Hawks, Kites, and Eagles  — Family Accipitridae

Osprey Pandion haliaetus

Through the middle of the 20^th century the Osprey suffered the ill effects of pesticide poisoning, contracted through the contamination of its staple food, fish.  After being released of much of this burden in the 1970s, the Osprey population resurged.  In San Diego County this resurgence is conspicuous: once rare, the Osprey has become regular year round in small numbers both along the coast and on inland lakes.  Several pairs have begun nesting.

Breeding distribution: The Osprey’s recolonization of San Diego County as a breeding species coincided with the beginning of field work for this atlas in 1997.  Each year from then through 2002 a pair nested in the northeast corner of North Island Naval Air Station (S9).  They fledged young in 1998 and 2000 and probably other years as well (G. Perkins et al.).  Another pair began nesting at Scripps Ranch High School (N10) in 1998 and continued annually, with success from 1999 at least through 2002 (G. Steinbach et al.).  The birds made uncertain or aborted attempts to nest near Torrey Pines State Reserve (N7) or UCSD (O7) in 1998 (carrying sticks or seaweed, M. C. Jorgensen, S. E. Smith), at Lake Murray (Q11) in 1997 and 2002 (carrying sticks to platform, N. Osborn, P. Famolaro), on the mast of a boat in Mission Bay (R7) 2001–03 (all attempts unsuccessful, D. Bittner), and near Pepper Park, National City (T10), in 1998 (R. T. Patton).

            With the conclusion of the atlas study in 2002 came a spurt of new Osprey nests: near Seaforth Landing in Mission Bay (R8), at Mesa College (Q9), San Diego State University (Q11), on some quarry equipment along Mission Gorge Road (Q11), and along the Tijuana River in Marron Valley (V16).  Some of these were only tentative attempts, but the nests in Marron Valley and at Mesa College were successful (J. Martin, J. Hannan).

Nesting: Ospreys build huge stick nests, often augmenting and reusing them year after year.  Their trend toward nesting on man-made structures is well illustrated in San Diego County: the most frequent nest site here is racks of floodlights for ball fields, used at Scripps Ranch High School, Mesa College, and North Island.  The only nest site in a tree was the one in Marron Valley, in a eucalyptus.  The nests at North Island and Scripps Ranch High School were used repeatedly, though the birds also built some new nests, some unused (typical for the species), accounting for nesting confirmations in adjacent atlas squares S8 and O10 (on a cell-phone tower at Alliant International University, D. Bainbridge).  Osprey nests are generally near water; the one at Scripps Ranch High School was 0.5 mile from Lake Miramar.  But the one at Mesa College was 2.3 miles from Mission Bay, and the one in Marron Valley was 8 miles from Lower Otay Lake, over the top of Otay Mountain, and over 10 miles from Presa Rodriguez on the outskirts of Tijuana.

            The schedule of Osprey nesting is tricky to ascertain because the nests are high, so the young are not visible until well grown, and the male feeds the female throughout incubation as well as the young after they hatch.  At Scripps Ranch High School, incubation had apparently begun by 8 February 2000; a chick was visible in this nest by 4 April (G. Grantham).  At North Island, the first of three young fledged in 2000 on 18 June, suggesting laying about 11 March (G. Perkins).

Migration: The Osprey is still more numerous in San Diego County during migration and winter than in the breeding season.  In regular surveys of north and central San Diego Bay 1993–95 Mock et al. (1994) and Preston and Mock (1995) found the Osprey most frequently in September and October, with a maximum of six per day.  From 1997 to 2001 numbers in fall ranged up to nine at San Elijo Lagoon (L7) 7 October 1999 (P. A. Ginsburg).

            Nonbreeding Ospreys are widely scattered over San Diego County through the spring and early summer, mainly along the coast (up to seven in the San Diego River flood-control channel, R8, 2 April 2000, Y. Ikegaya) and in the coastal lowland.  Single individuals have been seen repeatedly at this season farther inland around lakes Cuyamaca, Barrett, and Morena and even at Tule Lake (T27; one on 6 and 27 June 2001, J. K. Wilson).

            In the Anza–Borrego Desert the Osprey is a rare migrant, recorded on 1 and 25 October in fall and about nine times from 16 February to 12 April in spring (ABDSP database).

Winter: In winter the Osprey occurs more widely than in the breeding season.  During the atlas period we noted it wintering at almost every coastal wetland and at most inland lakes.  Sites getting the heaviest Osprey use in winter were Batiquitos Lagoon (J6/J7; up to 11 on 28 December 1999, R. and A. Campbell) and south San Diego Bay (U10/V10; up to six on 3 December 1999, B. C. Moore).  Especially favored lakes are Hodges, (K10; two counts of up to three, R. L. Barber), Sweetwater (S12/S13; up to five on 2 December 1998, P. Famolaro), and Morena (S21/T21; three counts of up to three, S. E. Smith).  The Wildlife Research Institute (2004) found 21 individuals wintering in the southwestern quadrant of San Diego County in 2002.  Perhaps because the Bald Eagle is so regular there, the Osprey is rare at Lake Henshaw, recorded on only one of 22 Lake Henshaw Christmas bird counts 1981–2002.

Conservation: The Osprey’s resurgence gained traction in the late 1980s and continued through the end of the century.  By 2001 it was more abundant in San Diego County than at any time in recorded history.  From 1953 to 1972 the San Diego Christmas bird count averaged 0.4 per year; from 1997 to 2002 it averaged 23.5.  The trend on Oceanside, Rancho Santa Fe, and Escondido counts is similar though less dramatic.  On monthly counts of San Elijo Lagoon 1973–83 King et al. (1987) found the Osprey only occasionally and almost always singly.  Before 1997 the only reported nesting attempts were on a boat in San Diego Bay, probably in the early 1860s (Cooper 1870), and on a beacon in the bay in 1912 (eggs collected 21 April, WFVZ 71019).

            Reduced shooting and, especially, the banning of nondegradable organochlorine pesticides are the factors allowing the Osprey’s increase.  In San Diego County, the building of dams and the stocking of reservoirs with fish created much new Osprey habitat.  Acclimatizing to human activity around nests and adopting man-made structures as nest sites allowed the Osprey to recolonize.  But the uses for which these structures were designed may be incompatible with Osprey nesting, possibly leading to a new generation of conflict.

Taxonomy: Ospreys throughout North America have long been ascribed to P. h. carolinensis (Gmelin, 1788).  On the basis of field observations, Blanco and Rodriguez (1999) reported Ospreys from Baja California Sur to have nearly white underwing coverts as in P. h. ridgwayi Maynard, 1887, of the West Indies.  But a one-year-old male and two one-year-old females in the San Diego Natural History Museum from San Ignacio and Scammon’s lagoons, Baja California, have the same amount of dark barring on the under primary coverts as specimens from farther north.

White-tailed Kite Elanus leucurus

The White-tailed Kite is not only one of southern California’s most elegant birds of prey, it is one of the most interesting, because of its communal roosting, history of steep rises and falls in population, and concentration on a single species of prey, the California vole or meadow mouse.  Though the kite is found in San Diego County year round, its numbers vary with those of the vole and the shifting of communal roosts.  Unfortunately, urbanization of the grasslands in which kites forage threatens the recovery the species enjoyed from the late 1930s to the 1970s.

Breeding distribution: The White-tailed Kite is widespread over the coastal slope of San Diego County, preferring riparian woodland, oak groves, or sycamore groves adjacent to grassland.  Regions of concentration are in the northwest from Camp Pendleton to Carlsbad and Vista, in the central region from Los Peñasquitos Canyon through Miramar to Poway, and in the south from the Tijuana River valley to Otay Mesa and Otay lakes.  In the foothills the species is less common than in the coastal lowland but still fairly widespread—substantially more widespread than I reported previously (Unitt 1984).  It is absent from most of the higher mountains but occurs uncommonly up to about 4200 feet elevation in the Julian area with up to six, including a pair at a nest, at Wynola (J19) 22 May 1999 (S. E. Smith).  It has been seen repeatedly at 4600–4700 feet elevation around Lake Cuyamaca (M20), where most birds are probably postbreeding dispersers (up to three on 6 August 1999, A. P. and T. E. Keenan).

The kite’s range spills over the east side of the mountains possibly in McCain Valley (S27; juvenile 14 May 1999, M. Sadowski), certainly in San Felipe Valley, where there are many reports, including a pair with a probable nest in the upper valley (H20) 1 May 1998 (A. P. and T. E. Keenan) and a pair in courtship display near Scissors Crossing (J22) 13 April 1998 (E. C. Hall).  Note that these suggestions of breeding were in the wet El Niño spring of 1998; monitoring at Scissors Crossing in dry to average 2002 and 2003 yielded only occasional postbreeding visitors (J. R. Barth).  The Anza–Borrego Desert State Park database has several records of the White-tailed Kite through the spring and summer, mostly of single individuals with no suggestion of breeding, but three were at Middle Willows (C22) 7 May 1972.

Nesting: White-tailed Kites build their nest in the crowns of trees, especially the coast live oak, or on clumps of mistletoe, both of which screen the nest well.  They use nonnative trees freely, especially orange trees, as citrus orchards often offer good foraging (Dixon et al. 1957).  Occasionally they use large shrubs such as scrub oak or toyon.

            Dixon et al. (1957) reported eggs from 6 February to 10 July, with 10-day-old young in one nest 22 February, suggesting laying about 12 January.  Our observations from 1997 to 2002 imply laying from late January to May, except that a brood of newly fledged young in lower Los Peñasquitos Canyon (N8) 27 February 1999 (B. C. Moore) must have hatched from a clutch laid around 1 January.  From 1997 to 2001, all our early nestings of the kite were in 1999, following the wet spring of 1998, when rodent numbers were still high.  Late nestings were in both 1998 and 1999.  Our number of nesting confirmations per year similarly shows the effect of variation in rainfall and presumably in prey: 15 in 1997, 41 in 1998, 72 in 1999, 7 in 2000, and 11 in 2001.

