Blue-gray Gnatcatcher Polioptila caerulea
The Blue-gray Gnatcatcher uses a wide variety of habitats: oak woodland, riparian woodland and scrub, pinyon/juniper, desert washes, chaparral, and sage scrub. It is migratory, occurring mainly in the foothills in the breeding season and at low elevations in winter. In open groves of the Engelmann oak and stands of the Tecate cypress it can be fairly common, but over most of the county it is still uncommon or rare as a breeding bird. The Blue-gray Gnatcatcher is perhaps the greatest beneficiary of the cowbird trapping intended to benefit Bell’s Vireo; it is now far more numerous and widespread as a breeding species than known in the 1980s.
Breeding distribution: The Blue-gray Gnatcatcher’s current distribution appears strangely patchy. The species is most numerous between 1000 and 4000 feet elevation but far from uniform even there. Zones of concentration lie around the base of Palomar Mountain (maximum count 20 in Agua Tibia Wilderness, C13, 18 May 2001, K. J. Winter), north and northwest of Hot Springs Mountain (C20/D19/D21), from Lusardi Canyon to Black Canyon west of Mesa Grande (G15/I16), around Viejas and Poser mountains (O17/O18), south of Pine Valley (Q20/R20/R21), from Mother Grundy Peak to McAlmond Canyon north of Barrett Junction (T17/T18/T19), and on Otay Mountain (U15/U16/V15). The Blue-gray Gnatcatcher ranges uncommonly as high as the tops of the county’s highest peaks with four, including a pair building a nest, near the summit of Hot Springs Mountain (E20) 29 June 2001 (K. L. Weaver). One area of concentration, however, Miramar Air Station, especially east of Interstate 15 (N11/N12/O11/O12), lies largely at an elevation of 500–1000 feet (O11; 12 on 20 June 1999, G. L. Rogers). There are a few breeding-season records right along the coast (e.g., Torrey Pines State Reserve, N7, two on 11 July 1998, K. Estey; Tijuana River valley, W10, one on 13 and 23 May 2000, W. E. Haas), but breeding is confirmed no nearer the coast than about 5 miles in southwestern Miramar (P9; nest with eggs 8 May 1997, K. Kenwood).
In the Anza–Borrego Desert the Blue-gray Gnatcatcher occurs sparsely in the pinyons of the Santa Rosa and Vallecito mountains (up to five in the Santa Rosas near the Riverside County line 3 May 2000, P. Unitt). At lower elevations in the desert it is even scarcer, irregular at any given site, occurring mainly in junipers, at oases like Lower Willows (D23), or in well-vegetated washes (maximum six, one building a nest, in Mine Canyon, J24, 15 April 2001, L. and M. Polinsky). Occasional birds, singing territorially, are found in scattered clumps of mesquite even in the emptiest desert (e.g., two, one singing, 2.7 miles northeast of Ocotillo Wells, H29, 27 April 2001, J. R. Barth).
Nesting: The Blue-gray Gnatcatcher builds its nest in either trees or shrubs, high or low. Heights of nests atlas observers described ranged from 4 to 50 feet. The most frequently reported supporting plant was Engelmann oak with five nests; coast live oak was second with four nests. Other trees were sycamore, black oak, Coulter pine, pinyon, and Tecate cypress; shrubs were scrub oak, redshank, chamise, white sage, and laurel sumac. Blue-gray Gnatcatchers often build in quite exposed situations with little or no foliage screening the nest. The Engelmann oak’s open branches offer little concealment. Two nests were in dead oaks; one was in a leafless burned snag of a shrub. This failure to hide the nest may contribute to the Blue-gray Gnatcatcher’s being so frequent a host of the Brown-headed Cowbird.
In San Diego County the Blue-gray Gnatcatcher lays mainly from mid April to early July. Nest building as early as 24 March (1998, near Angelina Spring, I22, P. K. Nelson) suggests laying earlier, though females may delay laying five to ten days after a nest has been completed (Ellison 1992). Adults gathering insects in Dameron Valley (C16) 4 April 1998 and near Wilderness Gardens (D12) 15 April 2000 (K. L. Weaver), however, confirm laying by about 22 March and 2 April, respectively, earlier than historically known for California. Bent’s (1949) earliest egg date for the state was 5 April.
