Pied-billed
Grebe
Podilymbus podiceps
Strange loud noises like choking, emanating from a marsh, are most likely coming from a Pied-billed Grebe. Large lakes, small ponds, and brackish lagoons offer habitat to this species year round as long as they support patches of marshes. Some birds disperse for the fall and winter onto protected salt water. The Pied-billed Grebe is adopting some of the habits of the megapodes of the Australasian region, using the heat and insulation from rotting wet nest material as an aid to incubation.
Breeding distribution: The Pied-billed Grebe occurs throughout San Diego County’s coastal slope wherever it can find suitable habitat. The largest numbers are in the coastal lagoons, especially Buena Vista Lagoon, site of up to 46 in the lagoon’s west basin (H5) 13 June 1999 (J. Ginger) and 30 in the east basin (H6) 18 June 1999 (L. E. Taylor). The birds are also numerous on lakes well equipped with fringing marshes; for example, daily counts at O’Neill Lake (E6) ranged up to 25 (21 August 1997, P. A. Ginsburg), those at Lake Murray (Q11) to 18 (N. Osborn).
In the foothills and mountains there are fewer suitable ponds and lakes, but the grebe nevertheless occurs at many of these, some quite isolated, such as Twin Lakes in Cooper Canyon (C18; three, including one chick, 15 June 2000, M. and B. McIntosh). It is confirmed breeding as high as 4600 feet elevation at Lake Cuyamaca (M20; up to ten, including fledglings, 26 June 1999, A. P. and T. E. Keenan) and recorded as high as 5400 feet at Big Laguna Lake (O23; up to two on 24 July 1998, E. C. Hall). The Pied-billed Grebe is also found at a few spots a short distance over the divide on the east slope of the mountains. Among these places, only at Tule Lake (T27) did we find more than two individuals and confirm breeding (up to 20 on 6 and 21 June 2000, adults feeding young 6 June 2001, J. K. Wilson).
Nesting: The Pied-billed Grebe builds a floating platform of marsh vegetation, tying it to emergent plants. Research in San Diego County, among other places, demonstrated that the grebes regulate the temperature and humidity of the incubating eggs by covering them with damp nest material, partially substituting it for the body of the adult (Davis et al. 1984). Sometimes the nest is at the edge of a marsh where it can be seen by a human observer, but often it is hidden within dense vegetation. As a result, the great majority of our confirmations of Pied-billed Grebe breeding were of observations of chicks. The young leave the nest and climb aboard the adult’s back shortly after hatching.
Almost all of our records of Pied-billed Grebe chicks fell between 11 April and 22 September. An abandoned newly hatched chick was picked up in Rancho Santa Fe (R8) 15 September 2001 (SDNHM 50574). These data show that in San Diego County the Pied-billed Grebe lays mainly from the third week of March to the third week of August. The species also nests occasionally in winter, as we noted once during the atlas period, with two young several weeks old but still begging from an adult at Wilderness Gardens (D11) 20 January 2001 (K. L. Weaver).
Migration: The degree to which movement of Pied-billed Grebes reflects arrival of migrants from the north versus shifting of the local population is unclear. On salt water, the species occurs mainly from September to March, only rarely in late spring and summer (no more than one per monthly survey of central San Diego Bay 3 May–13 October 1993–94, Mock et al. 1994; no more than one per weekly survey of the San Diego Bay salt works April–July 1993, Stadtlander and Konecny 1994). The latter authors found a maximum of 20 on 13 October 1993 and a monthly average peaking in December and January.
Juveniles may disperse soon after fledging while still retaining some of the chicks’ striped head pattern. Such a juvenile on a small pond in Greenwood Cemetery (S10) 31 May 1997 was not raised there (P. Unitt).
In the Anza–Borrego Desert the Pied-billed Grebe is a rare visitor. There are five records in the Borrego Valley in fall from 5 September (1998, one near Borrego Springs Country Club, F24, P. D. Jorgensen) to 15 November (1984, one at the Roadrunner Club, F24, A. G. Morley). In late spring there are two records, of two in Borrego Springs (G24) 22 May 2001 (P. D. Ache) and one at a pond in an orchard near the mouth of Coyote Creek Canyon 15 June 1973 (ABDSP database).