Migration: The White-tailed Kite is nonmigratory but nomadic and dispersive.  Dixon et al. (1957) reported a bird banded as a nestling in San Diego County recovered about 100 miles north two years later.  The birds invaded San Clemente Island in 1984, one year after El Niño rains (Scott 1994).  Records for the Anza–Borrego Desert in the state park database are scattered rather evenly through the year.

Winter: The White-tailed Kite’s winter distribution differs little from the breeding distribution.  The Wildlife Research Institute (2004) found the kite to constitute essentially the same percentage of the raptor fauna of southwestern San Diego County in winter as in summer.  Atlas observers did find the species a bit more frequently in marginal areas like Warner Valley, Ranchita, Lake Cuyamaca, and McCain Valley in winter than in summer.  Also in winter a few birds entered the developed areas of the city of San Diego where kites are lacking during the breeding season.

            The most notable aspect of the kite’s nonbreeding biology is its communal roosting.  From 1997 to 2002, flocks were reported from near the upper end of Agua Hedionda Lagoon (I6; up to 21 from 1 to 9 December 1998, L. E. Taylor, P. A. Ginsburg) and at the mouth of Spring Canyon, Mission Trails Regional Park (P11; up to 50 from 3 December 1998 to 7 February 1999, D. Mooney).  In the latter case the flock was observed through the middle of the day.  Another notable diurnal concentration was 113 (mostly juveniles) scattered over eastern Otay Mesa (V14) 31 July 1999 (S. D. Cameron).

Conservation: In the 19^th century the White-tailed Kite was probably uncommon in San Diego County; A. M. Ingersoll told Willett (1912) that “from 1887 to 1892 he saw White-tailed Kites frequently in the vicinity of San Diego and knew of two pairs nesting in that region.”  From 1892 to 1920, however, the species went unrecorded and was probably extirpated.  One pair nested 5 miles east of Del Mar (M8) in 1920, and one bird was shot at the Tijuana River mouth (V10) on 15 January 1930 (Huey 1931b).  Dixon et al. (1957) saw no kites in northern San Diego County from 1900 to 1935, but then they colonized and increased, becoming common by 1956.  San Diego Christmas bird counts show an upsurge in 1965, though this was due in part to the count circle’s being shifted to include the Tijuana River valley.  The population probably peaked in the 1970s and early 1980s.  Long-term trends are difficult to track because the kites’ numbers vary with rain and rodent numbers and the shifting of roost sites.  Nevertheless, results of the Oceanside Christmas bird count may be a gauge; its circle once consisted largely of prime kite habitat.  From 1976 to 1983 this count averaged 72; from 1997 to 2002 it averaged 30.

            Shooting was likely responsible for the kite’s first decrease.  Agriculture may have been a boon to the species, as it nests and feeds commonly in orchards.  But with citrus growing no longer profitable in San Diego County most groves have been developed into housing tracts.  Most grassland has also already been lost.  Poisoning of rodents may be affecting the kite by depressing its food supply.  The disappearance of kites from the long-developed areas of the city of San Diego is obvious from the map and suggests a scenario for the rapidly urbanizing areas of northern San Diego County.  Another possible negative factor is the proliferation of crows and ravens; Dixon et al. (1957) reported crows robbing kites of their prey repeatedly; D. Bittner has observed ravens doing the same.

Taxonomy: The subspecies of White-tailed Kite in North America is E. l. majusculus Bangs and Penard, 1920, larger than the nominate subspecies of South America.  The White-tailed Kite has sometimes been considered conspecific with the Black-shouldered Kite (E. caeruleus) of the Old World; at these times the name Black-shouldered Kite has been applied to the birds in the New World as well.

Mississippi Kite Ictinia mississippiensis

The Mississippi Kite breeds mainly in the southern Great Plains and southeastern United States; it is only a vagrant to California, where there are 29 well-supported records through 2000.  A population increase and westward range expansion in the 1970s led to the prediction that the kite would colonize southern California (Parker and Ogden 1979), but all that followed was a spike in its frequency here in the early 1980s.  This spike accounts for all records for San Diego County.

Migration: The California Bird Records Committee has accepted three identifications of the Mississippi Kite in San Diego County, of one seen in the Tijuana River valley (V11) 18 July 1982 (G. McCaskie, Morlan 1985), one photographed at Pio Pico Campground (T15) 12 June–30 July 1983 (D. W. Povey, Morlan 1985), and one seen at Point Loma (S7) 21 September 1985 (G. McCaskie, Bevier 1990).  The committee considered inadequately supported a fourth record also in the early 1980s.

Bald Eagle Haliaeetus leucocephalus

In San Diego County the Bald Eagle is a rare but annual winter visitor to lakes in the foothills and mountains, especially Lake Henshaw.  From 1997 to 2002 the number reaching the county varied from about 8 to 15 each year.  A few individuals have remained into summer, and in 2001 an unmated bird was carrying sticks, suggesting the Bald Eagle could colonize San Diego County as a breeding species.  Reintroduction programs, the banning of eagle shooting, and especially the ban on DDT have resulted in vigorous growth of the Bald Eagle population nationwide since the late 1970s.

Winter: Because its primary prey is fish, the Bald Eagle occurs mainly at lakes.  At Lake Henshaw (G17), the species’ favored site in San Diego County, at least two wintered each year from 1997 to 2002, and the maximum count was eight on 26 January 1999 (P. D. Jorgensen).  The highest ever was 16 from 13 December 1972 to 28 January 1973 (AB 27:663, 1973).  Cuyamaca (M20), Corte Madera (Q20/R20), and Morena (T21/S21/S22) are also fairly consistent sites.  Atlas observers noted only one or two at a time away from Lake Henshaw but encountered the species at least occasionally at almost all lakes in the foothills and mountains, even some rather small ones, as in Thing Valley (Q24; one on 7 January 2001, J. R. Barth). 

On lakes in the coastal lowland the Bald Eagle is infrequent, with only two noted during the atlas period (O’Neill Lake, E6, 1 December 2000–14 January 2001, P. A. Ginsburg; Sweetwater Reservoir, S12, 30 December 2000, C. H. Reiser).  It is even less frequent in coastal wetlands; 10 years of monthly surveys of San Elijo Lagoon (L7) yielded only one (King et al. 1987).  From 1980 to 2002, the San Diego and Rancho Santa Fe Christmas bird counts recorded the Bald Eagle only once, the Oceanside count not at all.  In the Anza–Borrego Desert the only records are of one over Whale Peak (L25) 18 February 1983 (A. G. Morley) and one over Indianhead (F23) 14 January 1997 (D. Waber).

Migration: Bald Eagles begin arriving in San Diego County in October, recorded as early as the 3^rd (1978, Sweetwater Reservoir, D. Thompson) and the 4^th (1982, Lake Henshaw, R. Higson, AB 37:223, 1983).  Most depart in March; the latest date recorded for a migrant is 4 May (1984, immature at Lake Henshaw, R. Higson, AB 38:960, 1984).  An immature at the Sweetwater River mouth on San Diego Bay (U10) 16 and 25 April 1998 (B. C. Moore) and an adult in Lost Valley (D21) 28 April 2000 (W. E. Haas) were unusual for both date and location.

            The Bald Eagle has been reintroduced to the Channel Islands, and a tagged subadult released at Santa Catalina Island showed up at Lake Morena 24 July 2001 (R. Roedell).

Breeding distribution: The only known nesting of the Bald Eagle in San Diego County was in 1936, when A. O. Treganza collected an egg from Little Tecate Peak (V17) 8 March (WFVZ 55005).  No summering birds were reported until 1988, when one was near Julian 18 May (J. Smith, AB 42:481, 1988) and 1996, when one was near the summit of Hot Springs Mountain (E20) 15 June (G. L. Rogers, NASFN 50:996, 1996).  In 2000 an adult was at Lake Henshaw 18 June (P. Unitt) and another was at Lake Cuyamaca 20–23 June (D. Bittner, NAB 54:423, 2000).  Then on 21 March 2001 an adult was carrying sticks to a sycamore tree at Lake Henshaw (W. E. Haas).  Evidently lacking a mate, this bird got no farther with its attempt to nest and did not remain through the summer.  But the episode suggests that the Bald Eagle could soon colonize San Diego County as a breeding species.  Its nearest current nest site is at Lake Hemet, Riverside County, where two young fledged in 2003 (D. Bittner).

Nesting: Bald Eagles build an enormous stick nest, usually in a tall tree.  They also nest on cliffs where trees are few or none; the nest on Little Tecate Peak was on a “pinnacle of rock.”

Conservation: The Bald Eagle suffered greatly from shooting and from poisoning by DDT and lead.  It was among the first species formally listed as endangered by the federal government in 1967.  With these adverse factors largely controlled, however, since 1980 the population has climbed steeply (Buehler 2000).   In San Diego County, which is peripheral to the species’ range, the most noticeable change is the occurrences in summer.  Lake Henshaw Christmas bird counts, begun in January 1981, show no strong trend.  But a change may be more likely detected in the number of lakes with Bald Eagles than in the number on a single lake.

Taxonomy: The Bald Eagle is usually divided into a larger northern subspecies, H. l. alascanus Townsend, 1897, and a smaller southern one, H. l. leucocephalus (Linnaeus, 1766).  Birds nesting in San Diego County would be the smaller subspecies, but winter visitors are more likely the northern one, and measurements of the single specimen, from Lake Cuyamaca 20 December 1922 (MVZ 144728), identify it as alascanus (Unitt 1984).  A Bald Eagle fitted with a radio transmitter and wintering at Lake Henshaw in 1999 and 2000 had migrated from Great Slave Lake, northern Canada (D. Bittner, B. J. Walton).

