Breeding Biology of the Santa Cruz Island Scrub Jay

INTRODUCTION The Santa Cruz Island Scrub Jay , Aphelocoma coerulescens insularis. is a genetically isolated populat ion limited in its geographic distribution to Santa Cruz Island, Santa Barbara County , Cal ifornia . This insular form is characterized by rather pronounced morphological different iat ion f rom mainland populat ions 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 s p e c i e s " (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 b iology. Most of the recent references to A. c. insularis in the literature deal with its morphological characterist ics (Pitelka 1951) or possible evolutionary history (Johnson 1972); Yeaton's (1974) ecological analysis of island and mainland bird communi t ies included a cursory examinat ion of the Santa Cruz Island Scrub J a y ' s foraging niche. Prior to the present s tudy, however , no longterm, detailed field observat ions of the populat ion had been made. This paper summar izes available data concerning the breeding biology of A. c. insularis and provides compar isons of this information with known mainland Scrub Jay populat ions . The results of an ongoing study of social interactions in the Santa Cruz Island Scrub Jay will be presented e lsewhere (Atwood in prep.) ; brief analyses of the popula t ion ' s vocal izat ions, 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 Commit tee (1957) considered the Santa Cruz Island Scrub Jay to be a well-marked subspecies of A. coerulescens. The question of what constitutes a biologically distinct species is frequently difficult to deal with and, in part icular, the lack of sympatry of A. c. insularis with any mainland subspecies prevents any natural testing of isolating mechanisms . Pitelka (1951) concluded that " i n 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 at tempts to c o n v e y . " Pitelka (1951) recognized eighteen subspecies of Scrub Jay which were divided into four major groupings. Numerica l ly , the largest of these is the " c a l i f o m i c a " group, consist ing o f t e n subspecies distributed in Oregon, Cal i fornia , and Baja California. A. c. insularis is clearly affiliated with the " "ca l i fo rn ica" group (Pitelka 1951). To appreciate the unique characterist ics of the Santa Cruz Island Scrub Jay, it is necessary to briefly evaluate its history as an insular populat ion. 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