Winter: In winter the Pied-billed Grebe ranges more widely than it does in spring and summer. Salt-water sites where it winters but does not breed are Agua Hedionda Lagoon (up to six on 27 December 1997 and 26 December 1998, C. Sankpill), Mission Bay (up to 20 in the northwest quadrant of the bay, Q7, 21 January 2001, L. Polinsky), the San Diego River flood-control channel and Famosa Slough (R8; up to 20 on 6 January 2000, J. A. Peugh), and San Diego Bay (up to 20 between downtown San Diego and Coronado, S9, 15 December 2001, Y. Ikegaya). Numbers on the lagoons of northern San Diego County are little changed from those in the breeding season, but on some inland lakes they can be considerably higher, e.g., up to 49 at Dixon Lake (I11) 2 January 2000 (C. Rideout) and 50 at Lake Cuyamaca 15 January 1998 (A. and. T. Keenan). In the Anza–Borrego Desert the only winter record is of one on the small pool at Butterfield Ranch (M23) 20 January 1973 (ABDSP database).
Conservation: There is no clear evidence for any trend in Pied-billed Grebe numbers in San Diego County, and data from other parts of the range are contradictory (Muller and Storer 1999). As an aquatic bird the grebe is exposed to water pollution, though it nests successfully in sewage ponds. In San Diego County, the grebe benefits greatly from the mass importation of water and the creation of the many reservoirs and ponds used to manage this water. The stocking of these ponds with fish and the introduction of the crayfish, not native to southern California, have supplied the grebe with its staple foods.
Taxonomy: All Pied-billed Grebes in North America are nominate P. p. podiceps (Linnaeus, 1758).
Horned
Grebe
Podiceps auritus
A winter visitor, the Horned Grebe is small and easily overlooked among the many other swimming birds along San Diego County’s coast. Even though San Diego is near the southern tip of the species’ winter range, the Horned Grebe is fairly common on San Diego and Mission bays, though outnumbered by the similar Eared Grebe.
Winter: In San Diego County, the Horned Grebe occurs most numerously on San Diego and Mission bays. Our highest count on San Diego Bay from 1997 to 2002 was on the Christmas bird count 15 December 2001, when 54 were found, 47 in the south bay, seven in the north bay. A high count for a more limited area was 38 in San Diego Bay off Crown Cove (T9) 9 December 1988 (Macdonald et al. 1990). Regular surveys of north and central San Diego Bay 1993–94 yielded no daily count higher than 17 on 1 February 1994 (Mock et al. 1994). Within the salt works Horned Grebes are fewer; Stadtlander and Konecny (1994) found no more than five there per weekly survey 1993–94. In Mission Bay, numbers ranged up to 12 in the northwest quadrant of the bay (Q7) 8 January and 12 February 2000 (L. Polinsky).
Elsewhere in the county the Horned Grebe is at best uncommon. A few occur on the ocean near shore (up to eight between La Jolla and Torrey Pines State Reserve, O7, 11 December 1997, S. E. Smith), in the Oceanside harbor, and the lagoons open to the tide (up to 11 in Batiquitos Lagoon, J7, 26 December 1998, R. Stone; four in Agua Hedionda Lagoon, I6, 23 December 2000, R. T. Patton). In lagoons with intermittent or no tidal input, such as San Elijo and Buena Vista, the Horned Grebe is rare.
There is considerable variation in the number of Horned Grebes found in San Diego each year, but it seems chaotic. The peaks and troughs on San Diego and Oceanside Christmas bird counts do not coincide. The highest total on an Oceanside count was 57 on 1 January 1977; since 1960 the highest total on a San Diego count was 176 on 26 December 1966.
Our finding wintering Horned Grebes a few times on inland reservoirs was unexpected because all but one of the few previous inland records for San Diego County were of spring migrants. Six at Lake Miramar (N10) 3 January 1998 dwindled to one by 12 February (P. M. von Hendy, P. Unitt, M. B. Stowe). Eight were on El Capitan Reservoir (O16) 20 December 1998 (S. Kingswood). At Loveland Reservoir, the species was found in two winters, with three on 31 January 2001 and 20 on 12 January 2002 (J. K. Wilson). The earlier record is of one in a borrow pit in the San Diego River (Q10) 1–9 January 1975 (J. L. Dunn).
