Falcons — Family Falconidae
American Kestrel Falco sparverius
Ideal habitat for the American Kestrel is a grove of tall sycamore or oak trees, which offer cavities for nests, adjacent to grassland or open ground, where the birds can forage. In the Anza–Borrego Desert a few kestrels nest in niches in eroded bluffs. The kestrel has also become a fairly successful urban adapter by nesting among the leaf bases of palms. It is a year-round resident in San Diego County but more numerous in winter, with the arrival of migrants from the north.
Breeding distribution: The American Kestrel is widespread in San Diego County but most numerous in valley floors and broad canyons on the coastal slope. Our counts during the breeding season ranged up to 13 (including fledglings) near Rincon (F13) 12 June 1999 (S. L. Breisch, J. Hargrove), but generally the kestrel must be rated uncommon as a breeding bird in San Diego County, as only slightly over 1% of our daily totals in spring or summer were of over six individuals. In regions of extensive unbroken chaparral and at higher elevations the kestrel is sparse or lacking. Nevertheless, it nests at least as high as 4500 feet on Palomar Mountain (D14; active nest on 24 June 1997, P. D. Jorgensen) and in the Laguna Mountains (N22; fledgling on 7 July 2001, G. L. Rogers), probably as high as 5500 feet around Laguna Meadow (O23; pair and probable nest 21 June 1997, A. Klovstad, C. Mann). The kestrel is absent from much of the Anza–Borrego Desert but breeds in small numbers at oases like Lower Willows (D23), Butterfield Ranch (M23), and Vallecito (M25), in developed areas, and in the Borrego and Carrizo badlands.
Nesting: Over most of the kestrel’s range it nests in tree cavities, either excavated by woodpeckers or resulting from decay. Such sites are common in San Diego County as well. But where fan palms or the Canary Island date palm have been planted, kestrels nest in the crevices among the leaf bases or under the skirts of dead leaves. Atlas observers described nests in palms more frequently than any other type of site. Other man-made sites we observed were the hollow arms of power poles, the aircraft-warning spheres hung on power lines, an abandoned building, and the eaves of the headquarters building for the Ocotillo Wells state off-road vehicle area (I28; nestlings on 24 April 1997, J. Rudley). Kestrels commonly use nest boxes if good hunting habitat is nearby (J. L. Lincer). We did not see the actual nests, but evidently the kestrel also uses crevices in bluffs in desert badlands. Such sites were especially likely near Font’s Point (F27; male carrying prey toward cliffs 1 May 1999, G. Rebstock, K. Forney) and between June Wash and Arroyo Tapiado (M27; male carrying lizard 26 April 2000, R. Thériault). One egg set collected at La Jolla (P7) in 1935 was from a hole in a cliff, and another from an unspecified location was from a granite cliff.
Dates of kestrel eggs collected in San Diego County 1894–1952 range from 21 March to 30 May, and the great majority of our observations 1997–2001 conform with this interval. A few, however, are earlier. Fledged young northwest of Santa Ysabel (I18) 16 April 2000 (S. E. Smith) imply egg laying as early as mid February; a nest with four nestlings in Los Peñasquitos Canyon (N8) 11 March 2000 (L. D. and R. Johnson) imply egg laying as early as late January or early February. Another nest with nestling just a short distance up the same canyon (N9) 9 August 1999 (A. G. and D. Stanton) implies laying as late as mid June. This pattern prevails among many raptors (Wildlife Research Institute 2004).
Migration: Though there is a winter influx of kestrels into San Diego County, the only concentration of migrants noted is of 17 in Blair Valley and 5 in nearby Little Blair Valley (L24) 9 April 1999 (R. Thériault).
Winter: The kestrel is even more widespread in winter (recorded in 396 atlas squares) than in the breeding season (recorded in 366 atlas squares). The pattern of winter distribution is similar, but the birds are even more concentrated in grassland. Our highest rate of encounter with the species was in the Santa Maria Valley on the northwest side of Ramona (K14). The factor affecting the variation in the number of kestrels from winter to winter is most likely the supply of prey as determined by rainfall. We found the species at the same relatively high rate in the wet winter of 1997–98 as well as the following year. Then in dry 2000–01 and 2001–02 the rate dropped to 54–62% what it had been at the beginning of the five-year atlas period.
