from 12 May to 30 June; in 2003, from 29 April to 4 June (R. T. Patton).

 

Migration: The Royal Tern is generally regarded as primarily a postbreeding and winter visitor to southern California.  During the atlas periods our highest counts were in December.  During their monthly counts at San Elijo Lagoon 1973–83, however, King et al. (1987) found numbers highest in spring (April–May) and fall (September–October), lowest in December and January.  During their weekly counts in and near the salt works February 1993–February 1994, Stadtlander and Konecny (1994) found no more than seven in winter but up to 36 in September.  Evidently local movement can obscure larger-scale migration.  There are no records inland.

 

Winter: In winter the Royal Tern is widespread along San Diego County’s coast but resting flocks aggregate at a few regular sites, especially beaches at estuary mouths (up to 35 at San Elijo Lagoon, L7, 22 December 2000, G. C. Hazard; 100 at Los Peñasquitos Lagoon, N7, 2 December 2000, K. Estey) and around San Diego Bay (up to 60 on the Silver Strand, T9, 19 December 1998, N. Osborn).  Foraging birds are often common on the ocean at La Jolla (P7; up to 40 on 28 January 2000, L. Polinsky).

 

Conservation: In spite of the Royal Tern’s decrease earlier in the 20^th century, the species’ colonization of San Diego Bay represents a northward extension of the breeding range.  This extension continued beyond San Diego as the Royal began nesting with the tern colonies at Bolsa Chica in 1988 and the Los Angeles harbor in 1998 (K. C. Molina unpubl. data).  The Royal Tern’s nesting biology is harnessed behind that of the other large terns; all the species depend on the same man-made colony sites and have the same need for security from disturbance and predators.

 

Taxonomy: Royal Terns on both coasts of North America are nominate S. m. maxima Boddaert, 1783.

Elegant Tern Sterna elegans

At the beginning of the 21^st century, the world’s 50,000–60,000 Elegant Terns were nesting in only five known colonies.  Isla Rasa in the Gulf of California is the most important of these, but the salt works of south San Diego Bay are important too.  With variation in the abundance of anchovies (the tern’s principal prey) and the birds’ shifting among colonies, the number of nests in the salt works has varied from none in 1990 to an unprecedented 10,300 in 2003.  After nesting, many of the birds nesting on Isla Rasa migrate to southern California, making the Elegant Tern an abundant visitor to San Diego County’s coast in late summer and early fall.

Breeding distribution: Gallup and Bailey (1960) discovered the Elegant Tern nesting in the salt works in May 1959, with 31 pairs.  The colony grew fairly steadily until the early 1980s, reaching at least 861 in 1981 (Schaffner 1986).  Until this time, San Diego Bay was the species’ only nesting site in the United States.  With the establishment of colonies at Bolsa Chica, Orange County, in 1987 (Collins et al. 1991) and the Los Angeles harbor in 1998, the number nesting in San Diego Bay became irregular.  The number of pairs nesting was 1870 in 1995 then dropped to only two in 1997 and about 100 in 1998 (Terp and Pavelka 1998)—perhaps as a result of El Niño reducing the fish supply, perhaps as a result of predation and disturbance. In 1999, with the onset of La Niña and improved management of predators—or just shifting of the population back from Bolsa Chica—the colony shot up to 3100 nests on 27 May.  On 3 May 2000 there were 81 active nests with eggs.  The count of 10,300 nests 28 May–4 June 2003 was over three times that in any previous year (R. T. Patton).

The only site outside the salt works where the Elegant Tern has attempted to nest in San Diego County is Zuñiga Point at the mouth of San Diego Bay (S8).  In 1998, two pairs had laid eggs there by 22 May, but both nests were depredated and abandoned by 5 June (R. T. Patton).

Even during the middle of the breeding season the Elegant Tern is seen all along San Diego County’s coast, sometimes in large numbers: up to 800 at the Santa Margarita River mouth (G4) 5 June 1998 (B. L. Peterson) and 2000 at San Elijo Lagoon (L7) 13 June 1998 (B. C. Moore).

Nesting: Within the salt works, the Elegant Tern nest in a few tight clusters, in association with Caspian Terns, on the bare dirt on top of the dikes.  The exact sites shift from year to year.  Within each subcolony, egg laying is usually synchronous, after the Caspians begin (Kirven 1969).  In 1980 and 1981, the terns laid from 4 April to 6 June (Schaffner 1982).  In 1999 and 2003 they were later, with first nests found 5 May and 29 April, respectively (R. T. Patton).  After hatching, the young cluster into crèches.  After fledging, they follow their parents far from the colony and are seen being fed all along San Diego County’s coast.

Migration: Elegant Terns begin returning to San Diego typically in the second week of March, exceptionally as early as 3 March, as in 1968 (AFN 22:478, 1968) and 1982 (AB 36:893, 1982).  Postbreeding dispersal from Mexico may begin as early as late May, depending on the success of the colony at Isla Rasa (Burness et al. 1999).  Numbers peak from July to September, then drop through October and November.  At the peak, flocks from 500 to 2000 are routine.  During the atlas period our highest estimate was of 3500 at San Elijo Lagoon 8 August 1998 (B. C. Moore).