Migration: The Horned Grebe occurs in San Diego County mainly from November to March, sometimes arriving in the last few days of October (earliest, one at Batiquitos Lagoon 27 October 1997, Merkel and Assoc. 1997). Surveys of San Diego Bay in 1993 and 1994 (Mock et al. 1994) recorded the species from 18 November to 6 April; Macdonald et al. (1990) recorded it as late as 17 April. From 1997 to 2001 our only record later than the third week of March was of one at Batiquitos Lagoon 16 April 1998 (C. C. Gorman). The only records later than 5 May are of one on San Diego Bay 25 May 1957 (AFN 11:376, 1957), two there 13 May 1984 (R. E. Webster, AB 38:957, 1984), one there 4 June 1967, and one on Mission Bay 11–18 July 1967 (G. McCaskie, AFN 21:604, 1967).
Horned Grebes migrate overland over southern California but rarely stop inland. In San Diego County, such migration has been noted mainly in spring 1983, when the many rainstorms associated with El Niño were an obstacle for migrants and Roger Higson monitored Lake Henshaw (G17), where many migrating water birds came down. That year, Higson noted at least 15 Horned Grebes from 4 March to 29 April, with a maximum of three on 31 March (AB 37:912, 1983). The only other records of inland migrants are of two at Lake Henshaw 7 March 1978 (D. W. Povey), one there 20–26 February 1979 (AB 33:312, 1979), and two at Lake Cuyamaca (M20) 1 March 1980 (AB 34:815, 1980).
Conservation: The Horned Grebe appears to be experiencing a slow decline, in both numbers and in its breeding range contracting (Stedman 2000). In San Diego County, which lies near the southern end of the species’ winter range on the Pacific coast, possible evidence of change comes from totals on Christmas bird counts in the 1950s, up to 661 in 1956 and 428 in 1957, that have not been approached since. These could, however, have been misidentified Eared Grebes. Siltation of the coastal lagoons degrades them as Horned Grebe habitat; conversely, the dredging of Batiquitos Lagoon in the mid 1990s led to the grebes’ using this site, where they had been absent previously.
Taxonomy: Parkes (1952) upheld the distinction of the Horned Grebe of the Old and New worlds as subspecies, with P. a. cornutus (Gmelin, 1789) in North America. But Cramp and Simmons (1977) reported too much individual variation for the subspecies to be valid.
Red-necked Grebe Podiceps grisegena
Uncommon as a winter visitor even in coastal northern California, the Red-necked Grebe is casual as far south as San Diego County, where there are three well-supported records. These are the southernmost for the species along the Pacific coast; the Red-necked Grebe is unknown in Mexico.
Winter: One was at Sweetwater Reservoir (S12) 20 December 1969–2 January 1970 (AFN 24:538, 1970), one was at the south end of San Diego Bay (V10) 14 March 1977 (AB 31:372, 1977), and one was at Santee Lakes (P12) 30 December 1984–6 January 1985 (D. and N. Kelly, AB 39:209, 1985). Other published reports from the 1950s and 1960s are more likely of misidentified Horned Grebes (Unitt 1984).
Eared
Grebe
Podiceps nigricollis
Though highly migratory, the Eared Grebe is also flightless for much of the year; its breast muscles atrophy except when needed for migration. Breeding birds use ponds and marshes with fresh to brackish water, but nonbreeders concentrate in water that is hypersaline. In San Diego County, such conditions are found in south San Diego Bay, where the Eared Grebe winters by the thousands. Though the grebe is still common on both fresh and salt water elsewhere, the numbers are much smaller. As a breeding bird the Eared Grebe is rare and irregular in San Diego County, which lies near the southern tip of the breeding range.
Winter: The salt works at the south end of San Diego Bay are the center for the Eared Grebe in San Diego County. Numbers recorded here on San Diego Christmas bird counts range as high as about 4100 on 19 December 1998 and are almost always several hundred. Because this count takes place in the third week of December and wintering grebes may not finish arriving until January (Jehl 1988), the count may not always represent the species’ peak abundance. Weekly surveys of the salt works from February 1993 to February 1994 yielded a maximum of 2359 on 17 March (Stadtlander and Konecny 1994). The Eared Grebe occurs in all other coastal wetlands too, in lagoons and estuaries (up to 60 at the Santa Margarita River mouth, G4, 27 December 1999, P. A. Ginsburg, 50 in the west basin of Buena Vista Lagoon, H5, 22 December 2001, J. Determan), on Mission Bay (up to 30 in the northwest quadrant, Q7, 8 January 2000, L. Polinsky), and throughout San Diego Bay (up to 262 in the central bay 10 March 1993, Mock et al. 1994).