Conservation: Christmas bird counts and historical literature from San Diego County suggest the kestrel population has been fairly stable through time, though the Breeding Bird Survey suggests a trend of decline over California as a whole (Sauer et al. 2003). The species has gained foraging habitat through the clearing of chaparral but lost it through the paving and landscaping of open ground. The changes humanity has made to the environment have meant a net increase in nest sites, which can be limiting to a secondary cavity user like the kestrel that is large in comparison to most of the primary cavity excavators, the woodpeckers.
Taxonomy: Almost all specimens of the American Kestrel from San Diego County, including 16 recent specimens of the local breeding population taken for predator control at Least Tern colonies, are the relatively large F. s. sparverius Linnaeus, 1758, widespread across North America. Just one specimen from San Diego 6 November 1921 (SDNHM 2288) is small enough to represent a wanderer of the smaller F. s. peninsularis Mearns, 1892 (Unitt 1984).
Merlin Falco columbarius
The Merlin is a rare winter visitor to San Diego County, seen most often in grassland, though it occurs occasionally in any habitat except dense woodland. Because it feeds largely on small birds, the Merlin may be attracted to any place where small birds flock, including mudflats and cattle pens. In the 1990s the number of Merlins wintering in San Diego County increased noticeably.
Winter: Wintering Merlins occur in all regions of San Diego County, though they are more frequent in the coastal lowland and less frequent in the Anza–Borrego Desert. The variation in their abundance from year to year is only moderate: the number per observer-hour each winter from 1997 to 2002 varied by up to 25% from the mean for the whole five years. Normally the species is seen singly; our maximum count per atlas square per day was three, southwest of Ramona (L14) 17 January 1999 (F. Sproul) and in the north Borrego Valley (E24) 20 December 1998 (P. R. Pryde). On San Diego County Christmas bird counts the maximum is nine on the San Diego count 19 December 1998, though some of these counts still occasionally miss the species. As an order-of-magnitude estimate of the number of Merlins reaching the county per year I suggest 100 (more pass through in migration).
Migration: The Merlin occurs in San Diego County mainly from October to March. Extreme dates are 23 August (1980, Tijuana River valley, AB 35:226, 1981) and 26 April (1998; San Elijo Lagoon, L7, C. G. Edwards; Otay Mesa, V14, S. D. Cameron; 2001, Point Loma, S7, P. A. Ginsburg).
From mid September to late October a few Merlins can be seen moving south along Point Loma every year. Unlike the hawks of the genus Buteo, Merlins do not hesitate when they reach the tip of the point, instead speeding south over the ocean, even when fog prevents them from seeing North Island (G. McCaskie).
Conservation: Early in the 20th century the Merlin was “common” according to Willett (1912), “rather common” according to Stephens (1919a). By the 1960s, however, it was rare, probably because of the same effects of DDT contamination that affected the larger birds of prey (Lincer 1975). From 1961 to 1980 San Diego Christmas bird counts averaged only 0.55 per year. A trend toward recovery did not become clear until the 1990s. From 1997 to 2002 the average on the San Diego Christmas bird count was 6.3. Similarly, the Escondido count went from an average of 1.3 from 1986 to 1994 to an average of 3.3 from 1997 to 2002.
Taxonomy: All three North American subspecies of the Merlin are known from San Diego County. By far the most frequent is the medium-dark F. c. columbarius Linnaeus, 1758, which breeds almost across the continent in boreal forests. All data above apply to it.
The Prairie or Richardson’s Merlin, F. c. richardsoni Ridgway, 1871, breeds on the northern Great Plains and is strikingly paler than nominate columbarius. It is casual as far west as San Diego County, with two sight records (Sweetwater Reservoir, S12, 5 February 1977, J. L. Dunn; Tijuana River valley, V10, 5 November 1977, P. Unitt) and five specimens: San Diego 10 January 1900 (Huey 1926b, SDNHM 360), Witch Creek (J18) 9 February 1904 (FMNH 1904), “about the end of September” 1915 (Grey 1925, SDNHM, mounted), Mission Valley (R9/Q10) 9 April 1925 (Bishop 1905, FMNH 157064), and Eichenlaub Ranch near Barrett Lake (S18) 23 September 2001 (SDNHM 50625).