            The only records even a short distance inland are from the San Diego River in Mission Valley (R9), from which one was picked up sick or injured and brought to rehabilitators 19 October 1990 (SDNHM 47145) and where I saw one on 3 May 1992.

Winter: A few stragglers sometimes remain as late as the third week of December, accounting for the Elegant Tern’s being recorded occasionally on San Diego Christmas bird counts.  Records from the 1950s and 1960s into early January could represent misidentified Royal Terns, but experienced observers reported single individuals at North Island (S8) 6 January 2000 (R. T. Patton) and on south San Diego Bay 5 January 2003 (D. M. Parker, NAB 47:257, 2003).  The only Elegant Tern that clearly wintered in San Diego County was at North Island 20 January–8 February 2002 (R. E. Webster, R. T. Patton, NAB 56:224, 2002).

Conservation: Before 1926, when Abbott (1927e) saw flocks off La Jolla, the Elegant Tern was known in San Diego County from just one specimen (Bishop 1905).  By the early 1950s, it had become regular and common as a postbreeding visitor (Monroe 1956). Though the long-term trend in the tern’s numbers in California has been up, the species faces many threats, acknowledged in its listing as a species of special concern by the California Department of Fish and Game.  An intensely gregarious species nesting at so few sites is inevitably vulnerable.  Several colonies in the Gulf of California were eliminated in the early 20^th century, and that at Isla Rasa was reduced by commercial egging.  In the salt works, disturbance and predation by dogs have been the principal threats, and policing is essential to ensure that any intruders, including people, are excluded.  The salt works were included in the San Diego National Wildlife Refuge in 1999, but continuing management will be necessary to maintain the dikes and protect the terns.  The failure of the nesting attempt at Zuñiga Point suggests that the Elegant Tern would establish new colonies if there were suitable sites but none are available.  Elegant Tern numbers and nesting success in San Diego Bay are linked to the abundance of the northern anchovy offshore, suggesting the tern could be affected by overfishing or changing oceanographic conditions that affect the anchovy (Schaffner 1986).

Sandwich Tern Sterna sandvicensis

The Sandwich Tern breeds only on Atlantic coasts but winters in part on the Pacific coast of southern Mexico and Central America.  In 1980 one joined the Elegant Terns moving along the Pacific coast and arrived in the Elegant Tern colony in south San Diego Bay.  Presumably the same individual accounted for sporadic sightings of the Sandwich Tern at San Diego through 1987.  The only other Sandwich Tern known from California (perhaps still the same individual) hybridized with Elegant Terns in the colony at Bolsa Chica, Orange County, from 1993 to 1995 (Collins 1997).

Breeding distribution: The Sandwich Tern in the south San Diego Bay salt works (U10/V10) 11–20 May 1980 unsuccessfully courted Elegant Terns in the colony there (Schaffner 1981).  It was seen and photographed again around San Diego Bay and the San Diego River mouth (R7/R8) 15 May–13 June 1982, 12–14 June 1985, and 18 April–16 May 1987 (Morlan 1985, Bevier 1990, Langham 1991).

Taxonomy: Sterna s. acuflavida Cabot, 1847, is the subspecies of Sandwich Tern in the New World.

Common Tern Sterna hirundo

As far as San Diego County is concerned, “common” is now a misnomer for the Common Tern, now far outnumbered at any season by the similar Forster’s Tern.  Abundant as a fall migrant as recently as the 1970s, the Common Tern is now uncommon.  Former regular summering and irregular wintering appear to have ceased.

Migration: In San Diego County, the Common Tern migrates primarily offshore, but the birds pause to rest on beaches and in coastal wetlands.  Fall migrants begin arriving by 12 July (1999, four at the Santa Margarita River mouth, G4, P. A. Ginsburg), peak in late August, then dwindle away through November.  In spring, arrival is normally in mid April, but two at the San Diego River mouth (R7) 21 March 1991 were most likely early spring migrants, as none had been in the area earlier in the winter (G. McCaskie, AB 45:496, 1991).  Some may still be moving north in early June.

            Though the Common Tern was long common at the Salton Sea (Patten et al. 2003), the only inland records in San Diego County have been from Lake Hodges (K10) in fall 1979 (two on 5 August, G. McCaskie; one on 2 September, D. M. Parker) and at Bonita (T11) 2 May 1931 (SDNHM 14529).

            In the 1960s and 70s, 30 to 60 immature Common Terns typically summered on the Silver Strand (T9), with a maximum of 150 on 23 June 1967 (AFN 21:605, 1967).  But the only recent report of a clearly summering bird was of one at the south end of San Diego Bay (V10) 25 June 2002 (M. Sadowski).

Winter: Formerly, a few Common Terns usually wintered on San Diego Bay or at the San Diego River mouth.  Numbers as high as 51 on the San Diego Christmas bird count 21 December 1968 may have included some misidentified Forster’s Terns, but up to 15 were at Shelter Island (S8) 18–31 January 1975 (J. L. Dunn, AB 29:742, 1975).  By 1980 only one or two were being found annually; after 1990 no more were reported.