The Eared Grebe can be quite common as well on large reservoirs, with up to 85 on El Capitan Reservoir (O16) 11 January 1998 (S. Kingswood), 102 on Barrett Lake (S18/S19) 2 February 2001 (R. and S. L. Breisch), and an exceptional 465 on Lake Hodges (K10/K11) 27 December 1998 (R. L. Barber, O. Carter). A few winter sometimes as high as Cuyamaca Lake (six on 15 January and 11 February 1998, A. P. and T. E. Keenan). One at Big Laguna Lake (O23) 6 December 1999 (D. S. Cooper) was likely a transient. In the Borrego Valley the Eared Grebe occurs rarely on artificial ponds, in most cases as a migrant but a few times in winter (G24; up to three on 9 February 1998, P. D. Ache).
Migration: The Eared Grebe’s migration pattern is unique: after breeding, most of the North American population gathers on Mono Lake and Great Salt Lake, remains through the fall, then migrates to the Salton Sea and Gulf of California. Thus, even though a few birds begin arriving in mid September, most arrive in October and November, and some arrive as late as January (Jehl 1988). In spring, most Eared Grebes depart in March and early April. Records from the Borrego Valley (Roadrunner Club, F24) range from 30 September (1986) to 28 April (1982; A. G. Morley).
Breeding distribution: The Eared Grebe breeds mainly in the intermountain region and northern Great Plains; southern California is marginal to its breeding range. During the atlas period, we confirmed the species’ breeding at three sites: the Ramona Water District’s pond 4.1 miles west of Ramona (K13; three young on 15 June 2000, W. E. Haas), in a borrow pit along the San Diego River in Santee (P13; young on 30 April 1998, W. E. Haas), and at the northeast corner of Lake Morena (S22; young two-thirds grown on 31 May 1998, R. and S. L. Breisch). In 2003, a pair with chicks was at Lake Murray (Q11) 27 April (N. Osborn). Typically, the Eared Grebe is colonial, and a few of the earlier records of nesting in San Diego County are of ephemeral colonies. In 1989, for example, 25 were on nests at Batiquitos Lagoon (J7) 8 July 1989 (J. Oldenettel, AB 43:1366, 1989). Unitt (1984) summarized other nesting records.
Nonbreeding birds are widespread through late spring and summer in small numbers; we recorded the species in 38 atlas squares in May, June, and July. These summering birds occur on both freshwater lakes (up to nine at Whelan Lake, G6, 17 June 1997, D. Rorick) and San Diego Bay (seven on the north bay, S8, 26 May 2000, R. T. Patton). The grebes are likely to nest irregularly at some of the freshwater sites, especially Sweetwater Reservoir (S12; up to six on 12 May 2000 and 16 May 2001, P. Famolaro), Loveland Reservoir (Q16; pair on 13 June 2001, J. K. Wilson), Lake Cuyamaca (M20; pair on 28 May 1998, A. P. and T. E. Keenan), and Tule Lake (T27; up to seven on 6 June 2000, J. K. Wilson).
Nesting: Like other grebes, the Eared nests on a platform of aquatic vegetation. And, like other grebes, it does not have a sharply defined breeding season. The family at Lake Murray implies egg laying as early as the beginning of April, yet R. A. Erickson noted a nest with eggs at the Stuart Mesa ponds, Camp Pendleton (G5), as late as 18–22 August 1978.
Conservation: With a total population of about 4 million, the Eared is the world’s most abundant grebe, evidently because of its unique ability to exploit the superabundant brine shrimp, alkali flies, and pile worms in salt lakes (Jehl 2001). There is no demonstrable trend in the North American population (Cullen et al. 1999). An increase at the Salton Sea is likely due to the birds’ shifting to this lake to take advantage of an increase in pile worms (Jehl and McKernan 2002). The grebe’s reliance on three lakes for staging during migration, however, makes it vulnerable at these bottlenecks; Owens Lake, formerly a major site, was eliminated by water diversion (Jehl 1996a), and the remaining staging sites are all foci of environmental controversy. The Eared Grebe has suffered some mass die-offs as a result of disease or migrating birds being downed by storms (Jehl 1996b, Jehl et al. 1999). A die-off along the coast of San Diego County in January 1983 (Jehl and Bond 1983) was most likely due to food shortage during El Niño (Jehl 1996b)
In San Diego County, the Eared Grebe doubtless benefited from the installation of reservoirs and sand mining that left borrow pits. The building of the salt works, more than anything, created prime wintering habitat for the Eared Grebe. With the salt works now a national wildlife refuge, it has become a question of public policy how much of this artificial habitat will be maintained for the sake of the Eared Grebe and other water birds that have capitalized on it.
