Breeding Biology of the Santa Cruz Island Scrub Jay

INTRODUCTION 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 spite 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 toA. 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 Jay' 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 ofA. 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). The Santa Cruz Island' Scrub Jay was initially described as a distinct species. Aphelocoma insularis, on the basis of strong morphological contrasts between the insular population and mainland Scrub Jay races (Henshaw 1886). More recently. the A.O. U. Check-list Committee (1957) considered the Santa Cruz Island Scrub Jay to be a well-marked subspecies of A. coerulescem. The question of what constitutes a biologically distinct species is frequently difficult to deal with and, in particular. the lack of sympatry ofA . c. insularis with any mainland subspecies prevents any natural testing of isolating mechanisms. Pitelka (951) concluded that "in the absence of natural or experimental proof of intersterility. specific segregation of the Santa Cruz Island form from its mainland relatives would seem to me to distort the facts of relationship which nomenclature attempts to convey. " Pitelka (1951) recognized eighteen subspecies of Scrub Jay which were divided into four major groupings. Numerically, the largest of these is the "calijimJica" group, consisting often subspecies distributed in Oregon. California. and Baja California. A. c. imularis is clearly affiliated with the "califomim" group (Pitelka 1951). To appreciate the unique characteristics of the Santa Cruz Island Scrub Jay. it is necessary to briefly evaluate its history as an insular population. Henshaw's (1886) early postulation that vagrant Scrub Jays had become established on Santa Cruz Island probably represents the opinion held by most ornithologists at the turn of the century. Dawson (1920) later stated that