Northern Harrier Circus cyaneus

Long known in America as the Marsh Hawk, the Northern Harrier is as much a bird of grassland as of marshes.  In San Diego County it is found year round but is more numerous and widespread as a winter visitor than as a breeding bird.  The Northern Harrier’s status as a breeding species in San Diego County is threatened by habitat loss and fragmentation, to which grassland birds that nest on the ground are especially susceptible.  The local breeding population undoubtedly varies much with rainfall and the abundance of prey but is between about 25 and 75 pairs.

Breeding distribution: In San Diego County breeding Northern Harriers are scattered, as patches of suitable habitat are separated by stretches of chaparral or urban development.  Camp Pendleton, with its extensive grasslands, functions as a refuge for the harrier; from 1997 to 2001 we noted possibly breeding birds in 17 of the 28 atlas squares fully or partly within the base.  Originally, this grassland spread over much of northwestern San Diego County.  Harriers nested in remaining undeveloped areas in Carlsbad (I7) at least until 2000 (fledglings on 15 June, D. B. Mayer) and at Guajome Lake (G7) at least until 2001 (female carrying a twig 25 June, K. L. Weaver).  In central San Diego County the most important area for breeding harriers is Los Peñasquitos Canyon (N8/N9); five pairs nested there 1998–99 (J. Hannan) but none remained 2001–02 (Wildlife Research Institute 2004).  The Tijuana River estuary and valley evidently have the largest concentration of nesting harriers in San Diego County, with up to 13 pairs in the Border Field State Park (W10) alone in 2002 (Wildlife Research Institute 2004).  In spite of sprawling scattered industrial development, perhaps four to six pairs still nest on Otay Mesa.  In southwestern San Diego County, north to the San Dieguito River and east to San Pasqual, Alpine, and Dulzura, the Wildlife Research Institute reported 11 pairs in 2001, 24 in 2002.  The harrier’s numbers vary greatly with rainfall.

            In the foothills and mountains harriers are few and scattered through the breeding season, and we did not confirm nesting there.  On the east slope of the mountains the harrier is apparently irregular in San Felipe Valley, where we noted it repeatedly in the springs of 1998 (after El Niño rains) and 1999, including a male in display flight at Sentenac Ciénaga (J23) 11 March 1999 (R. Thériault).  But after four years of drought, in 2002 and 2003, the harrier was absent there.  Similarly, an apparent pair in the Jacumba Valley (U28) 28 February 1999 (F. L. Unmack) was our only suggestion of breeding in southeastern San Diego County.  The only late spring or summer record from the Anza–Borrego Desert is of one at Lower Willows (D23) 4 July 1994 (M.Getty).

Nesting: The Northern Harrier nests on the ground, with the nest concealed within a marsh or other dense vegetation.  Our relatively few observations of nesting activity imply the birds lay eggs at least from 1 April to 1 May, an interval little different from the 5 April–11 May documented by 12 egg sets collected 1918–44.

Migration: Winter visitors occur mainly from September to March.  Except for the one July record, the harrier has been noted in the Anza–Borrego Desert from 6 September (1999, one at the Ram’s Hill sewage ponds, H25, P. D. Jorgensen) to 1 May (2001, one at the north end of Clark Valley, C25, H. E. Stone).

Winter: Though the Northern Harrier is considerably more numerous in San Diego County in winter than in spring or summer, it is still generally uncommon.  We found no communal roosts.  Our highest counts were of nine in Los Peñasquitos Canyon (N8) 1 February 1998 (D. K. Adams), eight in Sycamore Canyon (O12) 8 December 1998 (W. E. Haas), and eight at Border Field State Park (W10) 19 December 1998 (K. Aldern).  Wintering birds are strongly concentrated in the coastal lowland, especially in the same regions with breeding birds.  At higher elevations we found no more than three harriers per atlas square per day, even in extensive grasslands such as those near Lake Henshaw and Lake Cuyamaca.  In the Anza–Borrego Desert wintering harriers are scarcer still, with only a few sightings of as many as two individuals at a time.  The average on Anza–Borrego Christmas bird counts is 2.4.

            Wintering as well as breeding birds vary in number with rainfall, which controls the abundance of the harrier’s prey.  The number atlas observers reported in drought-plagued 2001–02 was barely a third that in 1998–99, the year following El Niño rains. 

Conservation: Early in the 20^th century, the Northern Harrier was a common breeding resident (Willett 1912, Stephens 1919a).  With urbanization, especially of floodplains, the harrier lost most of its habitat and became rare as a breeding species.  Loss of foraging habitat and disturbance of nest sites are both likely factors.  Unfortunately, data to quantify the change do not exist.  Christmas bird counts suggest that the numbers of wintering birds remained more stable through the final third of the 20^th century.

            The Northern Harrier exemplifies the conundrum of rare wildlife with conflicting needs being squeezed together into small remnants of habitat.  In the Tijuana River estuary, some harriers learned to specialize on the Clapper Rail as prey (P. D. Jorgensen); some prey on Least Terns (R. T. Patton).  The Wildlife Research Institute (2004) reported that the most frequent issue confronting nesting harriers is disturbance from people walking, dogs allowed to run free, and off-road vehicles.

Taxonomy: The Northern Harrier comprises an Old World and a New World subspecies; C. c. hudsonius (Linnaeus, 1766) inhabits the latter.

Sharp-shinned Hawk Accipiter striatus

The Sharp-shinned Hawk is a widespread but uncommon winter visitor to San Diego County.  It is found in a wide variety of habitats, though more frequently in areas with trees or tall shrubs than in those without them.  Because the Sharp-shinned Hawk feeds predominantly on small birds, any place that concentrates flocks of House Finches, House Sparrows, White-crowned Sparrows, or juncos is likely to attract the hawk.

Winter: The Sharp-shinned Hawk is rather uniformly distributed over the coastal slope of San Diego County.  On the scale of the atlas grid there is no clear region of concentration.  In the Anza–Borrego Desert the species is sparser than on the coastal side, found mainly at oases, in developed areas, and in mesquite thickets.  In winter 2002 the Wildlife Research Institute recorded the Sharp-shinned Hawk at only two of its 45 study sites in southwestern San Diego County.

            Likewise, there is no great variation in the Sharp-shinned Hawk’s abundance from year to year.  From 1997 to 2002 the number of Sharp-shinned Hawks atlas observers reported per hour varied by no more than 23% from the mean for the five-year period.

Migration: In fall, the Sharp-shinned Hawk returns to San Diego County usually in the middle of September and is most numerous as a fall migrant in late September and early October, when up to 15 in a day have been seen migrating down Point Loma.  In spring, it  begins migrating out in March, and after the first week of April it is rare. From 1997 to 2001 our dates for the species ranged from 12 September (1997, one at Lake Hodges, K10, R. L. Barber) to 26 April (three records).  Specimens range from 15 September (1889, Ballena, SDNHM 325) to 6 April (1930, Point Loma, SDNHM 12716).  The latest reliable date ever reported for a migrant is 28 April (1982; one at Point Loma, R. E. Webster, AB 36: 893, 1982).

Breeding distribution: The Sharp-shinned Hawk has been confirmed nesting in the San Bernardino Mountains and seen repeatedly in summer in the San Jacinto Mountains, but in San Diego County evidence for its breeding is slight.  J. B. Dixon (in Willett 1933) reported seeing the species “in the Cuyamacas and other mountains in San Diego County in summer.  In one instance, the actions of the birds indicated a nest nearby.”   No more specific information is available.  Through the latter half of the 20^th century the only records of possibly breeding or summering Sharp-shinned Hawks were an apparent pair in Banner Canyon (J20) 29 April 1978 (J. L. Dunn, AB 32:1208, 1978) and a single bird on Palomar Mountain (D14) 14 July 1982 (R. Higson, AB 36:1016, 1982).

Conservation: In contrast to the crash (due to slaughter of migrants) and recovery in the eastern United States, changes in the Sharp-shinned Hawk’s numbers in California have been modest.  Christmas bird counts in San Diego County suggest no long-term change.  The birds use urban habitats as freely as natural ones.  During the atlas period our highest daily count of four was in a heavily developed area, East San Diego (R10; 20 December 1997, J. A. Dietrick), as was our highest number per hour, in southeast San Diego (S10).

Taxonomy: Accipiter s. velox is the subspecies of the Sharp-shinned Hawk widespread across North America and the only one recorded in California.

Cooper’s Hawk Accipiter cooperii

Long a bird of oak groves and mature riparian woodland, Cooper’s Hawk adapted abruptly to city living in the last two decades of the 20^th century.  The species is now at least as numerous in urban eucalyptus trees as in natural habitats.  The breeding population increased to the point where the local birds probably outnumber winter visitors.  Though Cooper’s Hawk is still listed as a “covered species” under San Diego’s multiple-species conservation plan, the idea that natural habitats and Cooper’s Hawks are conserved together is now laughably obsolete.

Breeding distribution: Breeding Cooper’s Hawks are widespread over San Diego County’s coastal slope wherever there are stands of trees.  They are most numerous in lowland and foothill canyons and in the urban areas of the city of San Diego.  We found up to four pairs per atlas square, and densities higher than this are likely.  Asay (1987) reported distances between nests as low as 1.0 km in oak woodland in Riverside and San Diego counties.  David Bittner, J. L. Lincer, and J. M. Wells found a similar density in riparian woodland of the Tijuana River valley in 2002 (Wildlife Research Institute 2004).

Cooper’s Hawk is sparser in the mountains than at lower elevations.  On Hot Springs Mountain (E20/E21) it has been seen only as a postbreeding visitor (juvenile 17 August 1996, K. L. Weaver, J. Dillane), but has been found nesting as high as 5900 feet elevation in the Laguna Mountains (P23; occupied nest on 1 July 1999, E. C. Hall, J. O. Zimmer).  The eastern margin of the species’ breeding range corresponds closely to the eastern limit of the oaks but goes down the east slope of the mountains in Alder Canyon (C21; used nest on 20 June 2001, P. D. Jorgensen) and San Felipe Valley (nests with nestlings near Scissors Crossing, J22, 28 June 2000 and 28 June 2001, M. and P. D. Jorgensen).  Cooper’s Hawks are seen rarely through the breeding season in the Anza–Borrego Desert, mainly but not exclusively in the Borrego Valley.  Most of these are nonbreeding; our only nest of Cooper’s Hawk in the Anza–Borrego Desert had nestlings in the north Borrego Valley (E24) 31 May 2001 (J. Fitch).  In 1997 a pair may have nested at Vallecito (M25); two birds were both carrying prey 29 April (M. C. Jorgensen).