Perhaps as a result of general population increase, the Black Merlin of the Pacific Northwest, F. c. suckleyi Ridgway, 1873, has begun appearing in San Diego County. First was one at Lake Henshaw (G17) 6 October 1982 (R. E. Webster, AB 37:224, 1983); subsequent sightings have been at Whelan Lake (G6) 22 December 1987 (G. McCaskie, AB 42:321, 1988), Tijuana River valley (V11) 18 October 1991 and 13 December 1999 (G. McCaskie, AB 46:148, 1992, NAB 54:221, 2000), O’Neill Lake (E6) 20 February 2000 (L. J. Hargrove, P. Unitt), and probably in Moosa Canyon (F9) 3 December 1998 (“extremely dark,” E. C. Hall). There is also one specimen, from the North Park area of San Diego (R9) 9 March 1997 (SDNHM 49941).
Peregrine Falcon Falco peregrinus
The Peregrine Falcon was decimated in the 1950s and 1960s by DDT poisoning, which prevents birds from depositing adequate calcium in their eggshells. San Diego County’s small population of about 12 breeding pairs of Peregrines was extirpated. Then, with the ban on DDT in 1972 and an ambitious reintroduction program, the falcon recovered, enabling two of its three North American subspecies to be removed from the endangered-species list by 1999. By 2004, five pairs had recolonized San Diego County, nesting at both artificial and natural sites (Wildlife Research Institute 2004). Since the 1980s the number of winter visitors has increased as well, with about 15 to 35 being seen in the county annually by the beginning of the 21st century.
Breeding distribution: The reestablishment of the Peregrine Falcon in San Diego began with a captive-bred female, released at Point Loma (S7) in 1986, that mated with a wild male and began nesting annually at the San Diego end of the San Diego Bay bridge (S9) in 1989 (Pavelka 1990). She died at the age of at least 17 in 2000. Another pair nested on a crane along the waterfront of National City (T10) at least from 1995 to 1997 (A. Mercieca). Yet another pair colonized the cliff at the tip of Point Loma, nesting repeatedly, including at least 1997 and 1999; one of the birds from National City shifted to Point Loma in 1997 (A. Mercieca). A fourth pair nested in Ysidora Gorge along the lower Santa Margarita River (G5) in 2000, reoccupying a site used regularly before 1950 (S. Buck). In 2003, a pair nested on a ledge of the U. S. Grant hotel in downtown San Diego (S9; M. Sadowski).
Other nest sites are so far unconfirmed but possible. At Torrey Pines State Reserve (N7/O7), an apparent pair was calling to each other, one bird in display flight, 23 March 1998 (B. C. Moore), two were regular through the winter of 2000–01 (K. Estey), and one was harassing ravens repeatedly in February 2002 (S. E. Smith). In lower Tecolote Canyon (Q8) 16 April 1998 an apparent pair was in courtship flight, then flew off together toward Mission Bay (E. C. Hall). From 22 to 26 April 1998 an apparent pair along the Chula Vista bayfront (U10) was carrying Mourning Doves toward the salt works (B. C. Moore). An apparent pair was at the California Center for the Arts, Escondido (J12), 19 February 1999 but did not remain later in the breeding season (C. Rideout). Two birds were together around the courthouse in El Cajon (Q13) 5 December 1997–19 January 1998 (A. Mercieca), but only a single individual remained into the spring, staying at least until 30 March 2001 (K. Neal). A pair, including a bird escaped from a falconer, were back at this site in spring 2004 (D. Bittner).
Nesting: The Peregrine Falcon’s traditional nest site is on a cliff ledge, sometimes in an old nest of another bird of prey or a raven, more often in just a scrape in debris (White et al. 2002). With recolonization and adaptation to urban living, the falcons have adopted man-made structures like buildings, cranes, and bridges. The nest on the Coronado bridge in 1989 was on a ledge 12 to 14 inches wide, in a scrape in about 1 inch of pigeon feces and dust (Pavelka 1990). Sites recorded with collected eggs in San Diego County include cliffs of granite, clay, and eroded earth overlooking the ocean, as well as old nests of the Golden Eagle and Common Raven. A pair commonly rotates among nest sites on a single cliff, and some sites, including those in San Diego County, have been used for generations.
Dates of San Diego County egg sets, collected 1894–1950 or reported in the literature, range from 8 March to 25 May. The current population begins nesting somewhat earlier. In 1997 the clutch at Point Loma hatched around 3 April, in 1999 around 6 April (A. Mercieca), corresponding to egg laying in the last week of February. Similarly, Pavelka (1990) estimated the young on the Coronado bridge to be one and a half to two weeks old on 18 April 1989, corresponding to egg laying around 1 March. These young fledged from 14 to 20 May.