Conservation: The atlas study was not designed to sample migrating water birds, so the numbers of Common Terns occurring currently in San Diego County are unclear.  But even though the Common can be overlooked easily among the abundant Forster’s Terns the complete lack of Common Terns recorded by the various systematic studies of birds on San Diego Bay from the late 1980s to mid 1990s suggests a significant change.  Former numbers of Common Terns in fall ranged up to 1000 migrating south off La Jolla (P7) 17 August 1977 (J. L. Dunn) and 750 at the Santa Margarita River mouth 22 August 1978 (P. Unitt).  But no such flocks of this size have been encountered recently; from 1997 through 2003 the only large concentration reported was of 200 at the San Diego River mouth 5 September 2002 (M. Sadowski).

            The decrease of Common Terns migrating through San Diego County may be due to problems in the breeding range, in the winter range, or both.  Of the adverse factors identified by Nisbet (2002), the Common Tern’s high susceptibility to environmental contaminants and displacement of nesting colonies by burgeoning populations of gulls seem most likely to be affecting the birds breeding in central North America, the source for migrants reaching California.

Taxonomy: The only subspecies of the Common Tern collected in California is the red-billed S. h. hirundo Linnaeus, 1758, the subspecies breeding in North America.

Arctic Tern Sterna paradisaea

The Arctic Tern is famed for its migration that takes the birds from a breeding range in the Arctic to a winter range in the Antarctic.  Modest numbers pass over the ocean off San Diego County.  Only a few times have Arctic Terns—perhaps all injured or oiled—been seen on the county’s shores.

Migration: The Arctic Tern is most numerous in fall, with up to 350 between San Diego and San Clemente Island 11 September 1976 (G. McCaskie).  Fall records extend from 27 July (1968, one collected of seven seen 80 miles west of San Diego, SDNHM 36752) to 25 October (1959, 30 near San Clemente Island, AFN 14:173, 1960), except for a straggler at the 43-fathom bank 36 miles off San Diego 10 December 1977 (J. L. Dunn).

            The species is less numerous in spring, with a maximum of 15 on 16 May 1981 (E. Copper).  Spring records extend from 1 May (1987, four off San Clemente Island, R. R. Veit, AB 41:488, 1987) to 7 June (1978, one off San Diego, AB 32:1209, 1978).

            Arctic Terns seen on shore at the San Diego River mouth (R7) 6 October 1991 (G. McCaskie, AB 46:149, 1992) and at the Santa Margarita River mouth (G4) 4 June 2000 (D. M. Parker, NAB 54:423, 2000) were both stained with oil.

Conservation: As with most other ocean birds, numbers of Arctic Terns off San Diego County have decreased since the early 1980s.

Forster’s Tern Sterna forsteri

Forster’s is the most widespread tern in San Diego County, most abundant in the coastal lagoons and bays but found regularly on inland lakes as well.  Though migratory, it is common in the county year round.  Hundreds of pairs nest annually in the salt works, site of the oldest and largest Forster’s Tern colony in southern California.  Since 1990, the birds have attempted to establish new colonies elsewhere along the county’s coast, though these are still small and often unsuccessful.

Breeding distribution: First observed in 1962 (Gallup 1963), the Forster’s Tern colony on the dikes of the salt works (U10/V10) has fluctuated between 100 and 600 pairs since 1963.  Because of renesting and the nests’ dispersion throughout the salt works the population is difficult to census accurately, but some recent figures are 548 nests in 1991, at least 510 in 1993, at least 345 in 1994, 520 in 1997, at least 225 in 1998 (Terp and Pavelka 1999), 126 in 1999, and at least 203 in 2003 (R. T. Patton).  The Chula Vista Wildlife Reserve, a similar artificial site on the north side of the salt works (U10), had 173 nests in 2001 (R. T. Patton).  Within the salt works, the Forster’s nests are less clumped than those of the larger terns; in 1998 the salt works’ perimeter road was the section used most heavily (Terp and Pavelka 1999).

            At the west end of Batiquitos Lagoon (J6) Forster’s Terns began nesting in 1990 (mummified chick picked up 3 August, SDNHM 46863) and were still doing so at least in 1997, 1998 (M. Baumgartel), and 2002 (R. T. Patton).  The number that nest is apparently small, though the birds are common in the area (122 in the lagoon’s east basin, J7, 1 May 1998, F. Hall; 247 throughout the lagoon 10 July 1997, Merkel and Associates 1997).  Attempts by single pairs at San Elijo Lagoon (L7) 5 June 2000 (A. Mauro) and at the San Dieguito River estuary (M7) 5 June 2001 (D. R. Grine) were aborted.  In 2001 the terns colonized Kendall–Frost Marsh, Mission Bay (Q8).  On 10 July, scanning from Crown Point Drive, R. T. Patton noted one chick, two young fledglings, and 17 adults, including three incubating and six feeding chicks hidden in the marsh.  Some nests were lost to tidal flooding.  Nesting at Kendall–Frost continued through 2003; abandoned chicks were picked up there 7 and 15 June (SDNHM 50786, 50867).