Migration: Determining the Blue-gray Gnatcatcher’s migration schedule is difficult to do with precision because the species is so widespread in both summer and winter. On the basis of sightings in breeding habitat in winter, probably some fraction of the breeding population is nonmigratory. Most of the breeding birds, however, are migratory, beginning to arrive in mid March. Root (1969) observed arrival in Monterey County one year as early as 24 February. The species’ movement appears to peak in the first week of April, and spring migrants are largely gone by the middle of April. Exceptionally late records of apparent migrants were of one in the Borrego Valley (F25) 25 April 1999 (P. D. Ache) and one at Point Loma (S7) 16 May 2001 (J. L. Coatsworth). Fall migrants begin returning regularly in the first week of September, rarely in late August (one at Point Loma 26 August 1999, D. K. Adams).
Winter: At this season the Blue-gray Gnatcatcher occurs most commonly in riparian scrub, especially in the Tijuana River valley, where counts are as high as 20 near the Dairy Mart pond (V11) 16 December 2000 (G. McCaskie). Elsewhere in the coastal lowland the species is generally uncommon. The Blue-gray Gnatcatcher also winters widely if uncommonly in washes and on valley floors in the Anza–Borrego Desert; maximum counts there are of seven in the Borrego Valley’s mesquite bosque (G25) 20 December 1998 (P. Unitt), near Barrel Spring (H29) 15 January 2002 (J. R. Barth), and in Box Canyon (L23) 10 January 1998 (S. D. Cameron). The Blue-gray Gnatcatcher even occurs rarely in winter in the same oak-wooded and chaparral-covered foothills where it breeds most commonly. Maximum counts in this habitat are of three along Bear Valley Road (Q21) 20 February 1999 and along the Espinosa Trail (R19) 18 February 2001 (A. P. and T. E. Keenan). Such winter records range in elevation as high as 4600 feet near Lost Valley (D20; two on 12 December 1998, J. M. and B. Hargrove).
Conservation: Willett (1912) and Stephens (1919a) called the Blue-gray Gnatcatcher “common” as a breeding bird; Sharp (1907) called it “not uncommon.” The 37 egg sets collected in San Diego County 1890–1938 attest to the species’ abundance in the early 20th century as well. By the 1970s, however, breeding Blue-gray Gnatcatchers had become rare, retracting out of the coastal lowland entirely (Unitt 1984). The decline coincided with the Brown-headed Cowbird’s invasion of southern California. The Blue-gray Gnatcatcher is a frequent host (Friedmann et al. 1977). In the late 1980s and 1990s, however, the gnatcatchers resurged, and the coincidence of this recovery with the beginning of cowbird trapping suggests that the gnatcatcher benefited, perhaps even more so than Bell’s Vireo, whose sixfold increase over the same period is far better documented (Kus 2002). Yet this simple hypothesis raises further questions: the gnatcatcher’s repopulation of lowland riparian woodland, where cowbird trapping has been most intensive, is quite modest. The increase is far more noticeable in chaparral and oak woodland away from the cowbird traps. Is the difference due to the gnatcatcher’s habitat preferences? Has the reduction of cowbird numbers been widespread enough over San Diego County to have effects several miles from the traps? Data on both the Blue-gray Gnatcatcher’s biology and trends in cowbird numbers are insufficient to answer these questions.
Wintering of the Blue-gray Gnatcatcher in San Diego County at elevations above 1500 feet was unknown before the 1980s. Might climatic warming, reflected mainly in an increase of winter low temperatures, be allowing the species to winter at higher elevations and obviating the need for some individuals to migrate? If some birds adopt sedentary habits, they may account for nesting earlier than historically known and help the species avoid cowbird parasitism. On the other hand, if more extended droughts accompany climatic warming, all insectivorous birds will suffer from a reduction in their food supply.
