Purple Martin Progne subis
A rare and declining summer visitor now restricted almost entirely to the mountains, in California the Purple Martin struggles for survival. Its preferred nest site is specialized: holes in prominent isolated dead trees. It is loosely colonial, several pairs sometimes nesting in a single cavity-ridden snag, but in the Southwest it has failed to make the shift to the multicompartment birdhouses that now are its mainstay in the eastern U.S. In our area the European Starling, a more aggressive secondary-cavity nester, has now taken over most of the martin’s nest sites. Even as a migrant the Purple Martin is now very rare here.
Breeding distribution: The Purple Martin now nests only on and around San Diego County’s higher mountain ranges, and the birds are few and scattered. On Palomar, they occur not only at the higher elevations but at a few places around the base as well: along Magee Rd. (C11; up to six, with one or two pairs nesting annually in holes in a power pole, J. M. and B. Hargrove), Cutca Trail in Long Canyon (C14/15; one on 16 May 1999; J. M. and B. Hargrove), and near Rincon Junction (F13; four, including at least one pair, 12 June 1999, E. Wallace). Purple Martins are also widely but thinly distributed through the Volcan, Cuyamaca, and Laguna mountains. In this area, they may nest downslope as far as Santa Ysabel (J18; five, including fledgling, 17 July 2001, J. R. Barth), possibly down into the gorge of the San Diego River above El Capitan Reservoir (L17, two on 14 June 1997; M17, one the same day, R. C. Sanger). The only site south of Interstate 8 is in Rancho Corte Madera near Long Valley Peak (Q21; four, including at least one occupied nest, 20 June 1998, M. U. Evans). Around Hot Springs Mountain, the county’s highest peak, the only record is of a single female circulating among snags at the Lost Valley Boy Scout camp (D20) 25 June 1998 (P. Unitt). At least in 1998, there must have been a colony near Lake Henshaw (G17); the site was not located, but up to 20 birds, including adults feeding fledglings, were there from 17 to 19 July (C. G. Edwards, K. L. Weaver).
Colony sites shift with the availability of suitable snags and, undoubtedly, the intensity of competition for them with other hole-nesting birds. Over the years, Cuyamaca Peak has been the most consistent site. From 1997 to 2001, numbers here ranged up to 20 on 18 April 1998 (N20; possibly including some migrants on this date, E. Siegel), 10 (including fledglings) 14 July 2001 (M20; J. R. Barth), and 8 on 14 June 2000 (M20; P. D. Jorgensen). The only other areas with likely more than two pairs were near Lake Henshaw, Pine Hills (K19; 18 on 26 June 2001, M. B. Stowe), Mt. Laguna (O23; up to 16 on 13 and 14 June 1999, C. G. Edwards), and Volcan Mt. (H20/I20; up to 8, including apparent juveniles, 16 July 2000, A. P. and T. E. Keenan). The total San Diego County population during the atlas period was probably about 100 pairs.
Nesting: The Purple Martin is a secondary-cavity nester, in southern California usually selecting tall isolated trees, dead or partly dead, for its nest hole. The trees are typically on the upper third of a slope, in open woodland of under 20% canopy cover (Williams 2002). The cavities may be natural, the result of decay following a fire or lightning strike, or excavated by woodpeckers. The only artificial sites reported by our atlas observers were those along Magee Rd. and in a telephone pole in Sherilton Valley (N19; G. and R. Wynn). Only once has use of a birdhouse been reported in San Diego County, by three or four pairs at Palomar Mt. in June 1985 (J. Robinson, AB 39:963, 1985).
Observations in the mountains of nest building (the birds carry material into their cavities) on 22 April and feeding young on 28 May suggest the martin may lay there as early as the first half of May. At low elevations nesting starts even earlier, though by 2000 there may have been only a single pair left to exemplify this. In 1905 and 1906, respectively, at San Onofre, Dixon (1906) found a nearly completed nest in a sycamore on 27 March and the birds selecting nest sites on 31 March. Our latest record of young in the nest, on 28 July, agrees with eggs laid in mid June; collected egg sets (WFVZ) are dated 3, 18, and 22 June.
Migration: Currently, the Purple Martin usually returns to southern California in April. From then through mid May migrants are rarely seen far from nesting sites. The earliest date recorded by our atlas observers, 22 March 1998 (one along Magee Rd., C11, J. M. and B. Hargrove), matched the earliest reported previously (Unitt 1984) and, notably, was from the lowest elevation where the birds are currently known to nest. The latest in spring the Purple Martin was noted away from nesting habitat from 1997 to 2002 was 25 May (2000, one in Oriflamme Canyon, L22, J. R. Barth), but past records of stragglers extend to 6 June. Locally breeding birds may remain to 15 August (1999, seven in Sherilton Valley, G. and R. Wynn). Fall migrants from the north pass through mainly in September, and there is still only one record for October.