Nesting: Cooper’s Hawks nest high in trees but beneath the canopy.  Sometimes they nest in riparian willows, but oaks are the species’ traditional nest site in California (Asay 1987).  In San Diego County the hawks still use oaks commonly but atlas observers described more than twice as many nests in eucalyptus trees as in oaks.  Other reported nest sites were also in planted trees: pine, redwood, and avocado. 

            Most of our nesting confirmations of the Cooper’s Hawk 1997–2001 corresponded to egg laying from late March to mid June, much like the 31 March–21 June interval of 32 egg sets collected 1897–1953.  However, atlas observers also noted about 15 instances of earlier nesting, enough to suggest that a broader breeding season is part of the species’ recent adaptations.  An adult feeding a full-grown juvenile already out of the nest at Kimball Park, National City (T10), 3 April 1999 (P. Unitt) suggests laying as early as the end of January.

Migration: Winter visitors occur in San Diego County mainly from September to March, but with the increase of Cooper’s Hawk as a breeding bird the arrival and departure of migrants is seldom obvious.  Occasionally, in late September and October, small numbers can be seen migrating south over Point Loma and through the Cuyamaca Mountains (D. Bittner).

Winter: Cooper’s Hawk is just as widespread over the coastal slope in winter as in the breeding season but more strongly concentrated at low elevations and in developed areas.  One of the atlas squares where we found the species most frequently in winter, for example, was Q13 in El Cajon, which has no significant natural habitat.  Our maximum per square per day was nine in Imperial Beach and the Tijuana River valley (V10) 19 December 1998 and 18 December 1999 (P. K. Nelson, W. E. Haas, et al.). 

In the Anza–Borrego Desert Cooper’s Hawk is rare except at oases and in developed areas.  But in Borrego Springs the birds are just as numerous as on the coastal slope.  The Anza–Borrego Christmas bird count 19 December 1999 yielded the maximum of seven in one single atlas square (F24; P. K. Nelson et al.) and 16 in the count circle as a whole.

Conservation: In 1978 the California Department of Fish and Game listed Cooper’s Hawk as a species of special concern (Remsen 1978), on the basis of population declines probably due to shooting, destruction of riparian woodland, and pesticide contamination. As the principal “chicken hawk,” Cooper’s Hawk attracted the wrath of man; even Stephens (1919a) wrote “it deserves no mercy.”  One specimen from San Diego tested in 1968 was highly contaminated with DDT (Risebrough et al. 1968), although such contamination was not widespread (Snyder et al. 1973). 

In the 1980s, though, Cooper's Hawks began adapting to the urban environment, nesting in eucalyptus trees in Balboa Park (R9) and later elsewhere through the city.  In the 1990s this adaptation accelerated and the birds' numbers increased conspicuously. By the time the atlas period began in 1997, Cooper's Hawks had colonized many small parks and schoolyards in inner-city San Diego: Roosevelt Junior High School (R9), the Educational Cultural Center (S10), Emerald Hills Park (S11), Kimball Park in National City (T10), and Eucalyptus Park in Chula Vista (T10). At the same time reports of nests in suburban and rural areas proliferated.  Numbers on San Diego Christmas bird counts increased from an average of 11 from 1961 to 1985 to 30 from 1997 to 2002.

Why did Cooper’s Hawk adapt so suddenly?  A shift in society's attitudes toward birds of prey coincided with the maturation of urban trees over many square miles of formerly treeless scrub. Collisions with windows are now the most serious source of mortality the hawk faces at the hand of man. Once enough Cooper’s Hawks learned that people rarely pose a threat any longer, the barrier to their occupying a habitat to which they were preadapted fell.  Though hawks in secluded areas defend their nests against people aggressively, in settled areas they usually ignore people even directly below the nest.  The open "woodland" of a eucalyptus-planted park, school campus, or neighborhood may be even more attractive habitat for the hawks than a natural oak grove if the numbers of prey like Domestic Pigeons, Mourning Doves, Western Scrub-Jays, and California Ground Squirrels are inflated by a steady supply of lunch scraps or bird feeders.

Northern Goshawk Accipiter gentilis

The Northern Goshawk is extremely rare in southern California, with only sporadic nesting in the high mountain forests.  Surprisingly, one of the old nesting records is from San Diego County.  Otherwise, only three winter vagrants are known in the county.

Winter: One was collected at Lower Otay Lake (U13) 9 November 1916 (Stephens 1919b, SDNHM 11577), one was collected at Mesa Grande (H17) 5 January 1928 (Abbott 1928b, SDNHM 11756), and one was seen at Palomar Mountain (D15) 22 March 1984 (R. Higson, AB 38:960, 1984).  The winter of 1916–17 was an invasion year for goshawks throughout California and Arizona.

 

Breeding distribution: Kiff and Paulson (1997) reported a set of goshawk eggs, preserved at the Slater Museum of Natural History, University of Puget Sound, collected by E. E. Sechrist at 5000 feet elevation in the Cuyamaca Mountains 7 May 1937.  On the data card accompanying the eggs Sechrist noted that he saw two young at the same location in June of the following year.  There have been no subsequent reports from the area, and the next nearest known goshawk nests have been at Mount Pinos and Mount Abel in northern Ventura County, though there have been summer sightings in the intervening ranges (Kiff and Paulson 1997).

 

Taxonomy: The specimens from San Diego County are the relatively pale A. g. atricapillus (Wilson, 1812), widespread across North America and the only subspecies of the Northern Goshawk known from California.

Harris’ Hawk Parabuteo unicinctus

Through recorded history, Harris’ Hawk has been irregular in southeastern California, the northwestern corner of its range.  It died out in the mid 1960s, and efforts to reintroduce it failed, as almost no native riparian woodland is left along the lower Colorado River.  Then, in 1994, an incursion, apparently from Baja California, brought nearly 50 individuals north of the border, many of them to San Diego County. Over the next few years the numbers in the county dropped to about five, but one pair in McCain Valley nested repeatedly, achieving success in 2000, 2001, and 2002—the first known successful nesting of wild Harris’ Hawks in California for over 40 years.  By 2003, however, the birds had disappeared.

Breeding distribution: The irruption of 1994 brought nine Harris’ Hawks to McCain Valley (S26) just north of Boulevard.  Some arrived even earlier, according to local resident Randy West.   In December 1994 another local resident, Leslie Mauris, showed me a photo of six huddled together on a phone line and a nest the birds had built earlier that year.  Over the next few years four of the birds were killed in various mishaps—three electrocuted on one pole in 1996—but one pair attempted to nest annually, with no success until 2000, when it fledged three young from a nest about 2 miles from that of 1994.  The nest was successful again in 2001 and 2002, but one bird was found dead and mummified 11 September 2001 (R. West; SDNHM 50578).  While nesting the birds in McCain Valley moved little; we had only one sighting during the breeding season in an adjacent atlas square, of two near the fire station in Boulevard (T26) 11 May 2000 (F. L. Unmack).

            In the Borrego Valley three Harris’ Hawks arrived 15 April 1994 and increased to eight by 13 September (J. Ruddley, R. Thériault).  Two were carrying sticks in the valley’s mesquite bosque (G25) 8 May 1994, and a pair was copulating in a trailer park in Borrego Springs (G24) 1 March 1995 (P. D. Jorgensen).  No further nesting activity was seen, however, and the last report of all eight birds was near Borrego Palm Canyon campground (F23) 12 March 1997 (B. Zuehl).  From 1998 on there were no more than two, and by the end of the atlas period in February 2002 only one remained. 

Nesting: The nests in McCain Valley in 1994 and 2000 were in the crowns of coast live oaks.  Harris’ Hawk’s breeding season is notably flexible; if prey is abundant, the birds nest earlier and lay repeated clutches (Bednarz 1995).  In 2000, the clutch must have been laid in late April, as at least one chick hatched by 1 June and the brood began fledging on 1 July, though the young were still jumping in and out of the nest on 12 July (R. West, P. Unitt).  In 2001, three young were still in the nest on 23 July.  Late nesting that year may have been due to ravens preying on an earlier clutch (R. West).

Migration: Patten and Erickson (2000) proposed that the history of Harris’ Hawk in southern California is a cycle of colonizations and extirpations, of fluctuations at the margins of the species’ range.  In San Diego County the only records before 1950 are of one collected in Mission Valley 7 November 1912 (Grey 1913a; SDNHM 1842) and one seen near Oceanside 1–6 November 1942 (Kent 1944).  From the 1960s until the incursion of 1994 scattered sightings of Harris’ Hawks were assumed to be of escapees from falconers, with whom the species is popular.

            Besides the concentrations in Borrego and McCain valleys, the irruption of 1994–96 brought one bird to Santee (P13) 26 November 1994–29 October 1996 (D. C. Seals, R. Saldino, NASFN 50:114, 222, 1996; 51:120, 1997; McKee and Erickson 2002), one to Carrizo Creek (Q27/R27) 6–10 July 1995 (P. D. Jorgensen, C. Hayes), one to Tamarisk Grove (I24) 3 December 1995 (R. Thériault), one to the Sweetwater River above Sweetwater Reservoir (S13) 31 March 1996 (P. Unitt, Rogers and Jaramillo 2002), one at Kit Carson Park, Escondido (K11), 3 April 1996 (M. B. Stowe, NASFN 50:332, 1996), and one to Butterfield Ranch (M23) 8 September 1996 (E. Craven, NASFN 51:120, 1997). 