            During the breeding season, nonbreeding Forster’s Terns are common away from the colonies at both coastal wetlands (50 at San Elijo Lagoon 2 May 1999, B. C. Moore; 40 at Famosa Slough, R8, 2 July 1998, A. E. Klovstad) and lakes in the coastal lowland (50 at Sweetwater Reservoir, S12, 4 May 1998, P. Famolaro; 45 at Lower Otay Lake, U14, 29 May 2001, N. Osborn).  Even on foothill reservoirs the tern is fairly common at times, with up to 22 at Barrett Lake (S19) 4 June 2000 (R. and S. L. Breisch) and 20 at the upper end of Loveland Reservoir (Q17) 20 May 1998 (P. Famolaro).  If there were suitable secure islands in these reservoirs the birds would likely nest there; they engaged in courtship feeding at Barrett Lake 23 May 1999 and 4 June 2000 (J. Hannan, R. and S. L. Breisch) and at the upper end of El Capitan Reservoir (N16) 29 June 2001 (J. R. Barth).

Nesting: In the salt works, Forster’s Terns tend to nest on the shoulders of the dikes, often in vegetation like pickleweed and crystalline iceplant rather than on bare dirt on top of the dikes like the larger terns.  The birds lay eggs from late April to late June, occasionally to early August.  A fledgling at Batiquitos Lagoon 12 June 1997 must have hatched from an egg laid no later than 23 April (M. Baumgartel).  Some eggs were still being incubated in the salt works as late as 18 July in 2001 and 2 September in 2003 (R. T. Patton).

Migration: Seasonal variation in Forster’s Tern abundance in San Diego County is not well marked.  In the salt works, on the basis of weekly surveys February 1993–February 1994, Stadtlander and Konecny (1994) noted two seasonal peaks, in May and November, and a maximum of 368 on 3 November.  In central and southern San Diego Bay, on the basis of weekly surveys April 1993–April 1994, Manning (1995) noted larger numbers October–March than during the breeding season.  At San Elijo Lagoon, however, on the basis of monthly surveys 1973–83, King et al. (1987), reported the largest numbers from July through September.  At Lake Hodges (K10/K11) K. L. Weaver noted a peak in spring of 100 on 24 April 1982, a peak in late summer of 90, including many juveniles, 13 August 1983.  A Forster’s Tern at Borrego Springs (G24) 30 March 1998 (P. D. Jorgensen) is the only one recorded in the Anza–Borrego Desert.

Winter: In winter Forster’s Tern concentrates more along the coast, especially in San Diego Bay.  Counts there range up to 263 in the central bay 14 December 1994 (Mock et al. 1994).  Winter numbers at other coastal sites range up to 75 at the Santa Margarita River mouth (G4) 27 January 1999 (P. A. Ginsburg) and 74 at San Elijo Lagoon 26 December 1999 (R. T. Patton).  Inland, wintering Forster’s Terns are sometimes common on Lake Hodges (K10; up to 75 on 22 December 2000, R. L. Barber) but generally uncommon elsewhere in the coastal lowland (up to 27 at Lake Wohlford, I12, 4 January 2001, J. O. Zimmer; 20 at San Vicente Reservoir, N13, 28 January 2002, N. Osborn).  Above 1500 feet elevation, the only lakes where we found Forster’s Tern in winter were Sutherland (J16; one on 22 February 2000, M. B. Stowe) and Henshaw (G17; three on 2 December 2001, C. G. Edwards; noted on 7 of 22 Christmas bird counts 1981–2002, maximum count seven).

Conservation: Between the salt works and the reservoirs inland, Forster’s Terns exploit habitats not available in San Diego County until the 20^th century.  The attempts to colonize new sites suggest the population is vigorous.  The dearth of suitable nesting sites secure from predators and disturbance, however, probably limits the population.  Even within the salt works the intrusion of terrestrial predators is a constant problem for all the water birds nesting there.

Least Tern Sterna antillarum

The beaches that are now the summer playground of millions of southern Californians were once the home of the Least Tern.  With disturbance and development of its habitat, this once common bird became rare and was among the first species listed as endangered in 1970 by both the California Department of Fish and Game and U.S. Fish and Wildlife Service.  Intensive study, monitoring, and management have allowed San Diego County’s Least Tern population to increase from about 500 pairs in the late 1970s to about 2100–2800 pairs 1997–2002 and nearly 4000 pairs in 2003.  The tern’s future, however, is now in the hands of man: special protection of colony sites and control of weeds and predators are inescapable if a ground-nesting bird like the Least Tern is to survive in its now urbanized environment.

Breeding distribution: The Least Tern’s nesting sites in San Diego County in the late 20^th and early 21^st centuries are listed in Table 7, on the basis of data provided by Robert T. Patton and recorded by him, Elizabeth Copper, Brian Foster, and Shauna Wolf. 

