Extolled by conservationists for its ability to stop a bulldozer in its tracks and reviled by land developers as their worst enemy, the tiny California Gnatcatcher has become a symbol of the challenges of how to interpret and apply the U.S. Endangered Species Act. Commonly found throughout most of southern and central Baja California in a variety of arid scrub habitats, the range of this small, nonmigratory songbird extends northward into coastal southern California, where it occurs on some of the most expensive private real estate in the United States. Explosive human population growth and resultant suburban sprawl within the last 50 years has reduced and fragmented the species' coastal sage scrub habitat, and led to Federal protection of the northernmost subspecies as a Threatened species in 1993. Furthermore, concerns engendered by this decision catalyzed passage of legislation by the State of California intended to protect natural communities while allowing continued economic growth. Former U.S. Secretary of the Interior Bruce Babbitt described this novel initiative, known as the Natural Community Conservation Planning (NCCP) program, as a “breathtaking experiment” that “may become an example of what must be done across the country if we are to avoid the environmental and economic train wrecks we've seen in the last decade.” Conservation of the California Gnatcatcher continues to be the dominant factor in determining details of the NCCP program's implementation, thus emphasizing the importance of scientific information about the species' behavior, ecology, and distribution.
Little is known of the California Gnatcatcher's biology in the heart of its range in Baja California. In contrast, substantial research has been conducted on its distribution and biology in the United States during the last 10 years. Much of this recent work was done in response to various management and conservation decisions, resulting in a diverse but sometimes poorly conceived research agenda pursued by environmental consultants, academics, wildlife agency personnel, and conservation advocates. Many results were never peer-reviewed or formally published, leading Rotenberry and Scott ( Rotenberry and Scott 1998 : 238) to comment that “researchers had few avenues of communication, no one could build off the established work of others, and mechanisms for identifying reliable data went unused.” Publication of papers presented at the 1995 California Gnatcatcher Symposium began to correct this situation ( Rotenberry and Scott 1998 ), yet much of what is currently “known” about the species still is found only in scattered field notes, anecdotal reports, and “gray literature,” rather than rigorous scientific studies. Where warranted, this account refers to such unpublished data as a means of summarizing all currently available information, while readily acknowledging the need for more careful and controlled studies. All unpublished reports cited here have been deposited at the Wilson Ornithological Society's Van Tyne Memorial Library (Museum of Zoology, University of Michigan, Ann Arbor, MI 48109-1079).
California Gnatcatchers are small, slender, gray birds with long, black tails narrowly edged and tipped with white. During the breeding season, males have a glossy black cap. Dark-gray northern populations grade clinally into pale southern forms. Upperparts are darker gray than underparts, with females having warmer, brownish tones on back, flanks, and belly. The species' kittenlike mewing calls best distinguish it from the similar and largely allopatric Black-tailed Gnatcatcher, Polioptila melanura .
Although the species is abundant throughout much of Baja California, fewer than 5,000 pairs of California Gnatcatchers now persist in coastal southern California, where their distribution is mostly restricted to the coastal sage scrub plant community. Breeding pairs are generally monogamous and sedentary, and both sexes participate in nest construction, incubation, and care of young. High rates of nest predation are offset by rapid and persistent renesting efforts extended over a protracted breeding season; as many as 10 nesting attempts may occur in a single year, producing up to 3 successful broods. Juveniles generally disperse less than 10 km from their natal territories, and establish pair bonds within several months of fledging. Rates of annual survivorship are variable and likely influenced by poorly understood interactions between winter temperature and rainfall patterns.