An adult near Jamacha Junction (R13) 24 April 2001 (M. A. Patten, NAB 55:356, 2001) may have been one of these birds still lingering.

Winter: Sightings of single Harris’ Hawks in Coachwhip Canyon (F28) 4 October 1998 (R. Thériault), at Seventeen Palms (F29) 13 January 1999 (J. Meier), and at Warner Springs (F19) 17 December 2001 (N. Osborn, B. Siegel) suggest the birds at Borrego Springs moved up to 15 miles both east and west of their center.  In McCain Valley in winter we saw up to four on 17 February 2001 but found none farther than 3 miles from the nest site, as near the corner of McCain Valley Road and old Highway 80 (T27) 29 January 2001 (F. L. Unmack).

Conservation: The Harris’ Hawks in McCain Valley have suffered shooting and electrocution by utility wires, the latter a common problem for the species (Bednarz 1995, D. Bittner).  Randy West, who lives in McCain Valley, is a licensed falconer who keeps two Harris' Hawks of Texas origin. The wild Harris' Hawks regularly visit his captive birds.  West's birds may be responsible for the persistence and nesting of the wild individuals in a raptorial version of the "Wild Animal Park effect." That is, just as the wading birds kept in captivity at the Wild Animal Park and Sea World have served as nuclei for colonies of wild colonial wading birds, West's Harris' Hawks may have been the nucleus for the "colony" near Boulevard.  A third bird emerged as a nest "helper" when the chicks hatched in 2002. The Harris' Hawk's highly social habits, entailing cooperative breeding, mark it as a candidate for the "Wild Animal Park effect."

Taxonomy: All North American Harris’ Hawks are best called P. u. harrisi (Audubon, 1837); P. u. superior van Rossem, 1942, described from Imperial County, is not differentiated adequately by size or color (Bednarz 1995).

Red-shouldered Hawk Buteo lineatus

Once an uncommon resident of lowland riparian woodland, the Red-shouldered Hawk has more than compensated for the loss of much of its primitive habitat.  Over the 20^th century it spread into oak woodland at all elevations.  It began nesting in eucalyptus trees as soon as they were introduced and adopted rural ranches as a new habitat.  Through the last quarter of the century it became more and more of an urban bird, adding palms to its repertoire of nest sites. 

Breeding distribution: The Red-shouldered Hawk is widespread over San Diego County’s coastal slope, lacking only from areas like Otay Mountain devoid of tall trees.  The inland valleys of northern San Diego County are home to the most concentrated population; the patchwork of riparian woodland, scattered rural residences, orchards, and eucalyptus groves that typifies this area makes ideal Red-shouldered Hawk habitat.  Bloom et al. (1993) found that on average 39% of the home ranges of 17 Red-shouldered Hawks in Camp Pendleton and Orange County consisted of oak or riparian woodland.  But up to 25% of the home ranges consisted of water, asphalt, or buildings.  Bloom et al. found the average home range of seven paired males to be 1.21 to 1.70 km², according to the method of calculation.  In parts of northwestern San Diego County the territories of this size may be packed together with no breaks.  Our daily counts per atlas square in this region ranged up to 20 in northwest Escondido (I10) 16 May 1998 (E. C. Hall).  Five pairs nest within the town of Ramona (Wildlife Research Institute 2004).  In 1998 two pairs nested within 40 acres at Fallbrook (D8; E. Ashton).

            In other parts of the county the hawk’s population is sparser, though the species is still fairly common in many places.  We recorded it repeatedly up to about 5000 feet elevation, finding it regularly in the wetter Palomar and Cuyamaca mountains but rarely in the drier Hot Springs and Laguna mountains.  The hawk’s preference for moister habitats reflects its diet, dominated by reptiles and amphibians found in these habitats (J. L. Lincer).

The eastern edge of the Red-shouldered Hawk’s range corresponds closely with the eastern edge of oak woodland, but a few birds extend down San Felipe Creek to Scissors Crossing (J22; repeated sightings, including a probable nest and recently fledged juvenile on 14 and 20 May 1998, E. C. Hall; two juveniles 2 July 2002, J. R. Barth).  Also, since at least 1993, a pair has nested in the cottonwoods planted at Butterfield Ranch (M23; Massey 1998).  The nest was still occupied 21 March 2001 (P. K. Nelson).  A juvenile in nearby Vallecito Valley (M24) 27 April 2001 (P. K. Nelson) most likely originated from this pair.

Nesting: The Red-shouldered Hawk builds a stick nest high in trees; sycamores and coast live oaks were the typical primitive sites.  Soon after eucalyptus trees were introduced, Sharp (1906) reported the hawk adopting them, and atlas observers described nests in eucalyptus as often as in all other sites combined.  Another common novel site is the fan palm, reported eight times.  In palms the nest is placed either in the middle of the crown or straddled across leaf bases under the crown.  Red-shouldered Hawks frequently reuse their nests in successive years and take over old nests of other hawks.  We noted two instances of the Red-shouldered Hawk adopting old raven nests, including one on the bridge of old Highway 395 over Los Peñasquitos Creek, adjacent to the heavily trafficked Interstate 15 bridge (N10), using it in both 2000 and 2001 (K. J. Winter).

            Our observations from 1997 to 2001 implied Red-shouldered Hawks laying eggs from early March to late April, an interval similar to that of egg sets collected 1890–1952, 28 February–13 May.  Sharp (1906) reported a nest with two small chicks near Escondido 4 July 1906.  Dixon (1928) reported the hawk to begin nesting earlier after wet winters, as do many San Diego County birds.

Migration: The Red-shouldered Hawk is largely nonmigratory in California, but a few individuals move into the Anza–Borrego Desert, beginning in July: one at Middle Willows, Coyote Creek (C22) 5 July 1997, one (the same?) at Lower Willows (D23) the next day (ABDSP database), one injured juvenile at the Roadrunner Club, Borrego Springs (F24) 9 July 1996 (P. D. Jorgensen), and one juvenile at Tamarisk Grove Campground (I24) 9–10 July 1996 (R. Thériault).  Most desert records are October–December, but there are two as late as April at orchards and nurseries in the Borrego Valley (F25), which the species could colonize: two on 8 April 2001, one on 28 April 1999 (P. D. Ache).

Winter: The Red-shouldered Hawk’s pattern of abundance in winter is the same as during the breeding season; established pairs remain in their territories year round.  Escondido Christmas bird counts have recorded up to 90, on 4 January 1997.  The species is rare but fairly regular in winter in the developed areas of the Borrego Valley.  It has been recorded on 14 of 19 Anza–Borrego Christmas bird counts 1984–2002, with a maximum of three and usually just one.

Conservation: Both Sharp (1906) and Dixon (1928) considered the Red-shouldered Hawk “fairly common” in the inland valleys of northern San Diego County.  Dixon reported 23 nesting locations within a radius of 30 miles of Escondido, an area encompassing most of the hawk’s range within the county, and considered the Red-shouldered the most restricted in habitat of the area’s raptors.  Sharp, however, had already commented on its “great fondness” for eucalyptus groves, and the proliferation of these more than compensated for the loss of much of the hawk’s original habitat of riparian woodland.  The 23 nesting locations reported by Dixon might be equaled within a radius of 5 miles of Escondido today.

            Both Sharp and Dixon were explicit on the species’ upper limit in elevation, Sharp putting it at 1350 feet, Dixon at 1200 feet.  Sharp’s nest at Ramona was at the highest elevation recorded by any egg collector 1890–1952.  By the 1970s, however, the birds had spread to higher elevations, taking advantage of oak woodland as freely as riparian (Unitt 1984).  From 1999 to 2002 four pairs maintained territories in about one square mile between Highway 78 and Deer Lake Park Road, near Julian (K20), at elevation 4000–4400 feet (D. Bittner).

             Sams and Stott (1959) reported the Red-shouldered Hawk as “less common than formerly,” but the species’ adaptation to urban life had already begun by then, as they wrote that it “also nests in eucalyptus trees in Balboa Park.”  In the core range of the Red-shouldered Hawk there does not seem to have been any change in the species’ numbers through the final quarter of the 20^th century, but in metropolitan San Diego those numbers increased greatly.  From 1954 to 1973 the San Diego Christmas bird count averaged 3.1 Red-shouldered Hawks; from 1997 to 2002 it averaged 25.7.  When expressed on the basis of birds per party-hour the factor of increase was 6.8.  In spite of occasional collisions with windows and a susceptibility to eye injuries from the sharp-tipped seeds of exotic grasses (McCrary and Bloom 1984), the Red-shouldered Hawk has become one of San Diego County’s most successful urban adapters.

Taxonomy: The California subspecies of the Red-shouldered Hawk, the Red-bellied Hawk, B. l. elegans Cassin, 1856, differs grossly from the subspecies of the eastern United States.  Among other differences, its breast is solid rufous, and the more posterior underparts, including the thighs and undertail coverts, are heavily barred with deep rufous.

Broad-winged Hawk Buteo platypterus

The Broad-winged Hawk is common in the eastern United States but a rare visitor to California, mainly in fall migration.  Records from San Diego County are mostly from Point Loma.  The species’ frequency has declined since the 1980s, becoming less than one per year by the turn of the century.

Migration: In a faint echo of the raptor migration at the Marin Headlands on the north side of the Golden Gate, Point Loma (S7) guides and accumulates migrating birds of prey, which are reluctant to cross over water.  Thus it is the logical site for species like the Broad-winged Hawk moving south along the coast.  In San Diego County records away from Point Loma are few and largely near the coast, such as one at La Jolla (O7) 6 October 1999 (S. E. Smith, NAB 54:104, 2000).  Farthest inland was one at Palomar Mountain (D15) 31 October 1983 (R. Higson, AB 38:246, 1984).  Records are concentrated in the first two weeks of October; fall migrants extend in date from 18 September 1991 (Point Loma, D. and M. Hastings, AB 46:148, 1992) to at least 17 November (1999, Point Loma, J. R. Sams, NAB 54:104, 2000).  Some records in December may have been of late fall migrants as well.

