A taunting “ zhee-zhee-zhee ” by the male first betrays the presence of this species; then, once spotted, a slender, minuscule bird is seen incessantly whipping its tail from side to side as it flits and darts around in desert thorn scrub in search of insects. One of several small foliage-gleaning insectivores, including Lucy's Warbler (Vermivora luciae), Verdin (Auriparus flaviceps), and Bewick's Wren (Thryomanes bewickii), the Black-tailed Gnatcatcher is commonly found in dense thorn scrub and thickets of the Desert Southwest. Weighing only 5 to 6 g, this gnatcatcher is among the smallest of North American songbirds. Nevertheless, these nervous and high-strung little creatures vigorously defend their territories against much larger birds.
Individuals often forage in pairs during the breeding season and may remain paired year-round, but may also be seen in small flocks at other times of year. This is a year-round resident in central and western Mexico, northeastern Baja California, southern California, Arizona, New Mexico, and southwestern Texas. Once considered conspecific with the California Gnatcatcher (Polioptila californica), it is now treated as a full species based on morphology, vocalizations, habitat ( Atwood 1988 ), and mitochondrial DNA (mtDNA; Zink and Blackwell 1998a ). Its vocal repertoire has been well catalogued ( Atwood 1988 ), although context and function need further study. Feeding behavior has been investigated near Tucson, AZ ( Smith 1967a ), and near Las Cruces, NM ( Thomas 1975b ), as have habitat relationships along the lower Colorado River area in Arizona ( Laudenslayer 1981 ), but other geographic areas need attention (for example, Chihuahuan Desert). Also, details of nesting behavior (such as natal down, feeding rates) and Mexican distribution need further study.