Taxonomy: Phillips (1991) reported too much overlap between P. c. amoenissima Grinnell, 1926 (widespread in the western United States), and P. c. obscura Ridgway, 1883 (of southern Baja California), for the former to be recognized. Tail length is reported to be the primary defining character, but an adequate quantitative comparison remains to be done.
California Gnatcatcher Polioptila californica
Patrick J. Mock
Within the United States, the California Gnatcatcher lives only in coastal southern California’s sage scrub, a habitat threatened by the continuing spread of agriculture and suburbia. Even within the sage scrub the gnatcatcher is localized, preferring patches dominated by California sagebrush and flat-top buckwheat and avoiding those dominated by sage, laurel sumac, and lemonadeberry (Weaver 1998a). The east edge of its range appears constrained by winter cold rather than by vegetation type (Mock 1998). As an “umbrella” or flagship species for its habitat, the California Gnatcatcher has been the focus for regional habitat-conservation planning in San Diego County since before it was listed as a threatened species by the federal government in 1993 (Atwood 1993).
Breeding distribution: In spite of its habitat being much constricted by urbanization, the California Gnatcatcher still occurs widely in San Diego County’s coastal lowland. It prefers open sage scrub with California sagebrush as the dominant or co-dominant plant (habitat use summarized by Atwood and Bontrager 2001). In general it is more numerous near the sage scrub–grassland interface than where sage scrub grades into chaparral; it occupies dense sage scrub less frequently than more open sites. In the more open chaparral on the flat mesa of Miramar, however, many territories encompass both sage scrub and chamise (K. Fischer). Much of the gnatcatcher’s range in the United States has been surveyed according to a standard protocol approved by the U.S. Fish and Wildlife Service. Elevation appears to limit the distribution in San Diego County, where over 90% of locations are below 1000 feet (Atwood and Bolsinger 1992, Mock 1993, 1998). The only sites above 2000 feet are on the east side of the San Diego River near Cedar and Boulder creeks, where the species has occurred up to 2400 feet. “Core” population areas supporting 30 or more pairs include Camp Pendleton/Fallbrook, Oceanside, north Carlsbad, southeast Carlsbad, southwest San Marcos, Rainbow/Pala, Olivenhain/Lake Hodges/San Pasqual, Poway, upper San Diego River/El Capitan Reservoir, Mission Trails Regional Park/Miramar, Lakeside/Dehesa, Sweetwater River/Reservoir, Jamul Mountains, Otay Lakes/Mesa, west Otay Mountain, and Tijuana River mouth (Mock 1993). Probably the single largest population concentration is around Lake Hodges (K10): the highest daily count reported by an atlas observer was 36 there on 21 March 1998 (R. L. Barber). The greatest number of gnatcatcher locations in the database for the Multiple Species Conservation Plan, based largely on surveys 1987–95, is from the same area.
San Diego County’s California Gnatcatcher population exceeds 2000 pairs, but fires in 1996 and 2003 temporarily reduced the carrying capacity of several of the habitat cores: Lake Hodges/Olivenhain, Mission Trails/Miramar, Jamul Mountains, and west Otay Mountain (Mock 1993, USFWS 1996, Bond and Bradley 2004). The size of a breeding pair’s territory is highly variable but correlated with distance from the coast, ranging from less than 1 hectare along the coast to over 9 hectares farther inland (Mock and Bolger 1992, Braden 1992, Preston et al. 1998, Atwood et al. 1998). During the nonbreeding season, a pair’s home range is about 80% larger than during the breeding season (Preston et al. 1998, Bontrager 1991).
The easternmost locations for the gnatcatcher in the breeding season are the base of Nate Harrison Grade, Pauma Valley (E13; one on 16 May 1999, C. Sankpill), 0.5 mile east of Saddleback along Cedar Creek (L18; male 28 August 2001, pair building a nest 20 March 2002, J. Turnbull), the upper end of Loveland Reservoir (Q17; pair and four nestlings banded 26 May 1997, P. Famolaro), and Grapevine Creek (U19; one on 13 June 1999, M. and B. McIntosh). The species is irregular at these marginal locations.