Conservation: The Purple Martin has long been in decline in southern California (Garrett and Dunn 1981). Our atlas effort revealed several new sites for one or two pairs but no large colonies. The martin’s decrease on Palomar Mountain is particularly alarming. As recently as 1983 Roger Higson reported 45 pairs nesting there (AB 37:1028, 1983), with a colony in a snag just north of the observatory. That site is now deserted, and the largest one-day count on Palomar 1997–2001 was of a single family. In 2002, though, larger numbers returned to Lower French Valley (D15), where Paul Jorgensen saw 20 on 31 May, entering at least four cavities in three tall Jeffrey pines, and local resident Tony Jaramillo reported 60. The former low-elevation population in northwestern San Diego County, never large, known mainly from Dixon (1906) and Alice Fries’ observations from 1967 to 1978, was last reported in the latter year.
The central factor in the Purple Martin’s decline is generally acknowledged to be competition for nest sites with the European Starling. Williams (2002) reviewed the species’ California distribution and found that it survives only where starlings are rare or absent. Willett (1933) reported the birds’ beginning to nest in crevices in buildings in the Los Angeles basin, but this adaptation failed to reach San Diego before the starlings arrived in the early 1960s, and now has disappeared throughout southern California. Unfortunately, the martins’ predilection for nest trees with some open country surrounding them usually means the trees are in the middle of good foraging habitat for the ground-feeding starling. Any effort to encourage the martin to recover will have to entail aggressive starling control, accompanied by habitat enhancement that builds on what little remains. The species’ social system implies that its apparent coloniality is a by-product of the scarcity of suitable nest sites, not of innate gregariousness (Brown 1997). Therefore, the population could be rebuilt through the production of isolated pairs; the loss of larger colonies is not the irreversible calamity it would be with more social birds like the Tricolored Blackbird and Least Tern. The martins’ beginning to use nest boxes in the Pacific Northwest (Brown 1997) implies that enhancement in southern California could begin with the providing of nest boxes in or near existing nest trees, followed by monitoring and elimination of any starlings. Such boxes need not be the elaborate apartment houses now traditional in the eastern U.S., just a few simple wooden boxes installed in trees or snags that meet the martins’ criteria and as remote as possible from foraging starlings. Williams’ (2002) study in the Tehachapi Mountains suggests this distance should be at least 1 km from residential development, which favors starlings. Unfortunately, these criteria mean rugged terrain where the installation and monitoring of the boxes will be difficult.
Taxonomy: In their medium size and dusky-throated females, the Purple Martins of southern California are consistent enough with P. s. subis (Linnaeus, 1758) of the eastern U.S. to be called the same subspecies (Grinnell 1928a, Unitt 1984).
Tree Swallow Tachycineta bicolor
The Tree Swallow is primarily a migrant through San Diego County, seen by the hundreds especially in early spring. In winter it is our most common swallow but still localized and only irregularly numerous then. San Diego County lies at the southern tip of the Tree Swallow’s breeding range. Until 1980 it was rare here as a breeding species but since then it has increased, colonizing holes in snags around lakes and in riparian woodland.
Breeding distribution: As might be expected at the edge of a range, as a breeding bird in San Diego County the Tree Swallow is only locally common. It is most widespread in the lower valleys of the Santa Margarita and San Luis Rey rivers in northwestern San Diego County, more local farther south and east. Many nesting sites are at or near lakes, and most of the remainder are in riparian woodland. Breeding Tree Swallows are rare in the mountains, where we have only five records, none of more than two individuals. Nesting is confirmed south to the east end of Lower Otay Lake (U14), with a pair occupying a nest hole 20 May 2001 (O. Osborn), and likely in Marron Valley (V17), with two on 16 May and 12 June 2000 (P. Beck). Thus, even though the Tree Swallow is still unknown as a breeding species in Mexico, its spreading there now seems likely.
The species is not colonial but the population is nevertheless clumped. The largest concentrations of breeding birds reported 1997–2001 were at Barrett Reservoir (S19, up to 60 on 18 June 2000, R. and S. L. Breisch), Sutherland Reservoir (J16, up to 40 on 26 May and 22 June 1998, B. Travis), Lower Otay Lake (U14, up to 30 on 4 July 1999, S. Buchanan), Guajome Lake (G7, up to 30 on 11 May 1999, S. Grain), and Santee Lakes (P12, up to 28 on 5 May 1997, E. Post).
Nesting: Tree Swallows nest in tree cavities, usually old woodpecker holes. Drowned trees in reservoirs are especially attractive, though the birds nest also in snags at the edge of small openings in the middle of riparian woodland. At Whalen (G6) and Guajome (G7) lakes Tree Swallows used cavities in the flowering stalk of agaves (J. Smith, S. Grain). At Wynola (J19) one used a nest box (A. Mercieca), a common site elsewhere in the Tree Swallow’s range.
Because of the dearth of collected eggs of the Tree Swallow, our atlas data add considerably to knowledge of the species’ nesting in San Diego County. Tree Swallows claim nest holes as soon as they return in the spring, leading to reports of “occupied nests” as early as 14 February, before actual nesting begins. Fledglings as early as 29 April (2001, Barrett Lake, S18, R. and S. L. Breisch) imply that the adults may lay as early as late March, much earlier than the 18–28 May of three egg sets collected in the early 20th century. Similarly, fledglings along the lower San Luis Rey River (G5) 1 August 1999 (R. E. Fischer) must have come from eggs laid in late June and imply that Tree Swallows could easily raise two successive broods in San Diego County. Over most of the species’ range, second broods are rare (Robertson et al. 1992).