            The three spring records are all inland: Palomar Mountain 2 April 1969 (AFN 23:625, 1969), Pine Valley Creek 2 miles north of Noble Canyon (O21) 26 April 1995 (K. F. Campbell), and Scissors Crossing (J22) 20 April 2003 (J. R. Barth).

Winter: Of San Diego County’s eight December–March records of the Broad-winged Hawk, only one is since 1981, at Point Loma 28–29 December 1991 (R. E. Webster, AB 46:314, 1992).  Unitt (1984) listed earlier records.  McCaskie (1968a) collected the only specimen, the first Broad-winged Hawk reported from California, in the Tijuana River valley 11 December 1966 (SDNHM 36086).

 

Conservation: The Broad-winged Hawk was more frequent in San Diego County in the late 1970s and 1980s: the seven reported in 1977 and 1980 have not been equaled since.  In the 1980s, Guy McCaskie (pers. comm.) expected to see a Broad-winged if he watched migrating hawks at Cabrillo National Monument between 11:00 AM and 1:00 PM during the species’ peak season.  But his similar effort there at the beginning of the 21^st century returned an average of less than one individual per year.

 

Taxonomy: Broad-winged Hawks in North America are all nominate B. p. platypterus (Vieillot, 1823); other subspecies are confined to the West Indies.

 

Swainson’s Hawk Buteo swainsoni

Swainson’s Hawk performs one of the most spectacular of all bird migrations: on its voyages between western North America and Argentina, almost the entire population gathers into huge flocks as it funnels through Central America.  Unfortunately, the hawk has been afflicted by contraction of its breeding range in North America and mass death by poisoning with pesticides in South America.  Although a fairly common breeding bird in San Diego County early in the 20^th century, Swainson’s Hawk no longer nests anywhere in southern California and has been designated threatened by the California Department of Fish and Game.  Over most of San Diego County, Swainson’s Hawk is now a rare migrant, but the Borrego Valley is an important staging site in spring.

Breeding distribution: Sharp (1902) considered Swainson’s the commonest nesting hawk in the valleys of San Diego County’s coastal lowland.  The birds nested at the edges of riparian woodland and foraged in nearby grassland.  Only one specimen, from Campo (U23) 15 July 1877 (SDNHM 337), suggests breeding above 1000 feet elevation.

            The first summer sighting of Swainson’s Hawk in San Diego County since 1933 was one of the least expected discoveries generated by the atlas study.  Eight molting subadults were in Warner Valley just north of Lake Henshaw 17 June 2000 (K. J. Winter, S. E. Smith), and these had increased to 25 one week later, 24–25 June (G. Rebstock, J. R. Barth, J. E. Pike, NAB 54:423, 2000).  The birds were drawn to the vast swarms of grasshoppers then overrunning the valley.  There was no previous record of Swainson’s Hawks in flocks in southern California in summer.

Nesting: In San Diego County, Swainson’s Hawks built their nests at heights of 35 to 75 feet in cottonwoods or sycamores (Sharp 1902).  These two trees account for all but one of the 33 egg sets collected from San Diego County 1900–33.  They attest to the hawk’s attachment in southern California to riparian woodland, an attachment maintained by the remnant population in the Central Valley (Schlorff and Bloom 1983).  The egg sets range in date from 12 April to 16 May; Sharp (1907) reported eggs near Escondido as late as 1 June.

Migration:  Spring dates for Swainson’s Hawk in San Diego County range from 31 January (1999, one at Lakeside, P14, M. B. Stowe, NAB 53:208, 1999) and 15 February (1996, one near Leucadia, K6, K. and C. Radamaker, NASFN 50:222, 1996) to 15 May (1977, one in the Borrego Valley, E. Copper) and 28 May (1999, one at Lake Cuyamaca, M20, A. P. and T. E. Keenan), with most from mid March to late April.  Over most of the county the birds are seen singly, sometimes in small flocks.  The Borrego Valley is on a migration corridor, the birds stopping to roost in strips of tamarisk trees and at nurseries (F25).  In 2003 and especially 2004, Hal Cohen and Paul Jorgensen organized daily (2004) or nearly daily (2003) monitoring.  The watch yielded 2055 Swainson’s Hawks between 27 February and 26 April in 2003 and 5210 between 22 February and 24 April in 2004.  In 2004 the largest single concentration was of 1000–1500 arriving on the evening of 25 March.  The birds feed on flying ants or dragonflies and on the caterpillars of the white-lined sphinx moth.  When thermal air currents arise in mid morning, most of the hawks head northwest through Coyote Creek canyon.  A few follow other routes such as San Felipe Valley, exemplified most notably by 74 near Scissors Crossing (J22) 18 March 1999 (ABDSP database).

            In fall Swainson’s Hawk is less frequent than in spring, but the birds still make some use of the route across the Anza–Borrego Desert.  This was illustrated most dramatically on 20 October 1999, when a flock of at least 140 roosted at Ocotillo Wells (I29; P. D. Jorgensen), perhaps the same as 130 that arrived 35 miles to the southeast near El Centro four days later (K. Z. Kurland; NAB 54:104, 2000).  The only other flocks reported in San Diego County in fall were 50–60 at Warner Springs (F19) 29 October 1988 (D. MacKenzie, AB 43:167, 1989) and 78 at Borrego Springs 21 October 2001 (P. D. Jorgensen).  Fall dates range from 9 September (1975, Point Loma, S7, AB 30:126, 1976) to 1 November (1986, Wilderness Gardens, D13, C. G. Edwards, AB 41:143, 1987).

Winter: For many years, Swainson’s Hawk was accidental in the United States in winter (Browning 1974).  Since 1990, small numbers have begun wintering in the Sacramento–San Joaquin delta (Herzog 1996) and a few wintering birds have been noted elsewhere in California.  In San Diego County the only winter records are of one at Whelan Lake (G6) 27 December 1986–8 March 1987 (G. McCaskie, AB 41:328, 1987), one in the Tijuana River valley 2 December 1995 (P. A. Ginsburg, NASFN 50:114, 1996), and one in Murphy Canyon (Q10) 19 December 2000 (M. A. Patten, NAB 55:227, 2001).  The last two may have been delayed fall migrants.

Conservation: Swainson’s Hawk was in decline by the 1930s, when E. E. Sechrist reported to Willett (1933) that it was “now scarce near San Diego.”  The last eggs collected in the county are dated 1933 as well.  The factors extirpating Swainson’s Hawk as a breeding bird from southern California are unclear, but shooting, elimination of riparian woodland, urban development, rodenticides, and other pesticides may all have contributed.  Urbanization and changes in crops threaten the remaining population in the Central Valley, estimated at about 430 pairs in 1988 (California Department of Fish and Game 1993).  Given that this is the only population remaining west of the Sierra Nevada, the numbers seen in the Anza–Borrego Desert suggest that a substantial fraction of California’s Swainson’s Hawks migrate across San Diego County. 

            After a low point from the mid 1960s to mid 1980s, numbers of Swainson’s Hawk migrating through southern California have increased somewhat (Patten et al. 2003).  An increase seems counterintuitive, given recent huge mortality in Argentina (Woodbridge et al. 1995).  It may be related to a northward shift of the winter range to Mexico and Central America enabled by deforestation.  Such a shift could account for more frequent wintering and arrival in February, another recent change.  Sharp (1902) specified that Swainson’s Hawk arrived in San Diego County 10–20 March.

Zone-tailed Hawk Buteo albonotatus

San Diego County lies at the northwest corner of the Zone-tailed Hawk’s primarily Mexican range.  At the beginning of the 21^st century four or five were being seen in the county each year.  A pair on Hot Springs Mountain from 1986 to 1992 was one of only two pairs ever known to have nested in California.  Though sightings of the Zone-tailed Hawk are more frequent in winter than in summer, continued scattered individuals in San Diego County’s mountains suggest that further nestings are likely.  An apparent mimic of the Turkey Vulture, the Zone-tailed Hawk associates regularly with vulture roosts.

Winter: At least 30 Zone-tailed Hawks have been noted in San Diego County in fall and winter, mainly in the inland valleys of the coastal lowland.  Several birds have apparently returned in successive years to the same area.  During the atlas period one or two wintered regularly at the Wild Animal Park (J12), evidently attracted by the Turkey Vulture roost, and at nearby Oak Hill Cemetery, Escondido (I12), where the gardener put out the gophers he trapped for the hawks.  Other areas where Zone-tailed Hawks have wintered repeatedly are the San Luis Rey River valley from Oceanside to Vista and Bonsall (G6/G7/G8/F8; 1979–97) and Santee Lakes (O12/P12; 1991–96).  Only five or six have been seen in winter at elevations above 2000 feet, but one was near the summit of Palomar Mountain (D15) 26 February–7 March 1983 (R. Higson, Roberson 1993).

Migration: All but one of the low-elevation sightings of the Zone-tailed Hawk fall within the interval 19 August (2001, Wild Animal Park, M. Billings) to 16 April (1998, same location, Rottenborn and Morlan 2000).  The exception was a bird that wintered repeatedly, seen also 9–27 June 1984 at the Bonsall bridge over the San Luis Rey River (F8; C. Wilson, Roberson 1993).

The summering birds on Hot Springs Mountain were reported from 5 May (1991) to 10 August (1986).

Breeding distribution: A pair of Zone-tailed Hawks nested in a steep west-draining canyon of Hot Springs Mountain (E20) in 1986, 1987, and 1988, fledging one young at least in 1986 (R. Higson; AB 40:1255, 1986; 41:143, 1987; Langham 1991).  The pair was seen annually through 1992, with only a single individual noted in 1993 and none subsequently.  The only other nestings known in California, all unsuccessful, were by a single pair on north slope of the Santa Rosa Mountains of Riverside County 1979–81 (Weathers 1983), about 8 miles north of San Diego County.

