In 1875, naturalist Henry Henshaw completed a short visit to Santa Cruz Island, largest of southern California's northern Channel Islands off Santa Barbara and only 30 kilometers from the nearest point on the mainland (Figure 1). During his visit, Henshaw collected 4 large and brightly colored scrub-jays, which, after comparison with mainland specimens, he concluded represented a new species, Aphelocoma insularis ( Henshaw 1886a ). This island endemic population averages about 15% larger, 40% heavier, and with a bill about 20% longer than the largest mainland scrub-jays, now known as Western Scrub-Jay (A. californica).
Henshaw's large scrub-jays are known only from Santa Cruz Island, and, to this day, no scrub-jay has been found on any of the other Channel Islands, even as a vagrant. In addition, the Island Scrub-Jay is the most strongly differentiated of the endemic landbirds found in the archipelago ( Swarth 1918b ). Although isolated and distinctive, its close similarity with other scrub-jays has caused taxonomists to treat it in the past as a subspecies along with all other scrub-jays in western North America and with the highly disjunct populations in Florida ( Pitelka 1951d ). Accumulated information on behavior, morphology, and genetics has since supported recognition of at least 3 species of scrub-jays ( American Ornithologists' Union 1998a ), although up to 5 phylogenetic species likely exist within the entire complex ( Curry et al. 2002 ). Originally known as the Santa Cruz Jay ( Bent 1946a ) and often referred to as the Santa Cruz Island Scrub- Jay (Atwood Atwood 1980b , Atwood 1980a ), a name that is consistent with other endemics to the island and perhaps more appropriate, the bird Henshaw identified is now known as the Island Scrub-Jay. Its status as a distinct species is based on traditional morphological criteria bolstered by genetic analysis (Peterson Peterson 1990b , Peterson 1992a ; KSD and R. K. Wayne unpubl.).
Besides its endemism and large size, the Island Scrub-Jay is notable in several other respects. As is often typical of island birds, the jays of Santa Cruz Island are locally common, fearless of humans, and inquisitively opportunistic in their foraging habits. Intensive study of their evolution, demography, and social ecology by C. T. Collins and his students (Atwood Atwood 1978 , Atwood 1979a , Atwood 1980b , Atwood 1980a ; Isitt 1989 ; Atwood et al. 1990 ; Collins and Corey 1994 ; Corey 1994 ; Atwood and Collins 1997 ; Kelsey 1998 ; Kelsey and Collins 2000 ) has revealed that the endemic birds are intermediate between Western and Florida scrub-jays in several important respects: while Florida Scrub-Jays are renowned for extreme habitat specificity, highly restricted dispersal, and cooperative breeding (Woolfenden and Fitzpatrick Woolfenden and Fitzpatrick 1984 , Woolfenden and Fitzpatrick 1996 ), Western Scrub-Jays are more flexible in their habitat requirements and less social throughout most of their broad geographic range ( Curry et al. 2002 ); Island Scrub-Jays display limited dispersal imposed by the constraints of habitat and physical space in their island range and concomitant delayed breeding, but without any evidence of cooperative breeding indicated by observations of helpers at the nest. An important driver of the social system of the Island Scrub-Jay is an adult survival rate of more than 90% per year among both breeders and nonbreeders, levels that are extra-ordinarily high for a passerine bird.
Despite the intriguing combination of island endemism, gigantism, and unusual demography, many aspects of the Island Scrub-Jay's biology remain unknown, in part because of the logistical challenges of conducting field research on an offshore island. This account focuses on topics for which at least some in-formation exists, including especially comparisons with mainland relatives where possible. Ample opportunities exist for further research on the unique Island Scrub-Jay, which is joined by only the Yellow-billed Magpie (Pica nuttalli) as a bird species endemic to California.