Migration: Spring return of the Tree Swallow is usually in the first week of February, occasionally the last week of January (one flying northwest at Ocotillo Wells, I28, 25 January 2001, J. R. Barth). Thus the Tree Swallow is one of our earliest spring migrants. The birds quickly become common, reaching peak abundance in March. By the end of April they are rare away from breeding locations, and two northeast of Borrego Springs (F25) 7 May 1998 (P. D. Ache) were the latest that were clearly migrants. A few birds in unsuitable habitat on the coastal slope through May and June may have been unable to find nest sites. One at Agua Caliente County Park (M26) 4 June 1998 (E. C. Hall) made the only such record for the Anza–Borrego Desert.
Fall migration begins by 13 July (1999, one in the Tijuana R. valley, W11, P. Unitt), possibly as early as 3 July (1999, 30 at Turner Lake—a nesting location?—G11, S. L. and S. J. Farrow), but is often not noticeable until August. In fall, migrating Tree Swallows are less numerous than in spring, and the main migration route at that season is apparently more across the Salton Sea than along the coast (cf. Patten et al. 2003).
Winter: The Tree is by far the most numerous swallow in southern California in winter, but its distribution then is still quite patchy. The birds occur usually around lakes, lagoons, and ponds at low elevations. In San Diego County, winter records are concentrated in the lower Santa Margarita and San Luis Rey valleys, along Escondido Creek from San Elijo Lagoon to Lake Hodges, Sweetwater Reservoir, at Santee Lakes, Lower Otay Lake, and the Tijuana River valley, especially in the first three areas: up to 100 at the Santa Margarita River mouth (G4) 22 January 1999 and 15 January 2001 (P. A. Ginsburg), 177 at Lake Hodges (K10) 23 December 2001 (R. L. Barber), 500 at Sweetwater Reservoir (S12/13) 22 December 1998 (P. Famolaro). Nevertheless, the Tree Swallow is irregular even at these spots of concentration. Above 1000 feet elevation, atlas observers encountered the species only twice, at Barrett Lake (S19; 50 on 26 December 1999, R. and S. L. Breisch) and at the upper end of Lake Morena (S22; five on 20 December 1998; R. and S. L. Breisch). Over 21 years of Lake Henshaw Christmas bird counts, the Tree Swallow was recorded only twice, with no more than three individuals. In the Borrego Valley the Tree Swallow is rare. Two of the four records between 1997 and 2002 were on 23 January so possibly of very early spring migrants; the others are of one on 19 December 1999 (E24, P. R. Pryde) and three on 16 January 2000 (G24, P. D. Jorgensen). There is only one earlier record of a single Tree Swallow on an Anza–Borrego Christmas bird count.
Conservation: As a breeding species, the Tree Swallow increased dramatically in San Diego County over the last two decades of the 20th century. Before 1980, it bred only “very rarely and sporadically” here (Unitt 1984). Because many of the new sites are around reservoirs, it seems reasonable to infer that the building of these reservoirs, which drowned trees, allowed the Tree Swallow to spread its breeding range south. Yet there must be other factors too—most of the reservoirs were there decades before the spread began.
Violet-green Swallow Tachycineta thalassina
The Violet-green is the characteristic, common swallow of San Diego County’s mountains, easily seen cruising over meadows and nesting in cavities in nearby oaks or conifers. It has converged on the White-throated Swift’s ecology by nesting also in rock crevices in cliffs. It is common as a spring migrant throughout the county, and as a fall migrant in the mountains. In winter the Violet-green Swallow is irregular, usually rare, and strangely localized in northern coastal San Diego County.
Breeding distribution: The Violet-green Swallow is common and widespread in woodland in the mountains. It has the honor of being the bird nesting at the highest altitude in San Diego County—on 8 June 2001 in a cavity in the fire-lookout tower atop the summit of Hot Springs Mountain, at 6533 feet the county’s highest peak (E20; K. L. Weaver). Numbers in the mountains range up to 200 in a day, as near Stonewall Mine (M20) 26 May 1998, when the birds were discouraged from foraging on a cold foggy morning (B. C. Moore). In the foothills, outside the zone of coniferous and oak woodlands, the Violet-green Swallow is uncommon and local. Our atlas effort revealed several scattered outlying populations toward the coast: San Onofre Mt. area (D2, E3, E4), San Marcos and Merriam Mts. (G8, G9), hills south of San Marcos (J8, I9, J9), Mission Gorge (P11, Q11), Otay Mt. (U15, V14, V15), and Tecate Mt. (V17, V18). On Otay and Tecate mountains numbers as large as 30 in a day have been reported (V15, 25 May 1999, D. Seals; V17, 28 May 2001, A. P. and T. E. Keenan). At the more northern outlying sites the count per day is seldom more than two, maximum only six. In the Anza–Borrego Desert, the Violet-green Swallow summers along the ridge of the Santa Rosa Mts. (C27, D27, D28), with up to 20 around benchmark Rosa (D28) 2 May 2000 (L. J. Hargrove). Though at 3000–4000 feet the elevation zone is occupied by Violet-green Swallows elsewhere, in southeastern San Diego County east of Cottonwood Valley (R22) and Lake Morena (T21) there is only one record in the breeding season, of a single individual at Tule Lake (T27) 3 July 2000 (J. K. Wilson).
