In a monograph concerning speciation and geographic variation in black-tailed gnatcatchers, Atwood (1988) identified three principal areas of sympatry between the California Gnatcatcher (Polioptila californica) and the Black-tailed Gnatcatcher (P. melanura). The largest of these lies in northeastern Baja California, extending north along the coast of the Gulf of California from approximately 29°N (near Bahía de los Ángeles) to 30°N (near Bahía San Luis Gonzaga) and 30 km inland along the eastern base of the Sierra San Pedro Mártir to approximately 30°50'N, 115°15'W (Arroyo El Cajón). To the northeast, along the gulf coast, only P. melanura is known (Grinnell 1928, Wilbur 1987, Atwood 1988, Howell and Webb 1995).