Table 7            Estimated Minimum Number of Nesting Pairs of the Least Tern in San Diego County, 1997–2003

Site	Square	1997	1998	1999	2000	2001	2002	2003
Aliso Creek mouth	F4	17	33	53	36	37	39	87
Santa Margarita River mouth	G4	808	727	619	993	953	545	1091
Batiquitos Lagoon (west half)	J6	142	93	126	110	113	147	398
Batiquitos Lagoon (east half)	J7	129	86	28	26	62	72	176
San Elijo Lagoon	L7	9	1	8	15	8	1	0
Mission Bay: FAA Island	Q8	20	31	66	173	184	192	216
Mission Bay: north Fiesta Island	Q8	76	21	0	15	53	60	60
Mission Bay: Mariner’s Point	R7	268	528	562	282	227	220	250
Mission Bay: South Shores	R8	0	9	0	0	0	0	0
Naval Training Center	R8	0	0	0	0	2	0	0
North Island Naval Air Station	S8	22	59	75	128	105	71	165
Lindbergh Field	S9	102	17	20	25	35	48	46
Naval Amphibious Base	T9	410	495	570	541	664	534	954
D Street Fill	T10	38	5	30	28	30	23	79
Chula Vista Wildlife Reserve	U10	0	2	2	0	0	3	25
Salt works	U10/V10	36	39	15	35	35	26	39
Tijuana River mouth	V10/W10	211	81	87	178	252	146	358
Total		2288	2227	2261	2585	2760	2127	3944

            The colonies at Aliso Creek, the Santa Margarita River mouth, Naval Amphibious Base, and Tijuana River mouth are also notable because they are the county’s only sites where the tern still nests on dunes and flats still more or less in their natural condition.  At Batiquitos Lagoon, the terns nest on several artificial sand flats installed for them at the time the lagoon was dredged and reopened to the tide, as part of an attempt at restoration of this badly silted lagoon in the mid 1990s.  Around Mission and San Diego bays, most of the tern’s nesting sites are fills, islands, or dikes built of dredge spoil, sometimes covered with sand. 

            Least Terns have a high level of fidelity to colonies where they have established themselves as adults, less so to their natal colonies (Atwood and Massey 1988).  But high levels of disturbance and a high rate of nest failure mean that shifting among colonies is frequent.  Site fidelity may be lower in San Diego County than in Los Angeles and Orange counties (area of Atwood and Massey’s study) because in San Diego there are more colonies with less distance between them.  The terns sometimes take advantage of new sites (whether they were designed for the birds or not) as soon as they are created.  For example, during the building of Seaport Village in downtown San Diego in 1977, 17 pairs nested on new bayfill there, and the first year after the installation of the Chula Vista Wildlife Reserve in 1980, 55 pairs nested there.  The trend over time, however, with development of the coastline almost complete, has been for the tern’s distribution to become more stable from year to year.

            Least Terns forage in the bays and estuaries near their colonies, on the ocean near shore, and at inland lakes in the coastal lowland.  No nesting was reported inland during the atlas period 1997–2001, but in earlier years the birds established small, ephemeral colonies up to 4 miles from the coast.  For example, in 1981, nine pairs nested in Encinitas (K7) in a then-vacant lot 2 miles from the beach and 2 miles from San Elijo Lagoon.  O’Neill Lake, Camp Pendleton (E6), is the terns’ most heavily used inland foraging site, with up to 40 there 26 July 1999 (P. A. Ginsburg).  More birds go inland after the young fledge in late July and August and the birds are no longer tied to their nests.  But even in the middle of the breeding season some birds forage inland, with up to 15 at O’Neill Lake 15 June 1998 (P. A. Ginsburg) and 10 at San Dieguito Reservoir (K8) 14 June 1998 (J. Determan).  The most inland sites are Depot Lake in the Fallbrook Naval Weapons Station (D6; one on 16 May 2002, J. R. Barth), Lake Hodges (K10; two on 14 June 1999, R. L. Barber), and a pond in the San Luis Rey River valley at the mouth of Keys Creek just east of Interstate 15 (E9; two engaging in courtship feeding 10 June 2001, J. E. Pike).  At 15.8 miles from the beach the last locality is the farthest inland.  Apparently Least Terns move inland much more in the northern than the southern half of San Diego County.  South of the San Dieguito River we had no records during the atlas period, though in past years Least Terns have been seen in Mission Valley (R9), along the Sweetwater River at Bonita (T10/T11), and at the Dairy Mart pond in the Tijuana River valley (V11).

Nesting: The Least Tern’s nest is a simple scrape in the sand or dirt, usually lined with a few broken bits of shell or debris.  At Lindbergh Field and North Island, the birds nest in cracks in the pavement, other gravelly areas, or the small depressions for electric lights to guide airplanes at night, in spite of the heavy traffic of aircraft around them.  Within a colony, nests may be within a few feet of each other, as on the FAA Island when large numbers of terns use that site, or well separated, as in most colonies—a strategy for making the nests difficult for predators to discover.

            In southern California, the Least Tern lays its eggs generally from mid May to early July.  A second “wave” of laying by two-year-olds and birds that lost their first clutches follows the first by about four to five weeks (Massey and Atwood 1981).  At the Santa Margarita River mouth in 1997, the terns began laying 6 May (exceptionally early), began hatching 29 May, and finished hatching 13 July (B. Foster).  At the salt works in 1998, they laid from at least 15 May to 30 June and hatched from 12 June to 21 July (Terp and Pavelka 1999).  The terns may lay as late July, but these eggs are abandoned by mid August.  Eggs being incubated up to 8 August can still hatch into chicks that fledge (E. Copper).