Taxonomy: Podiceps n. californicus Heermann, 1854, is the subspecies of the Eared Grebe in North America, differing from those in the Old World by lacking white in the primaries.
Western
Grebe
Aechmophorus occidentalis
In the middle of the 20th century, the Western Grebe was just a winter visitor to San Diego County, common on salt water along the coast. It continues as one of the most abundant winter visitors on San Diego Bay and the ocean near shore. Since the 1950s it has taken on an additional role as a locally common breeding species, colonizing an increasing number of lakes and lagoons with fringing marshes. The Western Grebe is an outstanding example of a bird that has taken advantage of the need for San Diego County’s human population to import and manage vast quantities of fresh water.
Breeding distribution: As of 2001, the Western Grebe was up to six nesting sites in San Diego County. At O’Neill Lake, Camp Pendleton (E6), the birds were summering by 1997 and first confirmed breeding in 1999. The numbers there are still small, maximum 12, including a fledgling 30 July 2001 (P. A. Ginsburg). At Buena Vista Lagoon (H5/H6) the grebes have nested since at least 1997 (K. Messer), both east and west of Interstate 5. Numbers in spring and summer are sometimes large, up to 132 on 11 May 1999 (M. Freda). San Dieguito Reservoir (K8) was a new breeding site discovered as a result of the atlas effort in 1997. The maximum count there in summer was at least 40 on 21 June 1997, including five adults with chicks. The birds nested there in winter as well, with up to 50, including three chicks, on 28 December 1997 and 18 January 1998 (J. Determan). Lake Hodges (K10) has become a major population center for the Western Grebe, with up to 400, including chicks, 14 June 1999 (R. L. Barber). Sweetwater Reservoir (S12/S13), site of San Diego County’s first Western Grebe colony in 1956, continues to be important, with up to 40 on 4 May 1998 (P. Famolaro). Finally, a new but large colony is at Lower Otay Lake (U13/U14), with up to 60, including young, in square U13 on 25 June 1999 (V. Marquez) and 150 in U14 on 2 April 2000 (S. Buchanan).
From 1997 to 2001, we found Western Grebes summering at least irregularly on most of San Diego County’s other reservoirs. On Lake Henshaw (G17) the numbers were large, up to 150 on 17 and 18 June 2000 (P. Unitt). On other lakes where the species was not confirmed breeding the numbers were much smaller, with no more than ten from May through August. Because of the Western Grebe’s history, however, further increases and colonizations may be expected.
Nesting: For its nest, the Western Grebe makes a pile of cattail leaves and other aquatic vegetation, normally in the water if not actually floating. Some nests are visible from lakeshores, but most of our confirmations of Western Grebe breeding were of chicks, often riding on their parents’ backs. Shortly after hatching, Western Grebe chicks climb onto the adults’ backs and leave the nest.
The peak of the Western Grebe’s nesting in San Diego County lasts from May through early July, but observations of chicks show that this species perhaps more than any other breeds year round. Small chicks have been noted at Lake Hodges 23 October and 30 December 1998, 27 January and 2 March 1999, and at San Dieguito Reservoir 28 December 1997, 10 and 18 January 1998, and 21 February 1998. Winter breeding of the Western Grebe at Sweetwater Reservoir was noted as long ago as 1966 (Lee 1967).
Migration: Winter visitors from the north and northeast arrive in October and November and depart mainly in April. In the spring of 2000, however, unusually large numbers remained late into the spring, with 675 on the ocean off North Island Naval Air Station (S8) 26 May (R. T. Patton) and 160 off Torrey Pines State Reserve (N7) 4 June (S. E. Smith). In the other four years of the atlas period, records of late stragglers after 2 May were of eight or fewer individuals. In weekly surveys of north San Diego Bay 1993–94, Mock et al. (1994) found the Western Grebe year round but fewest in August (no more than 12 per day).
In the Borrego Valley, there are five records of single fall migrants, 15 October–29 November, and one record of a spring migrant at Lower Willows (D23) 15 April 1974 (ABSDP database).