Nesting: Abandoned woodpecker holes and natural cavities in trees are the Violet-green Swallow’s most common nest site, especially in semiopen woodland. Because of the difficulty of approaching them closely, nests in cliff crevices are probably far more common than the three our observers noted. Most of the nests at lower elevations are probably in cliffs, undoubtedly so in places like Otay and Tecate mountains that lack large trees and woodpeckers. Cliffs offer more sites in the Santa Rosa Mts. than do the small, sparse pinyons.
Atlas data suggest the Violet-green Swallow often lays its clutch earlier than the 28 May–22 June range of the seven collected egg sets. A pair evidently feeding young 12 April 1997 suggests eggs laid as early as late March, though this was at the comparatively low elevation of Fernbrook (M14; J. Savary). Many reports from higher elevations imply that laying is regular by mid to late April. Adults removing a fecal sac from a nest hole 29 July 2000 (O23; J. R. Barth) and a fledgling 5 August 2000 (C8; K. L. Weaver) imply laying as late as about 1 July.
Migration: In spring, Violet-green Swallows arrive in early February, always by 5–10 February, occasionally by 1 February (1998, two at Lake Morena, T21, R. and S. L. Breisch). Two in south Ramona (L14) 23 January 2000 (G. Moreland), farther inland than other winter records, may have been exceptionally early migrants. Spring migration peaks in March and continues to mid May, rarely as late as 24 May (1999, four near Yaqui Well, I24, P. Nelson). King et al. (1987) reported two early June sightings at San Elijo Lagoon.
In fall, migration begins by 19 July (1999, one at O’Neill Lake, E6, P. A. Ginsburg), peaks in August and September, and trails off through October to early November. Fall migration is mainly through the mountains and seldom conspicuous unless unseasonal stormy weather compels the birds to fly low.
Winter: At this season, the Violet-green Swallow is still regular only in the lower San Luis Rey River valley, around Whelan and Windmill Lakes (G6), as I reported in 1984. The maximum reported from 1997 through 2002, 20 on 20 January 2000 (J. Smith), is less than the high counts in some past years (maximum on Oceanside Christmas bird count 60 on 31 December 1977). A new wintering site, though, may be emerging at San Elijo Lagoon (L7), with records in three of the five years of the atlas period and up to 20 birds 28 December 2000 (A. Mauro). King et al. (1987) found no wintering Violet-greens at the lagoon from 1973 through 1983, but the species has been noted on 15 of 21 Rancho Santa Fe Christmas bird counts, 1980–2001. There were only single atlas records at other winter locations, but at Sweetwater Reservoir (S12) P. Famolaro noted 25 individuals 13 December 1999.
Conservation: The Violet-green is our only swallow not obviously affected by environmental change. Its core habitat of montane woodland is little disturbed. The birds’ use of nest boxes and exotic trees is known but still rare. Evidently the Violet-green can use cavities too small to accommodate starlings. The breeding distribution we recorded from 1997 through 2001 is considerably more extensive than that previously described, but the difference may be due only to more thorough exploration of regions once poorly known.
Taxonomy: Phillips (1986) found too much overlap in both size and color to uphold a distinction between T. t. thalassina (Swainson, 1827), of the mountains of mainland Mexico, and T. t. lepida Mearns, 1894 (type locality Laguna Mountains), from farther north. Eliminating lepida leaves the smaller Baja California form brachyptera Brewster, 1902, as the only other subspecies of the Violet-green Swallow.
Northern Rough-winged Swallow Stelgidopteryx serripennis
As the fan palm to the Hooded Oriole, so the box-frame bridge to the Northern Rough-winged Swallow—an accoutrement of southern California civilization opens a door of opportunity to a bird preadapted to capitalize on it. Armed only with the flexibility it shares with many species programmed to nest in cavities, the Rough-winged Swallow has maintained its status as a common summer resident. It is even more common in migration but rare in winter.
Breeding distribution: The Northern Rough-winged Swallow is widespread in San Diego County’s coastal lowland, becoming scarcer and patchier at higher elevations. It is absent as a breeding bird from Point Loma and Coronado. A few pairs nest as high in the mountains as about 4000 feet, as at Pine Hills (K19; nest building 5 May 2001, S. E. Smith) and near Green Valley Falls Campground (N20; nest building 23 May 1999, B. Siegel). In the Anza–Borrego Desert, the Rough-winged is widespread as a migrant but rare as a breeding species. Occasional pairs evidently nest in the eroded bluffs of the Borrego and Carrizo badlands: one pair at Font’s Point (F27) 27–28 May 2000 (G. Rebstock), another pair copulating in Arroyo Seco del Diablo (O28) 8 May 1999 (R. and S. L. Breisch). The species is regular in small numbers around Carrizo Marsh (O29), with up to 15 on 6 May 1998 (P. D. Jorgensen). Though the Rough-wing nests commonly in the Imperial Valley, and man-made structures are now its primary nest sites on the coastal slope, the only place in the Anza–Borrego Desert where the birds have been seen near buildings in the breeding season is around the Casa del Zorro (H25; two on 23 May 1998, H. L. Young).