Migration: On the basis of recoveries of banded birds, Anderson et al. (1988) demonstrated substantial migration of Peregrines along the Pacific coast. They reported an immature banded as a nestling near Aristazabal Island, British Columbia, found stunned on a hotel balcony in San Diego seven months later. With the species’ recolonization, its migration schedule in San Diego County is less clear. Nevertheless, the Peregrine still occurs here mainly as a winter visitor, being most frequent from October to February. There are still few sightings away from known or possible nest sites during the breeding season from March through June.
Winter: Wintering Peregrines are most numerous along the coast, where prey like shorebirds and ducks concentrate. San Diego Bay serves as a nucleus for the wintering birds as well as the breeding population. San Diego Christmas bird counts have returned up to 11, on 16 December 2000; no other San Diego County count has had more than three. The species is usually seen singly; from 1997 to 2002 our maximum count per atlas square per day was two.
Away from the coast, the Peregrine Falcon is seen most often around lakes, such as Sweetwater Reservoir (Wildlife Research Institute 2004), but occasionally far from water. During the atlas period we did not encounter the species in the higher mountains, on the Campo Plateau of southeastern San Diego County, or anywhere in the Anza–Borrego Desert except the Borrego Valley.
Conservation: In the early 20th century the Peregrine nested regularly in San Onofre Canyon (C3), at Las Flores (E3), in Ysidora Gorge, near Pala (D10), in Bandy Canyon (K13), at Torrey Pines, and at Point Loma. John Oakley estimates about 12 historic Peregrine territories in San Diego County. No nesting was reported between 1950 and 1989, corresponding to the widespread use of DDT beginning in the 1940s (Fyfe et al. 1987). By 1960 the falcon was nearly if not completely extirpated as a breeding bird from Santa Barbara to San Diego, including the Channel Islands (Walton et al. 1988). From 1956 to 1980 the San Diego Christmas bird count averaged just 0.4 per year. In 10 years of monthly counts at San Elijo Lagoon 1973–83 King et al. (1987) saw only two. An increase in wintering Peregrines became noticeable in the mid 1980s, and by 1997–2002 the average on the San Diego Christmas bird count was 6.3. The Wildlife Research Institute (2004) estimated 20 wintering in the southwestern quadrant of San Diego County in 2002.
The effort to restore the Peregrine Falcon through reintroduction included San Diego County, with 12 birds released at Point Loma 1982–88 (Pavelka 1990). Only the one nesting on the bridge, however, originated from these releases. With most or all of the historic sites still intact, and many new possible sites on man-made structures, San Diego County’s Peregrine population could easily grow beyond its historic level. Though there is less habitat for shorebirds than formerly, the introduction of the Domestic Pigeon and the continued abundance of the Mourning Dove, staples of the falcon’s diet, assure its food supply. A return to a small, stable population is likely unless new environmental poisons or other challenges intervene.
Taxonomy: The resident subspecies of the Peregrine Falcon in California, and most migrants, are the widespread medium-dark F. p. anatum Bonaparte, 1838. The very dark and, in immatures, heavily streaked subspecies of the Pacific Northwest, F. p. pealei Ridgway, 1873, migrates in some numbers to northern California (Anderson et al. 1988, Earnheart-Gold and Pyle 2001). Two have been reported from San Diego County, a specimen from San Diego Bay 31 March 1908 (Swarth 1933, CAS 11694) and the bird banded in British Columbia reported by Anderson et al. (1988). The pale F. p. tundrius White, 1968, likely occurs rarely, though there is yet no specimen or conclusive report from San Diego County. It nests in the tundra and migrates far to the south. White (1968) reported one specimen from Los Angeles County, and Earnheart-Gold and Pyle (2001) reported 10 sightings from Southeast Farallon Island.
Prairie Falcon Falco mexicanus
The Prairie Falcon is one of San Diego County’s scarcest breeding birds, with a population of 20 to 30 pairs. The birds nest on ledges on cliffs or bluffs and forage in open desert or grassland. They are somewhat more numerous in winter, enough so to be considered merely uncommon at that season in San Diego County’s largest grassland, Warner Valley. In spite of nesting birds’ sensitivity to human disturbance the San Diego County population seems stable.