Winter: As a winter visitor, the Western Grebe is most numerous on the ocean within a mile or two of the shore, especially in the bight extending from Point Loma to Imperial Beach. The birds are here each winter by the hundreds, and in some years by the thousands. On 18 December 1999, the total estimated here on the San Diego Christmas bird count was 5440 (D. W. Povey, R. B. Riggan, S. Walens). Large flocks are regular elsewhere along the coast as well, with up to 1100 off San Onofre (D2) 27 December 2001 (P. D. Jorgensen) and 1840 off Oceanside (H5) 26 December 1998 (S. Walens).
The Western Grebe is common on San Diego Bay, too, with up to 536 in the central and south bay 26 January 1994 (Manning 1995). Mock et al. (1994) found it to be the sixth most numerous bird in north San Diego Bay even when counts throughout the year were pooled. Their weekly counts peaked at 399 on 19 January 1993.
In winter, the grebe is found on inland lakes even more widely than in summer. Even where it breeds in large numbers, its numbers are even larger in winter, up to 918 on Lake Hodges 27 December 1998 (R. L. Barber), 900 on Sweetwater Reservoir 16 December 2000 (P. Famolaro). On lakes that the grebe has not yet colonized winter counts run up to 115 at Lake Henshaw 12 December 2000 (J. R. Barth) and 62 at Lake Morena (S22) 20 December 1997 (R. and S. L. Breisch). Above 3000 feet, the elevation of Lake Morena, the only record is of two at Lake Cuyamaca (M20) 4 December 1998 (A. P. and T. E. Keenan).
Conservation: The Western Grebe has long been common as a winter visitor along San Diego County’s coast; there has been no clear change through history in its status in that role. As a breeding species, however, the Western Grebe is a newcomer. It was found nesting first at Sweetwater Reservoir, long the only breeding site in the county, in May 1956 (AFN 10:409–410, 1956). All other colonies have established themselves since the mid 1980s. The numbers of nonbreeding birds on inland lakes have increased greatly since that time.
Clearly the building of dams, the filling of reservoirs, and the reservoirs’ being stocked with fish were prerequisites for the Western Grebe’s colonization and spread. Most of these were established, however, decades before the grebe began nesting. Sweetwater Reservoir, closed to the public, provided an undisturbed refuge and nucleus for the colonization. Lakes Hodges and Lower Otay, however, are open to small boats, however, so the grebes can adapt to some level of disturbance. Some factors not yet identified must be facilitating a population increase. The Western Grebe still faces threats, however, most notably coastal pollution. Because San Diego County’s coast is in the core of the Western Grebe’s winter range, a large oil spill here could kill many thousands. The effects of pollution on a small scale can be seen in the dead grebes contaminated with spots of oil that wash up regularly on the county’s beaches.
Taxonomy: Both the Western and Clark’s Grebes consist of two subspecies, a small one on the Mexican Plateau and a larger one farther north. The northern subspecies of the Western Grebe is nominate A. o. occidentalis (Lawrence, 1858).
Clark’s
Grebe
Aechmophorus clarkii
Long considered a color morph of the Western Grebe rather than a distinct species, Clark’s Grebe is only barely over the threshold of being a species. In San Diego County hybridization is frequent. In biology, so far as is known, the differences between the two grebes are trivial. The two occur in the same habitats, breed in mixed colonies, and flock together in winter. As a breeding bird in San Diego County, the Western tends to be somewhat more numerous than Clark’s, but the difference is not great. As a winter visitor, however, the Western far outnumbers Clark’s.
Breeding distribution: The distribution of Clark’s Grebe is closely similar to that of the Western. At O’Neill Lake (E6), Clark’s occurs in numbers similar to those of the Western, counts in summer ranging up to seven on 4 July 2000 (P. A. Ginsburg). A pair nested there apparently unsuccessfully in 1997, then successful nesting began in 1999, the same year as the Western. Indeed, many of the pairs at this site have been mixed. At Buena Vista Lagoon (H5/H6), by contrast, Clark’s Grebe is much less numerous than the Western and not confirmed nesting; our high count here during late spring or summer was only three on 2 May 1999 (J. Determan), and we did not confirm the species’ nesting. At San Dieguito Reservoir (K8), our only records were in winter, but the birds bred there in 1997–98, with up to ten individuals including one pair with two nearly grown young and others in courtship display 10 January 1998 (K. Aldern). Lake Hodges is as major a site for Clark’s as for the Western; counts here in spring and summer ranged up to 35 in square K10 on 16 April 1997 (V. P. Johnson) and 35 in K11 on 13 June 1998 (E. C. Hall). At Sweetwater Reservoir (S12/S13) Clark’s breeds regularly but in numbers smaller than the Western (eight on 4 May 1998, P. Famolaro). At Lower Otay Lake (U14) Clark’s is also confirmed breeding (young on 4 July 1999, S. Buchanan) but occurs usually in small numbers; the only count of more than ten was of 55 on 29 May 2001 (N. Osborn).