The Rough-winged Swallow is not colonial, so large numbers are usually of migrants rather than of breeding birds. But if suitable nest sites are clumped—as with several drain holes under a single bridge—multiple pairs often nest in amity. From 1997 to 2001, the largest numbers were reported consistently from lower Los Peñasquitos Canyon (N8), up to 150 on 2 May 1999 (P. A. Ginsburg). Such large concentrations during the breeding season may be of birds unable to find suitable nest sites—the availability of sites may limit the population (DeJong 1996).
Nesting: Burrows in banks and bluffs were the Rough-winged Swallow’s primitive nest site. The literature is inconsistent on the question of whether the birds dig the burrows themselves, as does the Bank Swallow. None of our atlas observers reported the species excavating. Clearly, the birds spare themselves the work of digging whenever possible, preferring existing burrows or artificial structures that mimic them. Of the 32 nest sites our observers described, seven were in banks, bluffs, or cliffs, three were in road cuts, three were in buildings, one was in a pipe in a water tank, and 18 were in drain holes under bridges. I investigated one reachable nest hole in the Hollister St. bridge over the 1993 channel of the Tijuana River (W11) and found that the nest was right at the lip of the hole—therefore far closer to the entrance than the mean 82.4 cm Hill (1988) reported for 44 nests in pipes.
Our observations imply that egg laying by Rough-winged Swallows is concentrated from mid April to early June, encompassing the range 1–23 May of the eight collected egg sets. Reports of a nest with nestlings at Fallbrook (D8) 1 April 1998 (M. Freda) and fledglings along the Sweetwater River at Interstate 805 (T11) 18 April 1999 (W. E. Haas) suggest that a few birds lay as early as mid March. Rough-winged Swallows raise only one brood per year (DeJong 1996).
Migration: The Northern Rough-winged Swallow is one of southern California’s earliest spring migrants. Even those seen in late December and early January may be returnees, as records in late October and November are so few. From 1997 to 2002 the earliest clear spring migrants were 12 north of Lake Morena (S21) 23 January 1999 (S. E. Smith). Scattered individuals at low elevations in late January and early February could have been wintering. The birds arrive by mid February every year, and by the last week of February they are common. Records from the Anza–Borrego Desert imply that spring migration continues regularly through the first week of May, possibly as late as 15 May (1999, two northwest of Clark Dry Lake, D25, K. J. Winter). Like that of spring migrants, the return of fall migrants is also early, in early July. Three in Borrego Palm Canyon (F23) between 2 and 5 July 1998 (L. J. Hargrove) and one at Big Laguna Lake (O23) 7 July 2001 (J. R. Barth) were remote from nesting locations. Numbers of fall migrants build to August then decline through September, until by mid October the species is very rare.
Winter: In December and early January the Rough-winged Swallow is rare but annual around lagoons, lakes, and ponds in the coastal lowland, with one to four records each year 1997–2002. The highest winter counts during the atlas period were of three individuals (Lake Hodges, K10, 22 December 2000, R. L Barber; upper Santee Lakes, O12, 5 January 2001, I. S. Quon). The only earlier records of larger numbers are of nine near Oceanside 16 December 1983 (R. E. Webster, AB 38:358, 1984) and at least six at Santee Lakes in winter 1992–1993 (C. G. Edwards, AB 47:301, 1993).
Conservation: Much of the Northern Rough-winged Swallow’s original nesting habitat has been eliminated with the conversion of wild meandering streams, with their eroding banks, into channels lined with riprap and concrete. But the species’ adaptability to man-made nest sites has probably more than compensated. Quarries, road cuts, and borrow pits are substitute habitat. The birds are opportunistic, quickly investigating new excavations—as I saw out the windows of the San Diego Natural History Museum when the pit dug for the building’s expansion stood idle for weeks. Above all, the building of bridges composed of box-frame girders of concrete, with their drain holes beneath, has created thousands of new nest sites. Though European Starlings also use these holes, apparently they do not prefer them, perhaps because of their vertical orientation, and leave many to the more agile Rough-winged. In winter the Rough-winged appears to be increasing gradually: eight records before 1982 (Unitt 1984) versus 12 from 1997 to 2002.
Taxonomy: The paler, more southern subspecies S. s. psammochroa breeds locally, while the darker, more northern S. s. serripennis passes through in migration (specimen collected 3 miles west of Santee, P11, 1 May 1921, SDNHM 32101).
Bank Swallow Riparia riparia
The Bank Swallow has long been extirpated from the site of its single known colony in San Diego County, and now it is rare even as a migrant. Its specialized nesting habits confine it to vertical sandy riverbanks, cut by erosion, or, as in San Diego County, bluffs overlooking the beach. The birds dig their own burrows, forming colonies. Unfortunately, in its dependence on a naturally unstable and shifting habitat, the Bank Swallow is losing out to the work of flood-control engineers and society’s demand for a static, tamed environment.
Breeding distribution: Long ago, the Bank Swallow nested in coastal bluffs at Las Flores (E3), then the southernmost nesting colony known in North America. Nelson K. Carpenter collected 11 egg sets there on 13 May 1917, eight on 2 May 1919 (WFVZ). Another set taken “north of Oceanside” on 9 May 1925 (SBCM) is the last record of the colony.