Breeding distribution: The Prairie Falcon has an inland distribution; all known or likely current nest sites are at least 23 miles from the coast. Five to ten pairs are in rugged areas of the coastal slope, down to an elevation of about 1000 feet. Six to ten pairs are on the steep east slope of the county’s mountains, and about seven pairs are in rocky hills or badlands within the Anza–Borrego Desert. Most nest sites are near grassland or desert plains where the birds forage, but some on cliffs on the coastal slope are surrounded by chaparral, sage scrub, and oak woodland for up to 2 miles in all directions. Clearly, the birds commonly range farther than this from their nests. In Idaho, Marzluff et al. (1997) found that Prairie Falcons commonly foraged over 4 miles from their nests and sometimes far as 24 miles. Such long commutes are likely in areas like the badlands of the Anza–Borrego Desert where the density of prey is low, especially in dry years.
Nesting: Prairie Falcons build no nest, typically laying their eggs directly on ledges, sometimes in caves. Both rocky cliffs and eroded earthen bluffs in desert badlands offer nest sites. Sometimes the birds reuse the stick nests of hawks or ravens; R. Thériault noted one such nest in the Anza–Borrego Desert 29 March 2001, and the only egg set collected in the county was from an old raven nest. Of seven nests checked by D. Bittner in the Anza–Borrego Desert in 2004, three were in rock cavities, two were in old raven nests, and two were in old eagle nests.
Data on the Prairie Falcon’s nesting schedule in San Diego County are still minimal. The one egg set was collected 4 April 1926 (WFVZ 63160). A fledgling in the Anza–Borrego Desert 24 April (R. Thériault) suggests egg laying as early to mid February, while large chicks still in a nest on the coastal slope 2 June (P. P. Beck) suggest egg laying as late as late April or early May.
Migration: The Prairie Falcon moves more in response to changes in the availability of prey than in a conventional migration. In summer, the falcons vacate deserts where rodents estivate (Steenhof 1998), so this pattern could be expected in the Anza–Borrego Desert. The state park database has only two records for August (two in Hawk Canyon, H27, 12 August 1991, J. Zemon; one at View of Badlands, N27, 16 August 1991, R. Thériault) and none for September. Along the coast, the Prairie Falcon occurs mainly from September to February, though it shows up exceptionally at other times (e.g., one in Cristianitos Canyon, B2, 17 May 1998, L. Allen).
Winter: In winter the Prairie Falcon is encountered more often than in the breeding season, especially in the Borrego Valley and in the larger grasslands of the coastal slope: Warner, Santa Ysabel, and Santa Maria valleys and Otay Mesa. From 1997 to 2002, however, we never noted more than two per atlas square per day. The maximum reported on any of San Diego County’s six Christmas bird counts was 10 on the Lake Henshaw count 19 December 1988; the Lake Henshaw count is the only one of the six in the county that has yielded more than five. Though more frequent along the coast in winter than in spring or summer, the Prairie Falcon tends to keep inland in winter as well; the Tijuana River valley is the only coastal location where the species occurs in winter with any regularity.
Conservation: The Prairie Falcon did not undergo the same DDT-induced population crash as some other birds of prey, and its numbers in California as a whole remained approximately stable through the 1970s (Boyce et al. 1986). The numbers we observed in San Diego County 1997–2001 are about the same as those reported from San Diego County in the 1970s as well (Unitt 1984). Evidently the breeding population here was always small. Records before 1997 list several sites where we did not see the falcon during the atlas period, but most of these are not far from current sites, which could be alternates. At least one former nest site on the fringe of the city of San Diego has apparently been abandoned, however: Fortuna Mountain (P11), active at least in 1980 (Calif. Dept. Fish and Game data).
The greatest threat to the Prairie Falcon currently is probably human disturbance near nest sites. Although in other parts of its range the falcon nests occasionally where there is some level of human activity, in San Diego County it may find it difficult to habituate to the intermittent disturbance of people hiking, driving vehicles, and especially climbing rocks near nest sites on weekends and holidays. Boyce et al. (1986) found that fledging success in areas of the Mojave Desert with heavy recreational use was less than in more secluded areas. Other threats are loss of grassland on the coastal slope to urbanization and a trend toward a drier climate, which could leave parts of the Anza–Borrego Desert with so little prey that the falcons can no longer nest successfully.