We confirmed nesting of Clark’s Grebe at two sites where the Western did not breed. On 17 May 1997 a chick was at the east end of Batiquitos Lagoon (J7; M. Baumgartel). In 1998, up to eight adults were at the east end of Loveland Reservoir (Q17) 29 April, but on 22 June, when one was on a completed nest with no eggs, it appeared to be the only bird left (P. Famolaro).
We noted just a few Clark’s Grebes summering at lakes where they did not breed. The only records of more than single birds were of two on Lake Murray (Q11) 16 July 1999 (N. Osborn) and three on Lake Wohlford (H12) 14 July 1999 (D. C. Seals). Some Clark’s, however, may have been overlooked among Western Grebes.
Nesting: No differences in nesting biology between the Western and Clark’s Grebes have been reported (Storer and Nuechterlein 1992). From 1997 to 2002, we noted winter breeding of Clark’s Grebe only once, at San Dieguito Reservoir. The other records correspond to egg laying from about mid April to early August.
Migration: The migration schedule of Clark’s Grebe appears to be the same as that of the Western. Migrants have departed largely by mid April though a few may straggle to early May. Six at Agua Hedionda Lagoon (I6) 20 April 1999 (P. A. Ginsburg) made our latest spring coastal count of more than two individuals. Two in Mission Bay (Q8) 28 June–8 July 1999 (J. C. Worley) and one in the Santa Margarita River estuary (G4) 30 June 2000 (P. A. Ginsburg) were the only summering Clark’s Grebes reported from salt water away from breeding sites.
Winter: In winter, Clark’s Grebe is considerably more numerous than in summer on Sweetwater Reservoir (up to 500 on 18 December 1999, P. Famolaro) and Lake Hodges (up to 83 on 23 December 2001, R. L. Barber), sometimes on Buena Vista Lagoon (70 on 27 December 1997, D. Rorick). Winter counts at other sites were of 15 birds or fewer. On salt water especially, Clark’s makes up a small minority of grebe flocks; for example, on 18 December 1999, when D. W. Povey and M. B. Mulrooney recorded 1500 Western Grebes off Coronado they noted only 15 Clark’s. During their weekly census of the salt works 1993–94, Stadtlander and Konecny (1994) recorded a maximum of 73 Clark’s on 17 February 1993, but this was their only count of more than 25. Ratti (1981) found 13% of the 332 wintering Aechmophorus grebes he observed around San Diego in January 1977 to be Clark’s. Part of the apparent difference between the species by season, however, may be due to the plumage difference between them being less well marked in winter and to the difficulty in distinguishing the birds at a distance in the large flocks seen offshore.
Conservation: Because Clark’s Grebe was widely recognized as a species distinct from the Western Grebe only in the early 1980s there are no substantial historical data on its status. In San Diego County the careers of the two grebes appear to be progressing in tandem.
Taxonomy: The extent of hybridization between the Western and Clark’s Grebes in San Diego County appears to be considerably higher than in the areas furnishing the data on which the decision to split the species was based (Ratti 1979). At Lake O’Neill mixed pairs are frequent, perhaps just as frequent as if the birds are selecting their mates without regard to species (P. A. Ginsburg). Of three nests at Sweetwater Reservoir observed by A. Mercieca in 2003, two belonged to mixed pairs. Possibly each grebe’s preference for its own species emerges only where the number of each in a colony is large. Possibly the degree of reproductive isolation between them varies regionally. The situation in San Diego County suggests that the relationship between these grebes could use further study.
Geographic variation in Clark’s Grebe parallels that in the Western, with a small subspecies on the Mexican Plateau. The larger subspecies of Clark’s found in the United States is A. c. transitionalis Dickerman, 1986.