Nesting: Carpenter (1918) described the colony at Las Flores as consisting of hundreds of tunnels (many abandoned) in a bed of sandstone sandwiched between layers of cobbles and clay. The bluff in which they were dug rose 25 to 100 feet above the narrow beach.
Migration: Even as a fall migrant the Bank Swallow is now very rare in San Diego County. In spring, when the species is still less frequent than in fall, we had only two records from 1997 through 2002, of one with other migrating swallows at a pond on Otay Mesa (V13) on 15 April 2000 (P. Unitt, S. D. Cameron), the other of one at sewage ponds near the Santa Margarita River mouth (G5) on 14 April 2002 (P. A. Ginsburg).
Winter: The Bank Swallow is casual at this season, with only three records, of one in the Otay River valley 21–22 December 1968 (AFN 23:522:1969), one at Old Mission Dam 26 January 1976 (AB 30:768, 1976), and up to four near Oceanside 27 December 1986–8 February 1987 (AB 41:331, 1987).
Conservation: With the wholesale channelizing of California’s rivers, eliminating most of the eroding banks the Bank Swallow needs for colony sites, the population collapsed and the range retracted north. Even colonies in unaltered habitat, like the coastal bluffs at Las Flores, were abandoned. The last reported nesting anywhere in southern California was in 1976 (Garrett and Dunn 1981). The decline continues with elimination of habitat for thousands along the Sacramento River (Garrison et al. 1987, Small 1994). The decline of the breeding population is reflected in the dwindling numbers seen of migrants, once “rather common” (Stephens 1919a). Like so many colonial birds, the Bank Swallow is more vulnerable than species that breed as dispersed pairs.
Taxonomy: The only subspecies of the Bank Swallow in North America, as across northern Eurasia, is R. r. riparia (Linnaeus, 1758).
Cliff Swallow Petrochelidon pyrrhonota
Bottles made of mud pellets and stuck to bridges and buildings, Cliff Swallow nests are perhaps more easily found and identified than those of any San Diego County bird. Living overwhelmingly around these man-made environments, usually in colonies, Cliff Swallows are familiar to San Diegans—sometimes too familiar, when the colonies are over doors, walkways, and patios. The species is still a common summer resident, locally abundant around some lakes and lagoons, but creeping signs of decline suggest that the Cliff Swallow’s bargain with humanity carries a price.
Breeding distribution: The Cliff Swallow is widespread though the coastal lowland, with colonies known from the great majority of atlas squares in the western half of the county. Along the coast, the only exception is Point Loma (S7), where the species is a rare migrant or nonbreeding visitor only. The largest colonies are on bridges over the lagoons, such as Buena Vista (H6, 1100 on 11 May 1999, M. Freda), Agua Hedionda (I6, 425 on 11 May 1999, W. E. Haas), and San Dieguito (400 on 31 May 1998, D. R. Grine). Farther inland, the reservoirs are home to substantial colonies, but the swallows also nest on buildings far from lakes, sometimes in large numbers. The species becomes more scattered at higher elevations and farther inland, but Cliff Swallow colonies trace the route of Interstate 8 as far east as Jacumba (U28). There are few colonies above 4000 feet elevation, but the highest site is at 6140 feet on the fire lookout tower atop High Point, Palomar Mt. (D15, up to nine on 1 July 1999, K. L. Weaver).
The Cliff Swallow’s breeding range barely extends onto the desert slope at Tule Lake (T27) and where Carrizo Creek crosses under Interstate 8 (T28). In the Anza–Borrego Desert, the species is only a migrant or nonbreeding visitor, with one interesting exception. On 25 June 1998, on a rock overhang near a tinaja where bighorn sheep come to drink in the north fork of Palm Wash (E29), Paul Jorgensen noted the remains of three old Cliff Swallow nests. The nests, in one of the most remote and arid parts of the Anza–Borrego Desert, could have been many years old—Mark Jorgensen had seen nests here in the 1970s. Though the Cliff Swallow began colonizing the Imperial Valley in 1977, and is now abundant there (Patten et al. 2003), it has not done so around Borrego Springs, even on buildings near golf-course ponds.
Nesting: The Cliff Swallow’s retort-shaped nest, built from pellets of mud, is unique in North America. Though cliffs were the species’ primitive nest site, the nests now are plastered typically under bridges, on concrete structures in reservoirs, and under the eaves of buildings—always where a vertical surface is sheltered from above. They often put the nests in corners and against neighboring nests, minimizing they amount of mud needed to enclose the nest. Our observers reported only a few colonies on natural cliffs or boulders, though another is on simulated rocks at the Wild Animal Park, San Pasqual (J12; K. L. Weaver).
Nest building begins usually in late March but began in mid March in the wet year 1998. The birds repair the nests whenever needed, accounting for nest building throughout the breeding season. Early in the season they may investigate and enter old nests, accounting for a report of an occupied nest as early as 17 February (P. A. Ginsburg). Checking old nests for parasites helps the swallows decide whether to build a new nest or refurbish an old one (Brown and Brown 1995). Atlas data imply that Cliff Swallows lay as early as the first week of April; otherwise they are consistent with the 29 April–3 July range of 51 egg sets collected 1896–1935.