Migration: Least Terns typically begin arriving in San Diego in mid April.  During the atlas period 1997–2001 the earliest report was 13 April (Batiquitos Lagoon, M. Baumgartel), but in 1995 it was 4 April (two on San Diego Bay, B. Foster, NASFN 49:308, 1995), and in 1994 it was 30 March, the earliest ever (E. Copper, NASFN 48:341, 1994).  Postbreeding dispersal begins immediately after the chicks start fledging in June.  By the first week of July, flocks can be seen away from colonies (52 at San Onofre State Beach, C1, 6 July 1997, P. D. Jorgensen), but many remain in San Diego County through August.  Most Least Terns leave the county in early September, and by mid September the species is rare.  There are two records of stragglers as late as October: of one near Imperial Beach 9 October 1993 (G. McCaskie, AB 48:152, 1994) and one at the San Diego River mouth 27–28 October 1981 (C. G. Edwards, E. Copper, AB 36:218, 1982).

            Birds banded as chicks in San Diego County have been seen in nesting colonies as far north as Alameda County, the northern limit of the Least Tern’s range in California.  Least Terns banded in southern California, including San Diego County, have been seen in winter along the Pacific coast of Guatemala, of Chiapas, southern Mexico, and of Colima, western Mexico (Massey 1981).

Conservation: The Least Tern’s increase over the final quarter of the 20^th century is a success, but there is no room for complacency.  The forces that made the tern an endangered species are as strong as ever and would overwhelm it without intensive management.  A basic problem is that the colony sites are now fixed: with the rest of the San Diego County coastline developed, no alternative sites are available if a site is overgrown with vegetation and so becomes unsuitable or when predators learn the location of a colony and return repeatedly.  Accelerated silting in of lagoons, the result of the vegetation being stripped from watersheds during development, has eliminated some former nesting sites, as at Los Peñasquitos Lagoon.  The tern’s strategy of shifting colony sites as conditions demand is no longer possible.

            Many techniques have been used to encourage and protect the terns.  Teams of volunteers organized by the San Diego Audubon Society and U.S. Fish and Wildlife Service have controlled invasive vegetation that would otherwise overrun sites such as Mariner’s Point, Fiesta Island, the FAA island, and the Tijuana River mouth.  Papier-mâché models of Least Terns have been set out as decoys to draw the birds to nest at sites where they can best be protected.  Sites that lack adequate protection for chicks have been supplied with pieces of Spanish roof tiles, under which the chicks can take refuge.  Some sites have been fenced and posted—fencing must be of sturdy chain link to be really effective, though this sometimes provides perches from which predators can survey the colony.  Predators have been controlled through both trapping and shooting.  The list of predators attacking the Least Tern in San Diego County is long and varied, including the Western Gull, American Kestrel, Common Raven, California ground squirrel, coyote, opossum, rat, ant, and domestic dog and cat.  Even species as apparently innocuous as the Western Meadowlark and Domestic Pigeon have destroyed Least Tern eggs (E. Copper).  In one week in 1999, a single coyote destroyed or caused the abandonment of about 340 of the 790 nests at the Santa Margarita River mouth, site of California’s largest colony Least Tern (B. Foster). Unfortunately, several known predators are themselves rare, declining, or endangered species, including the Peregrine Falcon, Northern Harrier, Burrowing Owl, Loggerhead Shrike, and Gull-billed Tern, making predator control for the sake of the Least Tern a balancing act.

            Adequate funding and coordinated management for activities this intensive is a constant challenge.  Sites must be cleared before the birds arrive—not after.  Vehicles, horses, and pedestrians must be kept out of colonies before they crush eggs and chicks.  Control appropriate for one predator is inappropriate for another, and which predator will strike which colony next is difficult to predict.

            Oceanographic variations may also affect the Least Tern’s food supply and nesting success.  From 1993 to 1997, San Diego County colonies produced an average of 1200 fledglings per year, but in 1999 the figure was only around 100.  That year, newly hatched chicks were smaller than normal by roughly 25%, and many died within just a couple of days of hatching. The nest-abandonment rate was also higher than normal, so many of the eggs laid never had a chance to hatch.   An ocean cooler than usual, as a result of La Niña, apparently delayed the breeding of the small fish on which the terns feed (B. Foster).  Conversely, the increased ocean temperature and reduced fish abundance associated with El Niño also have a negative effect: Atwood and Massey (1988) suggested that the dip in California’s Least Tern population in 1984 was due to high mortality in the winter range during El Niño.  The population did not recover to its 1983 level until 1988 (Massey et al. 1992).

Taxonomy: The type locality for the California Least Tern, S. a. browni Mearns, 1916, is in San Diego County, on the beach at the international border.  The validity of this subspecies has often been questioned, but Johnson et al. (1998) investigated the Least Tern’s variation in color and upheld the distinction of browni from subspecies athalassos of the Mississippi basin and antillarum of the Atlantic coast on the basis of a combination of color differences.  The trend of these is that browni differs from these two in its paler nape but duller back and darker (though still whitish) breast.  Johnson et al. did not compare browni with two subspecies in Mexico, but the more northern of these, S. a. mexicana van Rossem and Hachisuka, 1937, was described as still darker than browni, the difference evident in the field.

Sooty Tern Sterna fuscata

Among the tropical ocean birds that began to appear in southern California in the 1990s was the Sooty Tern.  The four reports for San Diego County include two pairs in tern colonies.  But the nesting attempt that got as far as incubation of an egg ended in failure when both adults were killed by a Peregrine Falcon.