Completely unexpected was the sighting of a male California Gnatcatcher apparently paired with a female Blue-gray on Jacumba Peak (U28) 23 April 2000 (C. Jones, J. Radtke). The photos taken are not adequate to distinguish the bird from a Black-tailed Gnatcatcher, but the observers were experienced with the species and heard the typical mewing calls. This record offers the only recent parallel to old specimens of the California Gnatcatcher from the desert slope at Palm Springs in Riverside County (Atwood 1988) and “San Felipe Canyon” (probably San Felipe Valley, I21) in San Diego County (13 February 1893, SDNHM 1678).
The map for the California Gnatcatcher expresses the species’ abundance in terms of number of points by square in the MSCP and Camp Pendleton databases. These sources include many records before 1997, many from sites where the species has been eliminated subsequently. Conversely, a single pair or territory may be represented by more than one point. Therefore, as with most other species, this feature of the map should be taken as an indication of only relative, not absolute, abundance.
Nesting: The California Gnatcatcher’s biology has been studied intensively; see the many papers in Western Birds volume 29, issue 4 (1998). The birds typically nest on slopes of a gradient less than 40%. The lower portions of gullies and drainages, when available within the territory, are frequently used as nest sites. Though nest success varies significantly by host shrub species, the birds are not selective, using shrubs in proportion to their availability, typically California sagebrush, flat-top buckwheat, California sunflower, and broom baccharis (Mock and Bolger 1992, Grishaver et al. 1998). Many other less common sage scrub species are used less frequently. Grishaver et al. (1998) found the nest’s average height from the ground to be 82 cm (range 30–292, n = 101), in shrubs of average height 135 cm (range 62–155, n = 103). The shrub cover around the nest is typically between 20% and 60%, with a gap between shrubs of 153 to 176 cm (Bontrager 1991, Mock and Bolger 1992, Grishaver et al. 1998). Site selection may influence risk of nest predation; nests within 70 cm of the ground are less successful than those placed higher (Sockman 1997).
Mid March to early July is the main season for California Gnatcatchers to lay in San Diego County. Frequent nest predation results in many replacement clutches during the nesting season. Roach (1989) found the California Gnatcatcher nesting as early as late February in San Diego County; Patten and Campbell (1994) reported two nests fledging young in Orange County as late as 12 and 25 August in 1991, implying laying as late as about 16 and 29 July. Patten and Campbell (1998) suggested that cowbird parasitism is responsible for a historical trend toward California Gnatcatchers nesting earlier in the year. Warming of the climate could contribute to this trend as well.
Migration: The California Gnatcatcher is nonmigratory. During postbreeding dispersal, in late summer and fall, juveniles typically move less than 3 km; their longest documented dispersal distance is 20 km (Hunsaker et al. 2000). Dispersing young cross riparian woodland, chaparral, and artificial landscapes, including major highways and residential development (Lovio 1996, Bailey and Mock 1998, Campbell et al. 1998, Galvin 1998, Haas and Campbell 2003). Nonbreeding California Gnatcatchers have been detected three times on Point Loma (Bailey and Mock 1998). The many examples of occupied habitat patches isolated by extensive development also attest to such movement. First-year birds establish territories by October and remain on them through the winter (Mock and Bolger 1992, Preston et al. 1998). Extensive movements by adults are relatively rare (Bailey and Mock 1998); the longest documented dispersal distance by an adult is 10 km (Hunsaker et al. 2000).
Winter: The gnatcatcher’s winter numbers and distribution differ little from those in the breeding season (maximum daily count 53 around the upper end of Lake Hodges, K11, 26 December 1999, E. C. Hall). The species is probably resident in low density in all atlas squares where we noted it in winter but not the breeding season, except for V10 on the floor of the Tijuana River valley (two on 16 December 2000, W. E. Haas). An exceptional winter sighting about 21 km from the nearest site where the species breeds was of one at the northeastern corner of Lake Morena (S22) 5 December 1999 (R. and S. L. Breisch).
