Breeding Biology of the Santa Cruz Island Scrub Jay

The Santa Cruz Island Scrub Jay, Aphelocoma coerulescens insularis. is a genetically isolated population limited in its geographic distribution to Santa Cruz Island, Santa Barbara County, California. This insular form is characterized by rather pronounced morphological differentiation from mainland populations of the same species; its uniqueness was quickly recognized by early observers, who described the jay as "the most interesting bird on the island" (Howell and van Rossem 1911) and "the most sharply differentiated of any of the island species" (Swarth 1918). Yet in spile of this early interest in the Santa Cruz Island Scrub Jay, remarkably little has been published concerning the details of its biology. Most of the recent references to A. c. insularis in the literature deal with its morphological characteristics (Pitelka 1951) or possible evolutionary history (Johnson 1972); Yeaton's (1974) ecological analysis of island and mainland bird communities included a cursory examination of the Santa Cruz Island Scrub J a y ' s foraging niche. Prior to the present study, however, no long-term, detailed field observations of the population had been made. This paper summarizes available data concerning the breeding biology of A. c. insularis and provides comparisons of this information with known mainland Scrub Jay populations. The results of an ongoing study of social interactions in the Santa Cruz Island Scrub Jay will be presented elsewhere (Atwood in prep.); brief analyses of the population's vocalizations, feeding ecology, and morphological characteristics are included in Atwood (1978, 1979).