Breeding distribution: Brian Foster and Patricia A. Campbell found a Sooty Tern at the Santa Margarita River mouth (G4) 6 July 1996 and photographed a copulating pair there the following day (McCaskie and San Miguel 1999).  Later that summer, probably the same birds accounted for reports of one at San Elijo Lagoon (L7) 28 July and two in the south San Diego Bay salt works (U10) 23 August 1996 (N. Shany, J. Konecny, NASFN 50:997, 1996).

The following year, in 1997, a pair nested in the salt works.  Smith (1999) noted the first adult 15 April, saw the pair together 30 May, and found the egg 3 June.  But on 6 June one adult, and on 10 June the other adult, were dead and partially dismembered, indicating predation by a Peregrine Falcon, which had been seen in the area.  The egg and remains of both adults are preserved (SDNHM 49807, 49966).

Migration: The California Bird Records Committee accepted the identification of an immature Sooty Tern, California’s first of this species, at the San Diego River mouth (R7) 27 September 1982 (R. E. Webster, Morlan 1985).  Another Sooty Tern was reported on the ocean off Coronado (T9) 2 September 1998 (B. Foster, FN 52:126, 1998).

Conservation: The appearance of the Sooty Tern and other warm-water birds in southern California presumably reflects the increase in ocean temperatures.  The Sooty Tern nests normally on predator-free oceanic islands; the pair at the salt works did not engage in the mobbing defense typical of the other terns nesting there (Smith 1999).

Taxonomy: The gray lower belly and crissum of the birds at the salt works are typical of the subspecies of Sooty Tern in the Pacific Ocean (Smith 1999).  Sterna f. crissalis (Lawrence, 1871) was described from Socorro Island in the Revillagigedo archipelago, one of the nesting sites nearest San Diego.

Black Tern Chlidonias niger

The Black Tern nests in inland marshes (no closer to San Diego County than California’s Central Valley), then heads to a pelagic winter range off southern Mexico, Central America, and South America.  Much of the population migrates by way of the Salton Sea, and few Black Terns now reach the coast of southern California.  The species’ population decline is reflected in greatly reduced numbers seen in San Diego County.

Migration: The Black Tern occurs in San Diego County primarily as a fall migrant, though by the beginning of the 21^st century it was averaging only about two birds reported per year.  In 1999 and 2001 it was missed completely.  It is most frequent from late July to mid September but also occurs occasionally in spring and early summer.  Extreme dates are 26 April (1991, San Diego, J. Brisson, AB 45:496, 1991) and 21 November (1958, Sweetwater Reservoir, S12, AFN 13:66, 1959), the six winter records excluded.  The maximum number seen together recently is three, at San Elijo Lagoon (L7) 13 June 1998 (B. C. Moore) and the San Diego River flood-control channel (R8) 11 May 2002 (M. B. Mulrooney).  Black Terns generally occur along the coast or at ponds and lakes in the coastal lowland, inland to Lake Hodges (K10).  Exceptions are two at the Borrego Springs sewage ponds (H25) 12 May 1990 (A. G. Morley) and a remarkable 41 at Lake Henshaw (G17) 10 August 1984 (R. Higson, AB 39:103, 1985).

Winter: Unitt (1984) summarized the six winter records, all coastal, and none more recent than 1977.

Conservation: No numerical data on the Black Tern’s former status are available, but Willett (1933) considered the species a “common migrant” in coastal southern California, and Stephens (1919a) considered it “rather common” in San Diego County.  Even in the 1970s it was seen much more frequently than at the turn of the millennium, with up to 13 in the Tijuana River valley 17 August 1977 (J. L. Dunn).  Even though Black Terns take advantage of agriculture, making rice fields their principal nesting habitat in northern California (Shuford et al. 2001), their population a whole has decreased seriously, possibly because of pesticide contamination, degradation of wetlands, and overfishing in the winter range (Dunn and Agro 1995).

Taxonomy: Chlidonias n. surinamensis is the subspecies of the Black Tern in the New World.

Black Skimmer Rynchops niger

Through the final third of the 20^th century, this avian oddity expanded its range greatly, moving northwest from the coast of mainland Mexico and colonizing California.  The salt works of south San Diego Bay have become a major colony with hundreds of pairs.  The skimmer resides in San Diego County year round but most of the birds shift between the salt works in summer and Mission Bay in winter.  Elsewhere along the county’s coast the skimmer is much less abundant, but a small colony has established itself at Batiquitos Lagoon.

Breeding distribution: Within the salt works (U10/V10), the skimmers nest on several dikes spread throughout the system; the locations vary year to year and shift through the course of each season.  The number of distinct nest clusters or subcolonies has ranged from 7 in 2002 to 13 in 1998 (R. T. Patton, Terp and Pavelka 1999).  During the atlas period the total number of nesting pairs varied from about 485 in 1997 to at least 280 in 1998 (Terp and Pavelka 1999) to about 200 in 1999 to 187–216 in 2000 to 268–280 in 2001.  In 2002 and 2003 it went back up to 331 (R. T. Patton). 