Migration: In spring, the Cliff Swallow typically arrives from mid February to early March—well before the birds’ fabled return to San Juan Capistrano on St. Joseph’s day, 19 March. The earliest arrival recorded 1997–2002, at Fallbrook (D7) 7 February 2001 (K. L.Weaver), differs little from the earliest arrivals recorded in the past, in the first week of February (Unitt 1984). Most Cliff Swallows pass through in March and April, with only a few stragglers noted far from breeding colonies by the first week of May. Three in the northeastern Borrego Valley (E25) 13 May 2000 (P. D. Ache) were apparently our latest spring migrants. Five at Ocotillo Wells (I29) 27 May 2000 (R. Miller) and one in Bow Willow Wash (O28) 7 June 2001 (P. D. Jorgensen) may have been ranging from colonies in the Imperial Valley, while one in San Felipe Valley (I21) 16 June 2000 (J. O. Zimmer) may have been ranging from those in the Lake Henshaw region.
After nesting, Cliff Swallows desert their colonies in August, but fall migrants show up in large numbers in mid July, sometimes in early July (86 at Sentenac Ciénaga, J23, 7 July 1999, R. Thériault). Their numbers drop rapidly in early September, and records from late September through October are few.
Winter: Only three records ever: San Diego Christmas bird count, six on 23 December 1967 (AFN 22:394, 1968), Otay, one on 15 December 1973 (J. L. Dunn, AFN 28:536, 1974), and Oceanside, one on 23 December 1984 (G. McCaskie, AB 39:211, 1985).
Conservation: The Cliff Swallow presents a paradox: a species that relies for a nest site almost totally on man-made structures—yet shows signs of decline. At many colonies atlas observers reported that only a fraction of the nests were active, and other colonies were abandoned entirely. Results of the Breeding Bird Survey suggest a significant decline in southern California since 1982 (Brown and Brown 1995). At San Elijo Lagoon (L7), from 1973 to 1983, King et al. (1987) recorded an April–July average of 230 and a maximum of 620 on 3 June 1979. From 1997 to 2002, with 26 records, the maximum reported there was only 100, on 2 May 1999 (B. C. Moore). Similarly, at Batiquitos Lagoon (J7), Mona Baumgartel’s maximum monthly count decreased from 920 in 1994 to 32 in 2001.
The erection of thousands of bridges and millions of buildings has given Cliff Swallows countless new colony sites, and the importation of huge quantities of water has given them new sources of mud. But the paving over of so much of the coastal lowland, and the landscaping of so much of what has not been paved, must be more than compensating, by reducing the supply of both mud for nests and insects for food. Homeowners and building managers annoyed with the mess underneath a Cliff Swallow colony may knock or hose the nests down; atlas observers occasionally noted nests deliberately destroyed. A citation from the health department obliged the Cuyamaca school camp (N21) to destroy nests over the door to its cafeteria (P. D. Jorgensen). San Diego State University adopted a policy of “no swallows,” so the former large colonies there are now history (P. R. Pryde). One of the most frequent public inquiries I get is how to prevent Cliff Swallows from nesting on buildings. Few birds test the limit of human toleration of wildlife more than the Cliff Swallow, and the evidence is growing that intolerance is winning.
In the northeastern U.S., House Sparrows usurping nests are responsible for Cliff Swallows declining (Brown and Brown 1995), and this problem is likely a factor in San Diego too. The proliferation of ravens and crows may be a threat as well—observers at Barrett Reservoir (S19/T19) repeatedly reported ravens destroying Cliff Swallow nests and eating the young.
Taxonomy: The small subspecies P. p. tachina Oberholser, 1903, is the one nesting in San Diego County, as elsewhere in southern California. Though there are as yet no specimens, the larger, more northern nominate pyrrhonota may pass through in migration, as through the Salton Sink (Patten et al. 2003), or it may bypass San Diego County. At 104 mm, the wing chord of a juvenile fall migrant picked up near Lake Henshaw 21 September 2000 (W. E. Haas, SDNHM 50553) is equivocal (cf. Phillips 1986).
Barn Swallow Hirundo rustica
The Barn Swallow is one of the most widespread and familiar birds of North America, but San Diego County is peripheral to its breeding range. Here the species occurs mainly—though commonly—in migration. Along the Pacific coast, Los Coronados Islands off Tijuana are its southernmost nesting site. The small San Diego breeding population still clings to its ancestral nesting habitat in sea caves, though nesting on man-made structures, now the rule over most of the species’ range, may be on the increase. In winter the Barn Swallow is rare but increasing.
Breeding distribution: Sea caves at La Jolla (P7) and Point Loma (S7) were the Barn Swallow’s original breeding habitat in San Diego County and still consistent sites for small numbers, such as six at La Jolla 27 May and 27 June 1999 (L. Polinsky) and eight at Point Loma 15 June 1998 (V. P. Johnson). As long ago as 1912, though, when a pair nested on the Hotel del Coronado (T9; WFVZ), Barn Swallows began taking to man-made structures over or near the water. From then through 1980 there was little evidence of population increase, but this trend may now be accelerating. Between 1997 and 2002 we recorded the species as at least possibly breeding, outside its migration seasons, in over two thirds of the atlas squares along the coast. There is some spread a short distance inland from Oceanside. Farther inland, as throughout the 20th century, we noted breeding Barn Swallows only sparingly: near Pala (D11), two at an active nest 18 May 2000 (V. Dineen), Hidden Meadows (G10), three on 26 May 2000 (J. O. Zimmer), and Kearny Mesa (P9), three on 25 June 1997 (K. Kenwood). There are also a few scattered records of single individuals in midsummer, not coded for “suitable habitat,” at Oak Grove (D16) 13 June 2001 (K. L. Weaver), San Pasqual Valley (K12) 27 May 1998 (E. C. Hall), Sweetwater Reservoir (S13) 23 June 1997 (P. Famolaro), and, most surprisingly, Borrego Sink (G25) 4 June 1998 (R. Thériault), although the species has bred in the nearby Salton Sink.