            Other spring and summer sightings in San Diego County are from Nate Harrison Grade, southwest slope of Palomar Mountain (E13), 17 April 1999 (C. Sankpill), near Angel Mountain (G16) March 1999 (D. Bittner) and 31 May 1999 (W. E. Haas), 1.5 miles east-southeast of Angel Mountain (H17) 13 June 1998 (P. Unitt, Erickson and Hamilton 2001), near Warner’s Ranch (G19) 24 June 2000 (E. C. Hall, NAB 54:423, 2000), at Mount Laguna (O23) 22 July 2000 (D. W. Aguillard, NAB 54:423, 2000), 1.25 miles northwest of Cameron Corners (T22) 7 May 2000 (R. and S. L. Breisch), and one at Campo (U23) 27 May 2001 (D. S. and A. W. Hester, NAB 55:356, 2001).

Nesting: Both California nests of the Zone-tailed Hawk have been in tall conifers on steep slopes, in a sugar pine in the Santa Rosa Mountains (Weathers 1983), in a conifer of unrecorded species on Hot Springs Mountain.  In 1986 the young fledged in July.

Conservation: From 1862 to 1972 only six Zone-tailed Hawks were reported from San Diego County.  Since 1973 the hawk has become ever more frequent, prompting the California Bird Records Committee to drop the species from its review list in 1998.  The nestings in southern California are part of the evidence for a slight northward spread of the species’ range (Johnson 1994).  But urbanization and desiccation of riparian woodland have eliminated the species from other former habitat in Arizona and Texas (Johnson et al. 2000).

Red-tailed Hawk Buteo jamaicensis

The Red-tailed Hawk is not only San Diego County’s most widespread bird of prey, it is one of the most widespread of all the county’s birds.  During the atlas study we found it in 472 of 478 covered atlas squares.  It favors grassland with scattered trees that serve as lookout perches and nest sites but uses all of the county’s terrestrial habitats to some extent. It tolerates considerable urbanization.  From the hawk’s point of view the additional nest sites afforded by eucalyptus trees and towers for electric lines have offset the reduction of foraging grounds.

Breeding distribution: Though the Red-tailed Hawk nests in all regions of San Diego County, the breeding population is concentrated in areas of extensive grassland like Camp Pendleton, Warner Valley, and the Santa Maria Valley around Ramona.  Atlas observers noted up to three active nests per square, as along 1 mile of the San Diego River above El Capitan Reservoir (M17) in March and April 1999 (R. C. Sanger) and along 3 miles of Dulzura Creek northwest of Dulzura (T16) on 23 February 1998 (D. W. Povey).  The Wildlife Research Institute (2004) reported 26 pairs in and around the roughly 5000 acres of grassland in Santa Maria Valley, a density of over three pairs per square mile.  The total county population may be on the order of 1000 pairs.

            In the Anza–Borrego Desert the Red-tailed Hawk is considerably sparser than on the coastal slope, but we found active nests in some of the most remote and dry desert wildernesses, as in Wonderstone Wash (D29) 29 March 2001 (R. Thériault) and in the north fork of Fish Creek Wash (L28) 13 April 1999 and 6 May 2001 (L. J. Hargrove, D. C. Seals).   The Anza–Borrego Desert probably supports around 25 to 30 pairs after wet years, fewer after dry ones.

Nesting: The Red-tailed Hawk tends to build its nest in more exposed situations than other local hawks, though it participates with other hawks, the Common Raven, and the Great Horned Owl in the musical-chair reuse of each other’s nests.  Of 55 nests that atlas observers described, 10 were in sycamores, 4 were in coast live oaks, 19 were in eucalyptus, and 3 were in other species of trees.  Seven, mainly in the Anza–Borrego Desert, were on bluffs or cliffs.  Ten were on power-line towers, one was on a wooden telephone pole, and one was on a platform for floodlights for the ball field at Mount Carmel High School (M10). 

            Red-tailed Hawks may begin refurbishing their nests in December and lay eggs mainly from late February to mid April.  The earliest date among 160 egg sets collected 1890–1964 is 22 February.  From 1997 to 2001, and especially in 1998 and 1999, however, about 20 observations suggested laying from early January to mid February.  The date of our latest nest with nestlings, 29 June 1998, suggests laying about 30 April.  Sharp (1907) reported eggs at Escondido as late as 4 May, and J. B. Dixon collected a set there as late as 3 June in 1926, following a wet winter (WFVZ 10082).   The Red-tailed Hawk’s breeding season tends to begin earlier and last longer after wet winters, like 1997–98, and the following year, when the abundance of prey is still high.

Migration: On the basis of monthly censuses 1973–83 at San Elijo Lagoon (L7), King et al. (1987) found numbers of Red-tailed Hawks to be at their low from May to September, at their high, about 3 times larger, from December to February.

Winter: The Red-tailed Hawk is only slightly more common as a winter visitor than as a breeding species; from December to February counts per day per atlas square ranged up to 23 (between Santee and Lakeside, P13, 8 January 2000, D. C. Seals) but from March to July the maximum was only to 13.  In winter it spreads to areas where it does not nest such as the Silver Strand (T9; up to six on 16 December 2000, P. R. Pryde).  It is most numerous in the coastal lowland and scarcest in the Anza–Borrego Desert.  We missed it in winter in only 15 atlas squares, one completely urbanized (Ocean Beach, R7), 14 in the desert.

Conservation: The Red-tailed Hawk uses any open area for foraging, however disturbed, giving the species great flexibility.  Even though it has little use for heavily developed areas, it takes advantage of even small scraps of undeveloped habitat.  It acclimatizes to human activity near nests, allowing it to breed in places like Switzer Canyon at the east edge of Balboa Park (R10).  From at least 1997 to 2000 a pair nested in this canyon surrounded by urbanization on three sides for nearly a century (J. M. Wells, J. A. Dietrick).  The majority of the hawk’s nests are now in eucalyptus trees and on power towers and other man-made structures, suggesting that the supply of nest sites limited the population, at least in the coastal lowland, before people introduced these features into the environment.  Even with explosive growth of the human population, the Red-tailed Hawk population is still remarkably stable.

Taxonomy: Red-tailed Hawks breeding in San Diego County, and nearly all winter visitors, are the widespread subspecies of western North America, B. j. calurus Cassin, 1856.  In calurus the light-morph adult has rufous-barred thighs and a rather distinct band of streaking on the lower breast and upper belly.  Two specimens match the whiter subspecies B. c. fuertesi Sutton and van Tyne, 1935, and so are apparently dispersers from northern mainland Mexico or the southern Great Plains.  One of these was at Ocean Beach (R7) 24 January 1921 (SDNHM 2235), the other in Pringle Canyon (T16) 10 February 1935 (SDNHM 17074).  There is one sight record of Harlan’s Hawk, B. c. harlani (Audubon, 1830), the melanistic subspecies with a mottled grayish tail that breeds mainly in Alaska and winters mainly in the southern Great Plains: near Lower Otay Lake (U13/U14) 14 January 1992 (J. C. Lovio, AB 46:315, 1992).

Ferruginous Hawk Buteo regalis

Open plains are home to the Ferruginous Hawk, an uncommon winter visitor to San Diego County.  About 100 individuals reach the county annually.  Conservation of the county’s most extensive grasslands is essential to maintenance of these numbers.

Winter: The Ferruginous Hawk’s patchy distribution in San Diego County corresponds largely to the larger tracts of grassland, especially those more than 12 miles inland.  Our highest counts 1997–2002 were from such areas: nine in the east arm of Warner Valley (G19) 15 March 1998 (P. D. Jorgensen), eight in the upper basin of Lake Cuyamaca (L21) 25 December 2001 (L. and M. Polinsky), and eight just southwest of Ramona (L14) 17 January 1999 (F. Sproul).  The largest numbers of Ferruginous Hawks in San Diego County have been found on Lake Henshaw Christmas bird counts, with up to 34 on 19 December 1988.  The years of high counts correspond with discoveries of Ferruginous Hawks roosting communally in trees and on telephone poles at Warner Springs (F19), up to 13 in a cluster (King et al. 1988).

Within 12 miles of the coast our only count of more than two individuals was of four in the Fallbrook Naval Weapons Station (D7) 28 February 2002 (K. L. Weaver).  The maximum recorded on any of San Diego County’s three coastal Christmas bird counts is four also.

            The Ferruginous Hawk also occurs fairly regularly in the Borrego Valley, where our highest counts per atlas square per day were of two individuals.  The species was recorded on 15 of 19 Anza–Borrego Christmas bird counts 1984–2002, with an average of 1.5.  One or two individuals are seen regularly in Earthquake Valley (J23/K23) but the species is rare in other desert valleys like Clark, Culp, Blair, and Vallecito.

Migration: The Ferruginous Hawk occurs in San Diego County mainly from October to March; extreme dates are 14 September (1962, San Diego area, AFN 17:68, 1963) and 17 September (year not specified, Lake Cuyamaca, J. G. Peterson in Grinnell and Miller 1944) in fall and 21 April (year not specified, Lake Cuyamaca, J. G. Peterson in Grinnell and Miller 1944) and 25 April (1997, between Carlsbad and San Marcos, J8, J. O. Zimmer) in spring.

Conservation: The Ferruginous Hawk has lost some habitat to urbanization, as at Rancho Otay (U12/U13).  Conservation of the threatened Ramona grasslands in Santa Maria Valley is critical to sustaining the hawk’s numbers in San Diego County.  Overgrazing could also degrade the Ferruginous Hawk’s foraging habitat by reducing the supply of prey.  Indiscriminate use of rodenticides could reduce the hawk’s food supply as well.  It appears from Lake Henshaw Christmas bird count results that the numbers in Warner Valley were greater from 1983 to 1989 (average 28) than from 1997 to 2002 (average 8).  Bad weather and no searches for communal roosts during the more recent interval, however, may account for some of the difference. 