Conservation: Historical data on the California Gnatcatcher’s abundance are minimal; its listing as a threatened species is based on the high fraction of its habitat already lost to agriculture and urbanization and the pressure to develop what remains. The primary strategy for its conservation is the establishment of a network of habitat reserves, encompassing enough of the remaining “core” regions to sustain a viable population, connected by habitat linkages. The strategy is being pursued through the state of California’s Natural Communities Conservation Plans, entailing negotiation among many public agencies, landowners, and environmental organizations. Land for the reserves is being acquired through mitigation agreements for developments and public agencies’ purchase of privately owned lands under a “willing seller only” policy. Although the design of the network is incomplete, as of early 2004 over 65% of the 172,000-acre reserve network in the area covered by southern San Diego County’s Multiple Species Conservation Plan (MSCP) was in place. In the incorporated cities of the north county a 19,900-acre network has been proposed, and the participating cities need to develop their detailed plans for approval by the wildlife agencies. As of early 2004, the county of San Diego had initiated an amendment to the MSCP to cover the unincorporated areas of the north county not included in the original plan but the area to be conserved in this region had not been formally proposed. San Diego County’s major military installations (Camp Pendleton, Fallbrook Naval Weapons Station, Marine Corps Air Station Miramar) support important gnatcatcher populations and are mandated under the Sikes Act and Endangered Species Act to manage for this and other formally listed endangered species through “integrated natural resource management plans” that are updated every five years.
Though the California Gnatcatcher is eliminated by development of its habitat, it does not appear especially sensitive to fragmentation of that habitat at the landscape scale (Bailey and Mock 1998), in spite of the report to the contrary by Crooks et al. (2001). Data supporting this conclusion include the species’ persistence in patches of sage scrub long isolated from extensive stands, as in Florida Canyon, Balboa Park (R9; pair with two fledglings 1 June 1998, J. K. Wilson) and Chollas Valley near Fairmount Avenue (S10; seven, including three pairs, 11 May 1997, P. Unitt).
Fire and the invasion of exotic vegetation, especially grasses and annual forbs, interact to threaten the gnatcatcher’s habitat. In much of coastal southern California, where these exotic plants are well-established and where the irreversible conversion of shrublands to grasslands is likely, fire frequency and burn size should be kept low (Zedler et al. 1983). Where possible, flammable exotics should be removed or reduced. The wildfires of October 2003 affected 4% of known gnatcatcher occurrences, 16% of designated critical habitat, and 28% of the area the U.S. Fish and Wildlife Service’s model for suitable habitat (Bond and Bradley 2004).
Disturbances that reduce sage scrub cover, such as frequent fire, mechanical disruption, livestock grazing, off-highway vehicles, and military training appear to reduce habitat suitability for the gnatcatcher (Bontrager et al. 1995, Mayer and Wirtz 1995, Beyers and Wirtz 1997, Wirtz et al. 1997, Atwood et al. 1998). Construction-monitoring studies suggest that California Gnatcatchers tolerate adjacent construction (Atwood and Bontrager 2001, URS Corp. 2004) and high noise levels (Famolaro and Newman 1998). Over 16% of the point locations recorded for the gnatcatcher in San Diego County are within 500 feet of major roads.
Predation is the most common cause of gnatcatcher nest failure. Information on whether predation rate is influenced by anthropogenic factors is lacking, but the species’ nest success along habitat edges is no less than that within the interior of habitat blocks (Mock and Preston 1995, Atwood et al. 1998). Depending on the adjacent habitat’s suitability to cowbirds, cowbird parasitism affects some populations of the California Gnatcatcher more than others (Braden et al. 1997, Grishaver et al. 1998, Atwood and Bontrager 2001). But the net demographic effect of nest parasitism may be small, parasitism just substituting for other forms of predation on gnatcatcher nests (Braden et al. 1997). The low rate of parasitism (3 of 134 nests) observed around Rancho San Diego 1989–92 suggests that the gnatcatchers benefited from cowbird trapping along the nearby Sweetwater River (Grishaver et al. 1998).