At most of the known or presumed nesting sites, Barn Swallows are few, sometimes only an isolated pair. The largest numbers during the breeding season are around the Chula Vista Nature Center in the Sweetwater River estuary, with up to 20 between 10 and 14 June 1998 (B. C. Moore).
Nesting: The Barn Swallow’s nest is built of mud pellets, like the Cliff Swallow’s, but when finished is only an open-topped bowl. Thus the nest of the Barn Swallow closely resembles that of the Black Phoebe and is built in similar situations, on a solid support and sheltered from above. Outside the sea caves, our observers noted nests under the eaves of nearby buildings (along Coast Walk in La Jolla) and underneath boat docks at the Catamaran Hotel and Mission Bay Yacht Club (Q7) and the San Diego Yacht Club (S8).
Our rather meager records of Barn Swallow breeding from 1997 to 2001 imply egg laying from about 1 May through mid June and thus fit within the spread of the equally meager previous records (eggs as early as 24 March, nestlings as late as 6 August).
Migration: In spring, Barn Swallows arrive typically in early March, occasionally late February. Four near Hills of the Moon Wash (G27) 22 February 2001 (D. Seals) and two in Borrego Springs (G24) 24 February 1998 (P. D. Ache) were clearly spring migrants. Perhaps because it is less gregarious, the Barn Swallow appears less numerous as a spring migrant than some of the other swallows; the maximum reported 1997–2002 was 50 at O’Neill Lake (E6) 3 March 1999 (P. A. Ginsburg). The last of the spring migrants depart in the third week of May: 23 May 2000, three in the Tijuana R. valley (W10; W. E. Haas); 23 May 2001, four near Tecolote Canyon (Q9; T. Plunkett).
Fall migrants begin returning in early July, giving the Barn Swallow a migration schedule like that of shorebirds nesting in the Arctic. Early fall dates are 2 July 1997 (Poway, M12, P. von Hendy) and 3 July 2000 (west base of Otay Mt., V14, S. D. Cameron). Fall migrants increase through July and August, peak in September, and remain common until the end of October, later than the other swallows except the Tree.
Winter: At this season the Barn Swallow is rare but annual and increasing, as throughout the Southwest. From 1982 to 1992 the total number of Barn Swallows reported on San Diego County Christmas bird counts was 13, whereas from 1992 to 2002 it was 81. Most winter records are along and near the coast from O’Neill Lake (E6) in the lower Santa Margarita River valley south to the San Dieguito River estuary, Del Mar (M7), and the largest winter counts are in this region: 12 at Whelan Lake (G6) 22 December 2001 (D. K. Adams, G. L. Rogers, and J. L. Coatsworth), 8 at Buena Vista Lagoon (H6) the same day (J. C. Lovio), and 15 at San Elijo Lagoon 23 December 2001 (E. Garnica). Nevertheless, a few have shown up farther inland, as far as Ramona (K15, 1 on 2 January 2000, D. and C. Batzler), Lindo Lake (P14, 1 on 20 January 2002, M. Sadowski), and Lower Otay Lake (U13, 3 on 3 January 2001, V. Marquez). Most notable are the first winter records ever for the Anza–Borrego Desert, of two at Borrego Springs (G24) 25 December 2001 (P. D. Ache) and two at Carrizo Marsh (O29) 9 February 2001 (P. D. Jorgensen). The Barn Swallow has become a regular winter visitor in small numbers to the Salton Sink, though, just 25 to 30 miles farther east (Patten et al. 2003).
Conservation: With a trend of increase and adaptability to man-made structures as nest sites, the Barn Swallow appears unlikely to present a conservation problem. The natural nest sites are inaccessible. The species’ recent colonization of the drainage culverts in the city of Riverside (Lee 1995) suggests that further spread is in store for San Diego County. The Barn Swallow’s noncolonial habits suggest it is less likely to suffer to the same degree from the problems afflicting the Cliff Swallow. The Barn Swallow’s increase in winter could be a symptom of climatic warming, which is seen primarily in an increase in winter low temperatures. Nevertheless, trends can be reversed: the Barn Swallow’s colonization of the Imperial Valley in the 1970s, by a few pairs, proved ephemeral (Patten et al. 2003).
Taxonomy: Some geographic variation is expected in a species like the Barn Swallow with a nearly worldwide distribution. But the subspecies H. r. erythrogaster Boddaert, 1783, distinguished (in adult plumage) by its deep rufous underparts and virtual lack of a dark breast band, is the only one nesting in North America.