Breeding male Black-tailed Gnatcatchers with their black caps, gray upper-parts, white below and mostly black tails, are distinguished from the more widely distributed Blue-gray Gnat- catchers (P, caerulea) by their plumage as well as their preferred habitat, range and vocalizations. Non-breeding males, females and immatures, lacking the black cap, take more care in identification (Pyle 1997, Farquhar and Ritchie 2002).
Black-tailed Gnatcatchers, whose range extends from Mexico only into the warm, brushy arid regions of the southwest United States, have been studied most extensively in California where they were once considered conspecific with the California Gnatcatcher (P. californica), a Federally Threatened species. These studies have given us a much better understanding of these primarily Mexican species than we have of most other birds whose ranges just extend into the United States (Atwood and Bontrager 2001, Farquhar and Ritchie 2002).
Black-tailed Gnatcatcher is apparently a combination of two taxa, representing eastern (Texas and eastern Mexico) and western (Arizona, California, Nevada and Sonora, Mexico) populations. These two taxa, presently considered subspecies, differ in morphology, genetics and vocalizations (Pyle 1997, Farquhar and Ritchie 2002).
DISTRIBUTION. During the 1987-1992 field work season of the TBBA project, volunteers found Black-tailed Gnatcatchers breeding records mostly from the Big Bend area of the Trans-Pecos with a few reports elsewhere in that region and some in the Edwards Plateau and South Texas Brush Country regions (see the region map in Lockwood and Freeman ). Since the population is resident in Texas, possible and probable records may represent actual breeding as well as the confirmed records. The North American Breeding Bird Survey (BBS) relative abundance map for this gnatcatcher agrees with the TBBA map and shows route averages of 3-10 birds per year for the southern Trans-Pecos (Big Bend area) dropping rapidly to less than one moving down the Rio Grande River (Sauer et al. 2005).
From Texas the range of black-tails extends south in Mexico in the central highlands and on the Atlantic slope to Guanajuato. In the Sonoran Desert this gnatcatcher is resident from Sonora and northeast Baja California to the Mogollon Rim in Arizona and in eastern California, southern Nevada and southwest Utah (Howell and Webb 1995, Farquhar and Ritchie 2002, Sauer et al. 2005).
SEASONAL OCCURRENCE. Black-tailed Gnatcatchers are resident in Texas, breeding from late March to late August, based on egg dates from April 13 to August 13 (Oberholser 1974) and TBBA confirmed breeding evidence from March 30 to June 15 with half of the records found in May.
BREEDING HABITAT. Black-tailed Gnat- catchers breed in Texas to about 1700 m (5500 ft) elevation in mesquite-acacia thorn-brush near dry watercourses and sometimes even in creosote bushes (Oberholser 1974). In Mexico this species is resident in open and partly open arid and semi-arid areas with scattered bushes and brushy riparian woodlands (Howell and Webb 1995). In Arizona 78% of breeding records came from upland and Colorado River Sonoran Desert habitats and Sonoran riparian woodlands (Wise-Gervais 2005).
The nest is often placed in a well-shaded fork of several branches in a shrub or low tree, 0.6-1.2 m (2-4 ft) above ground, usually lower than the nests of Blue-gray Gnatcatcher. The compact, deep cup is built of fine materials such as leaves and plant fiber and down, wrapped on the outside with spider silk. The cup is not decorated with lichens as blue-gray nests often are. The outside diameter is 5-5.7 cm (2-2.3 in), the height is 3.2-6.4 cm (1.3-2.6 in), and both the inside diameter and cup depth are 2.5-3.8 cm (1-1.5 in; Harrison 1979, Farquhar and Ritchie 2002).
The usual clutch size is 4 (range 3-5) bluish or greenish-white eggs, evenly marked with fine reddish-brown spots or dots and indistinguishable from the eggs of Blue-gray Gnatcatcher. Both sexes incubate the eggs for 14-15 days and the young birds leave the nest 9-15 days after hatching and are fed by the parents for about 3 weeks more. Two broods per season are not unusual. Parasitism by Brown-headed Cowbirds (Molothrus ater) at rates as high as 100% has been reported (Harrison 1979, Farquhar and Ritchie 2002).
STATUS. Data points from the Coastal Sand Plain and extreme South Texas Brush Country regions of south Texas seen on Oberholser’s (1974) map are mostly absent on the TBBA map below. The situation is apparently complex for this species as Lockwood and Freeman (2004) report recent range expansion on the Edwards Plateau.
BBS data from 18 routes in Texas om which this gnatcatcher was detected suggest a yearly population change of about -3.5% for the period 1980-2005, but this trend is not statistically significant and is based on a relatively small data sample (Sauer et al. 2006). Continuing study of Black-tailed Gnatcatcher’s range and status in Texas is needed especially since so little information is available about its status in the major part of its range in Mexico.
Text by Robert C. Tweit (2006)
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Farquhar, C. C. and K. L. Ritchie. 2002. Black-tailed Gnatcatcher (Polioptila melanura). In The birds of North America, No. 690 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
Harrison, H. H. 1979. A field guide to western birds’ nests. Houghton Mifflin, Boston, MA.
Howell, S. N. G. and S. Webb. 1995. A guide to the birds of Mexico and northern Central America. Oxford University Press, New York.
Lockwood, M. W. and B. Freeman. 2004. The TOS handbook of Texas birds. Texas A&M University Press, College Station.
Oberholser, H. C. 1974. The bird life of Texas. University of Texas Press, Austin.
Pyle, P. 1997. Identification guide to North American birds, part 1. Slate Creek Press, Bolinas, CA.
Sauer, J. R., J. E. Hines, and J. Fallon. 2005. The North American Breeding Bird Survey, results and analysis 1966-2005. Version 6.2 2006. USGS Patuxent Wildlife Research Center, Laurel MD < http://www.mbr-pwrc.usgs.gov/bbs>
Wise-Gervais, C. 2005. Black-tailed Gnatcatcher (Polioptila melanura). In Arizona breeding bird atlas. pp. 424-425 (T. E. Corman and C. Wise-Gervais, eds.). University of New Mexico Press, Albuquerque.