Rough-legged Hawk Buteo lagopus

Breeding on arctic tundra, the Rough-legged Hawk reaches the southern edge of its winter range in San Diego County.  The only well-supported record for Baja California is of one soaring over the border fence in the Tijuana River valley (Erickson et al. 2001).  The Rough-legged seeks the same open grasslands as the Ferruginous Hawk.  Though the Rough-legged’s abundance in California as a whole varies in a cycle of 3 to 5 years (Garrison 1993), the cycle is ill marked in San Diego County, where the average since the mid 1990s has been less than one per year.

Winter: Records of the Rough-legged Hawk are widely scattered over San Diego County’s coastal slope as well as the Borrego and San Felipe valleys on the desert side.  They are concentrated, however, in Warner Valley.  This region, the county’s largest block of grassland, is the only site of reports of more than two individuals: three on 21 March 1976 (AB 30:765, 1976) and five on the Lake Henshaw Christmas bird count 23 December 1985, with three remaining to 1 February 1986.  But the three seen during the atlas period 1997–2002 were from other locations: Dameron Valley (C15) 6 February 1999 (K. L. Weaver), San Dieguito Valley (M8) 27 February 1999 (R. T. Patton), and Galleta Meadows (E23) 18 January 2001 (P. D. Jorgensen).

Ascertaining the Rough-legged Hawk’s status in San Diego County has been bedeviled by misidentification since Huey (1924) misreported two specimens of the Ferruginous as the Rough-legged.  There is still no specimen or photograph from the county.

Migration: Dates for the Rough-legged Hawk in San Diego County extend from 8 October (1985, one at Lake Henshaw, R. Higson) to 27 March (1982, one at Lake Henshaw, R. E. Webster, AB 36: 894, 1982) and 28 March (1976, two at Lake Henshaw, AB 30:765, 1976).  One at Point Loma (S7) 20 November 1984 was likely the same as one at Silver Strand State Beach (T9) four days later and a migrant heading south (D. Parker, E. Copper, AB 40:158, 1986).  Garrison and Bloom (1993) found that of five Rough-legged Hawks banded in the breeding range and recovered in California, four originated in Alaska, one in arctic Canada.

Conservation: From the early 1990s through 2002 the Rough-legged Hawk was less frequent in San Diego County than in the 1970s and 1980s.  Climatic warming could allow the winter range to shift northward so that the hawk no longer reaches its current southern limits.

Taxonomy: The single New World subspecies of the circumpolar Rough-legged Hawk is B. l. sanctijohannis (Gmelin, 1788).

Golden Eagle Aquila chrysaetos

As a top predator, the Golden Eagle has the largest territory and the lowest population density of any San Diego County bird.  Pairs remain in their territories year round, though the young disperse widely.  Most pairs nest on cliff ledges, the rest in trees on steep slopes, hunting in nearby grassland, sage scrub, or broken chaparral.  San Diego County’s Golden Eagle population has dropped from an estimated 108 pairs at the beginning of the 20^th century to about 53 pairs at the century’s end, mainly as a result of urban development of foraging habitat.   Many of the territories persisting at the beginning of the 21^st century lie near the edge of the urban growth front, a shadow over the future of the capstone of San Diego County’s ecosystem.

Breeding distribution: The Golden Eagle’s distribution in San Diego County is known better through history than that of any other bird, thanks to study by generations of San Diegans: James B. Dixon, John Colton, John Oakley, Thomas A. Scott, David Bittner, and their collaborators through the Wildlife Research Institute.  Since 1988, Bittner and Oakley have organized a team of observers to monitor the county’s nesting eagles annually and have checked some inaccessible nest sites via helicopter.  This account is based largely on data kindly provided by Bittner.

From 1997 to 2001, about 50–55 pairs nested in the county.  Fewer than 20 pairs fledge young each year, averaging 1.5 young per successful nest.  Only four of these territories lie west of Interstate 15, three in Camp Pendleton, one around Lake Hodges (K10).  Most of the remaining pairs nest within a band 20 to 25 miles wide through the foothills.  In southern San Diego County, San Miguel Mountain (S13/S14) and Otay Mountain (U15/V14/V15) mark the western limit of the current breeding range.

            In and along the edges of the Anza–Borrego Desert there are 10 known nest sites or clusters of nest sites, though some of these went unused during the entire atlas study, even following the wet winter of 1997–98.  Only seven of these territories were active during the atlas period 1997–2001, and at most three were active in any given year.  In some nests new material was added but no eggs were laid.  Since 1998, drought has suppressed numbers of the eagle’s principal prey in the Anza–Borrego Desert, the Black-tailed Jackrabbit.  Only two young eagles fledged in the Anza–Borrego Desert in 2003 (D. Bittner). 

The Golden Eagle is absent from some surprisingly large yet little disturbed areas of San Diego County, such as Cuyamaca Mountains and the Campo Plateau between Lake Morena and Jacumba.

The map of the species’ breeding distribution somewhat overrepresents its abundance. A few pairs straddle two atlas squares. Nesting in three squares (F19, M13, R15) has ceased since 1997. And nesting in two squares (D27, L28) is based on addition of new material to old nests but no egg laying.  It somewhat underrepresents its former breeding distribution because nonbreeding birds were recorded during the atlas period in some squares where the species formerly nested.

Nesting: Scott (1985) found about 80% of San Diego County’s Golden Eagle nests built on cliff ledges, 20% in trees, usually on steep slopes.  A pair typically rotates among several nest sites, including both cliff and tree nests.  Many of the cliff sites have been in regular use since the early 20^th century and undoubtedly long before that.  Though the giant stick nests are reused for years, the birds refurbish them annually.  In San Diego County, fallen yucca leaves, with their tough fibers, are a common ingredient in the nest’s lining (Dixon 1937, D. Bittner).

            The Golden Eagle’s schedule of nesting in San Diego County is also supported by abundant data.  Nest building begins with the first heavy rain of fall (Dixon 1937).  Copulation begins as early as 5 January (D. Bittner).  Dates of 407 egg sets collected or observed from 1891 to 1957 range from 2 February to 26 April, except for one on 7 May and another on 16 June.  The mean date is 4 March, standard deviation 17 days.  Eggs laid after the first week of March, however, are probably replacement clutches (Dixon 1937).  During his recent surveys, Bittner has found most eggs laid in mid February, most chicks hatching in late March or early April, and most young fledging in June.  Occasionally, however, he encounters nestlings on dates suggesting they hatched from eggs laid in mid January (e.g., chicks five weeks old on 15 April 2004).

Migration: Once a Golden Eagle acquires a mate and a territory, it remains with them year round, except for occasional swapping (Kochert et al. 2002).  Young birds, however, may disperse considerable distances: birds banded in San Diego County have been recovered in Ojai, Ventura County, in Apple Valley, San Bernardino County, in Utah, in the Grand Canyon, Arizona, and near Guadalajara in central Mexico (T. A. Scott, D. Bittner).

Winter: In spite of the mobility of immatures and nonterritorial adults, the nonbreeding distribution of the Golden Eagle in San Diego County does not differ greatly from the breeding distribution.  In southern San Diego County a few birds often spread west to the Otay and Tijuana River valleys, accounting for the near regularity of the eagle on the San Diego Christmas bird count (noted on 16 of 20 counts 1983–2002).  One on the fill north of the Sweetwater River mouth, National City (T10), 15 December 2001 (S. M. Wolf) was our only sighting during the atlas period of a Golden Eagle that must have flown several miles over developed areas.  The count circles other than San Diego include at least one nesting territory.  Our maximum winter count per atlas square per day was three, all within a few miles of nest sites.

Conservation: Following studies by Dixon (1937) and Scott (1985), David Bittner and John Oakley estimate the Golden Eagle population of San Diego County in 1900 at 108 pairs. It remained near 100 pairs until the rapid growth of the county’s human population following World War II.  In the 1970s, following the building of the interstate highways and the spread of avocado and citrus orchards along Interstate 15, the decline became precipitous.  By 2004, the population had dropped to about 53 pairs, with some uncertainty because of a few territories straddling the county line and long vacancy of some territories in the Anza–Borrego Desert.  Since 1988, the surveys organized by the Wildlife Research Institute have located about 15 previously unknown pairs in remote parts of the county, accounting for the variation from the estimate of 40–50 pairs reported by Unitt (1984) on the basis of studies by T. A. Scott (pers. comm.).

            The eagles abandoned four territories just within the five-year atlas period, and the Wildlife Research Institute estimates that nine more are in imminent danger of abandonment.  Without better planning for habitat conservation, the institute estimates the county’s eagle population could be halved again by 2030. 

The most important factor in this decrease has been urban sprawl covering former foraging habitat.  From 1900 to 1936, when eagle territories still filled northwestern San Diego County, Dixon (1937) found the territories of 27 pairs in that region to range from 19 to 48 square miles and average 36.  Thus the area needed to support the species is considerably greater than for any other San Diego County bird.  The viability of territories that become isolated from the main block of the species’ range is also questionable.  Of the 27 territories mapped by Dixon (1937), only nine were occupied at the beginning of the 21^st century.

            Other factors affecting the eagle are human disturbance, especially rock climbing on nesting cliffs, but also shooting (both recreational and for military training on Camp Pendleton), and agriculture (avocado orchards planted near nest sites).  Electrocution on power lines is now the biggest source of mortality: 37 of 55 dead eagles picked up in and near San Diego County 1988–2003 and reported to Bittner had been electrocuted.  The Golden Eagle was less subject to poisoning by insecticides like DDT than other birds of prey but has suffered poisoning by scavenging prey killed by rodenticides.  Three of the 55 dead birds recovered had been killed through such secondary poisoning.  Ever more prolonged droughts could depress the population further, a factor Hoffman and Smith (2003) suggested as affecting raptors throughout the western United States.

Taxonomy: Aquila c. canadensis (Linnaeus, 1758) is the only subspecies of the Golden Eagle in North America.