Taxonomy: The subspecies of California Gnatcatcher in the United States is nominate P. c. californica Brewster, 1881, the dark extreme of the species. It ranges south along the coast to Ensenada, south of which it is replaced by P. c. atwoodi, in which the back and flanks of the female are paler brown (Mellink and Rea 1994), and by still paler subspecies from El Rosario south to Cabo San Lucas. Analysis of mitochondrial DNA shows little geographic structure of genetic variation in the California Gnatcatcher throughout its range (Zink et al. 2000), suggesting that the subspecies’ characteristics are being maintained by natural selection for plumage suitable to their local habitat rather than by restricted gene flow.
Black-tailed Gnatcatcher Polioptila melanura
The Black-tailed Gnatcatcher is one of the Anza–Borrego Desert’s characteristic birds, fairly common within the desert, essentially absent elsewhere. Like the better-studied California Gnatcatcher, the Black-tailed apparently remains in pairs year round. It is a permanent resident among spiny trees and shrubs like mesquite, smoketree, and palo verde, the preferred sites for nests. We found no current overlap between the California and Black-tailed Gnatcatchers in San Diego County, though there is one such site just north of the county line near Aguanga, and strays of both species have been seen near the Mexican border in the gap between their ranges.
Breeding distribution: The Black-tailed Gnatcatcher occurs almost throughout the Anza–Borrego Desert, being absent only from the higher mountains, stands of low halophytic scrub, and the most barren badlands. Within the desert it was missed during the breeding season in only five atlas squares: in the Santa Rosa Mountains (C27), from Peg Leg Road to Font’s Point (F26/F27), and around Ocotillo Wells (I28/I29). Even in the latter area it should be expected occasionally in the scattered ironwood trees. The birds are most concentrated in mesquite thickets and in well-vegetated washes and bajadas. The largest numbers were reported from the west end of Clark Dry Lake northwest along Rockhouse Truck Trail (D25; 40 on 14 April 2000, K. J. Winter) and from Mescal Bajada (J25; 30 on 26 April and 12 June 1998, M. and B. McIntosh). As spiny vegetation becomes sparser, so do the gnatcatchers. The species is confirmed breeding up to an elevation of about 3100 feet in Smuggler Canyon (L25, building nest 21 March 2000, R. Thériault) and recorded in the breeding season up to about 3400 feet at the western edge of Culp Valley (G22; one on 28 May 2001, P. D. Jorgensen). Points on the west edge of the range are Alder Canyon (C21; one on 3 May 2000, G. Rebstock), about 3200 feet elevation on the northeast side of San Felipe Valley (H21; seven on 22 May 2001, E. C. Hall, J. O. Zimmer), the northwest corner of Mason Valley (L22; two on 2 May 2001, R. Thériault), and near Jacumba (U28; up to four on 20 March 1998, C. G. Edwards).
The Black-tailed Gnatcatcher occurs on the coastal slope within 2 miles of the San Diego County line northeast of Aguanga in south-central Riverside County (Weaver 1998b). But, unlike the Ladder-backed Woodpecker and Black-throated Sparrow, it does not extend into the semidesert scrub of nearby Dameron Valley (C15/C16). Two completely unexpected sightings, however, suggest that the Black-tailed Gnatcatcher may be a rare resident in the arid chaparral about 3800 feet elevation along Lost Valley (Indian Flats) Road north of Warner Springs. The first report was of a pair building a nest 28 April 1998 (E19; W. Pray, O. Carter). Then, on 6 June 2001, a short distance farther north in square D19, southeast of Pine Mountain, a pair appeared agitated, as if disturbed near a nest (K. J. Burns, E. Sgariglia). These observations are unprecedented, and the area was surveyed repeatedly on other occasions with no other observations. Yet rare occurrences in this area could connect the population seemingly isolated near Aguanga with the species’ main range.
Another sighting on the coastal slope out of the known range was of two males, still with black caps and making short buzzy calls typical of the Black-tailed and dissimilar from the California, about 3200 feet elevation near Morena Village (T22) 1 July 2000 (R. and S. L. Breisch). Might the Black-tailed Gnatcatcher, like the Verdin whose habitat it shares, extend locally onto the coastal slope in northern Baja California, providing a source of dispersers across the border? (See also under Winter.)