In the west basin of Batiquitos Lagoon (J6), 14 pairs of skimmers colonized in 1995 on dredge spoil installed as nesting habitat for the Least Tern and Snowy Plover in 1994.  Ten pairs nested there in 1996 (Whelchel et al. 1996).  Subsequently, the birds shifted to an island in the east basin (J7), where they nested at least in 1997, 1998, and 2001–03.  In the last three years the number of pairs increased from 8 to 10 to at least 26, fledging 19, 5, and 15–19 young, respectively (S. M. Wolf).

 

Nesting: Black Skimmers nest in a scrape on bare ground, using no nest material.  In the salt works, they nest either near the large terns or in separate clusters of their own, the nests spaced a few feet apart.  The birds may begin selecting their nest sites and scraping as early as 5 May (1999).  From 1997 to 2003, egg laying began between 16 May and 2 June (R. T. Patton, Terp and Pavelka 1999).  In 1998, the great majority of the birds laid between 10 and 20 June.  Some nesting, however, presumably laying of replacement clutches by birds losing first clutches, continued to late August in 1998 (Terp and Pavelka 1999) and 22 August in 2001 (R. T. Patton).  From 1999 to 2003, incubation continued through 22 August to 18 September.  Chicks began hatching from 7 to 18 June and nonflying young were observed as late as 4 September to 7 October.  Newly hatched chicks were found as late as 11 September.  The first flying fledglings were observed each season between 18 and 28 July, and some remained through 19 September to 7 October.

The year the skimmers first colonized Batiquitos Lagoon they nested late, laying from 30 July to 21 August and fledging from 25 September to 17 October (Whelchel et al. 1996).  From 2001 to 2003 egg dates their ranged from 23 May to 2 August with latest fledging in 9 September (S. M. Wolf).

Migration: Recoveries of banded birds show that the skimmers move among the colonies on the coast of southern California, at the Salton Sea, and at the head of the Gulf of California (Collins and Garrett 1996).  In San Diego County the birds are most concentrated at the nesting colonies from late May through the first week of July.  During that interval atlas observers saw no more than four at other locations, as at San Onofre State Beach (C1) 1 July 1999 (P. D. Jorgensen) and at the San Diego River mouth (R7) 28 June 1999 (F. Shaw).  Through July skimmers are seen increasingly at other coastal wetlands and occasionally flying past places where they neither rest nor feed (two at La Jolla, P7, 17 July 1999, E. Wallace).  The wintering flock at Crown Point, Mission Bay (Q8), peaks in fall and early winter (up to 140 on surveys 1992–95, Gazzaniga 1996; up to 278 on 18 December 2002, K. C. Molina), but it may remain large until late April (90 on 15 April 1999, E. Wallace)

            Only one skimmer has been reported inland in San Diego County, at Upper Otay Lake (T13) 19 July 2000 (T. W. Dorman).  Because of population exchange across the mountains and sightings at Lake Elsinore and Mystic Lake in western Riverside County (Garrett and Dunn 1981), such a record is not too surprising.

Winter: Mission Bay is the skimmer’s primary winter site in San Diego County; the birds concentrate on the east side of Crown Point.  Counts there during the atlas period ranged up to 130 on 13 February 2000 (B. Hendricks).  But the birds move around the area; our highest winter count was of 200 in the San Diego River flood-control channel and at Fiesta Island (R8) 28 February 1999 (B. C. Moore).  The number remaining in south San Diego Bay is variable, sometimes large (up to 70 on 19 December 1998, D. C. Seals), sometimes small or zero.  Other sites where skimmers often winter in some numbers are Agua Hedionda Lagoon (I6; up to 35 on 9 February 2001, P. A. Ginsburg), Batiquitos Lagoon (up to 16 on 4 December 1998, C. C. Gorman), and the Tijuana River estuary (V10/W10; up to 50 on 19 December 1998, S. Walens).  Winter sightings elsewhere along the coast were of six or fewer.

Conservation: The skimmer’s range expansion has been facilitated by its use of artificial habitats like the salt works.  The species was first seen in the county 18–19 September 1971 (AB 26:121, 1972) and first nested at the salt works (a single pair) in 1976 (AB 30:1004, 1976).  For the next 20 years the population increased fairly steadily; the 485 pairs estimated in 1997 is the highest figure yet.  Subsequently the population seems to have stabilized at around 300 to 400 pairs (K. C. Molina unpubl. data).

            Even though its numbers have increased, the skimmer is vulnerable to several threats.  The heavily urbanized coast of southern California offers few secure nesting sites to colonial water birds that nest on the ground.  The concentration of the population at just three or four locations means that a disruption of one site has a large effect on the entire population.  In the first years of the 21^st century, skimmer nesting at the Salton Sea began to fail as the sea could no longer sustain fish (K. C. Molina, NAB 56:486, 2002; 57:545, 2003).  The incorporation of the San Diego Bay salt works into the San Diego National Wildlife Refuge was an important step, facilitating monitoring and management of a site critical to many nesting water birds, but threats remain.  The site must be policed with vigilance; disturbance by people, dogs, and predators getting onto the dikes is a continuing problem.  Disturbance at a critical time could eliminate an entire year’s production of young.

Taxonomy: Black Skimmers on both the Pacific and Atlantic coasts of North America are nominate R. n. niger Linnaeus, 1758.  It is distinguished from other subspecies in South America by having the tail white except for the central black pair of rectrices.