The nominate subspecies of the California Gnatcatcher (Polioptila californica) has recently been listed as a threatened species under the federal Endangered Species Act (Salata 1993). Atwood (1992) estimated the subspecies' population in the United States, as of 1991, at between 1,811 and 2,291 pairs. Critical to management and protection of the California Gnatcatcher is a clear understanding of its nesting phenology.