Nesting: The Black-tailed Gnatcatcher has a strong preference for nesting in spiny shrubs or trees. Of the 17 nests whose placement atlas observers described, four were in smoketrees, four in palo verde, three in mesquite (two of these in or under clumps of mistletoe), two in desert thorn, two in desert lavender, one in desert apricot, and only one in a nonspiny plant, a California juniper. The heights of the nests ranged from 3 to 12 feet but were mainly around 5 feet.
The Black-tailed Gnatcatcher begins nesting in March. Our earliest record of nest building was 6 March 1997 at Yaqui Well (I24; P. K. Nelson), of an occupied nest (incubation probably begun) was 17 March 2000 at the west end of Clark Dry Lake (D25; K. L. Weaver), and of a nest with eggs 19 March 1998 along Borrego Sink Wash (G26; P. D. Jorgensen). If the Black-tailed Gnatcatcher’s incubation and nesting periods are the same average 14 and 13 days, respectively, as the California Gnatcatcher’s (Grishaver et al. 1998), fledglings near Halfhill Dry Lake (J29) 10 April 1998 (L. J. Hargrove) hatched from eggs laid by 14 March. The close of the nesting season is more difficult to gauge and probably varies from year to year, variation in rainfall governing the abundance of insects and whether the birds can raise more than one brood. Our later observations of nesting activity, however, were not disproportionately concentrated in the wet year of 1998. Birds building nests in lower Carrizo Valley (O28) 31 May and 1 June 2001 (P. Famolaro, P. D. Jorgensen) suggest laying in early June.
Winter: The Black-tailed Gnatcatcher’s dispersal in winter out of its breeding range is almost nil. In the Anza–Borrego Desert, we noted the species in seven squares with marginal habitat where we did not find it in the breeding season, though it could be a very sparse or irregular resident even in those. Winter records extend west to the edge of the range of the honey mesquite at Banner Queen Trading Post (K21), a well-covered site where the only sighting was of one on 2 December 1999 (P. K. Nelson). Two reports from chaparral in Potrero County Park (U20), of one on 25 January 1998 and two on 29 December 1999 (R. and S. L. Breisch), however, lie 23 miles west of the Black-tailed Gnatcatcher’s nearest known locality of residence at Jacumba. The birds’ calls were heard and the undersides of their tails were seen well. Like the July record from Morena Village, these observations could reflect an unknown extension of the range in Mexico.
Numbers of the Black-tailed Gnatcatcher varied less over the five years of the project than those of many other desert birds, but even this species was not immune to the cycle of wet and dry years. The count per hour in the drought-plagued final two winters of the project was 69% of that in the first three.
Conservation: The Black-tailed Gnatcatcher has always been considered common in the Anza–Borrego Desert, and most of its habitat is conserved in the state park. In spite of its nesting in a habitat unsuitable for cowbird foraging, the Black-tailed Gnatcatcher is subject to cowbird parasitism; we noted two instances of gnatcatchers feeding fledgling cowbirds as well as two instances of pairs mobbing cowbirds. Also, overpumping of groundwater could kill mesquites, eliminating one of the Black-tailed Gnatcatcher’s primary habitats.
Taxonomy: All Black-tailed Gnatcatchers in California are P. m. lucida van Rossem, 1931. The Black-tailed Gnatcatcher has had a tortured taxonomic history. Aside from being lumped with the California Gnatcatcher as a single species from 1926 to 1988, the Black-tailed was long known as the Plumbeous Gnatcatcher, Polioptila plumbea (Baird, 1854). When it was realized that the name plumbea had been applied earlier to the species now called the Tropical Gnatcatcher, the next oldest name, Polioptila melanura Lawrence, 1857, had to be substituted, and the common name Black-tailed Gnatcatcher, already in use for what we now call the California Gnatcatcher, was applied to the entire complex. When P. melanura and P. californica were shown to be distinct species (Atwood 1988), the English name Black-tailed went with the former, in agreement with the scientific name, which means black-tailed in Greek. This agreement between the scientific and common names, however, reverses traditional usage and contradicts the birds’ actual characters—the Black-tailed Gnatcatcher has less black in the tail than the California.