The California Scrub-Jay is a familiar jay occurring in coastal states of the western United States, from Washington to California and south to the southern tip of Baja California. This species is the result of a recent taxonomic split of the Western Scrub-Jay (Aphelocoma californica) complex, leading to recognition of both the California Scrub-Jay (A. californica) and Woodhouse’s Scrub-Jay (A. woodhouseii) ( Gowen et al. 2014 , Chesser et al. 2016 ), the latter species occupying inland areas of the southwestern United States, extending into southern Mexico.
Beyond differences in their geographic ranges, California Scrub-Jays generally occupy dry scrub and oak woodland, often in residential areas, whereas Woodhouse’s Scrub-Jays occur in dry scrub and woodland, especially pinyon–juniper (Pinus–Juniperus). The California Scrub-Jay differs outwardly from the Woodhouse’s Scrub-Jay by having brighter blue upperparts, paler underparts, and a darker, more distinct breast-band. Relative to the latter species, the California Scrub-Jay has a heavier and more hooked bill ( Pitelka 1951d ), which is believed to be an adaptation for consuming acorns ( Peterson 1993 , Bardwell et al. 2001 ). Further, the two species exhibit differences in behavior and vocalizations ( Dunn and Garrett 2001 , Curry et al. 2002 ). Calls of the California Scrub-Jay are typically harsher, 1-syllabled and lower-pitched than the 2-syllabled calls of Woodhouse’s Scrub-Jay ( Dunn and Garrett 2001 ).
The California Scrub-Jay is sister to the Island Scrub-Jay (A. insularis), endemic to Santa Cruz Island, California ( Peterson 1990a , Peterson 1992a ), and is closely related to Woodhouse’s Scrub-Jay ( Gowen et al. 2014 ). Species limits among the scrub-jays are not clear. Geographically isolated populations on Santa Cruz Island have fixed allelic differences from other populations. However, California and Woodhouse’s scrub-jays have few or no fixed differences in alleles and interbreed where their geographic ranges contact in western Nevada and east-central California, and in desert ranges of eastern California ( Peterson 1990a , Peterson 1990b , Peterson 1992a ). Despite ongoing gene flow, the hybrid zone is narrow, and there is evidence for selection against hybrids ( Gowen et al. 2014 ).
Evidence from studies of breeding biology and demography of the California Scrub-Jay has presented a general picture of a less social, non-cooperative species ( Ritter 1972 , Ritter 1983b , Ritter 1984 , Carmen 1988 , Carmen 2004 ). Along with being a model species for studies on behavioral evolution, the California Scrub-Jay is a model in studies of foraging behavior and cognitive abilities, including spatial memory ( Clayton et al. 2000 , Clayton et al. 2001 , Emery and Clayton 2001a , Emery and Clayton 2001b ). The California Scrub-Jay is adept at locating and selecting food and in dealing adaptively with social competition from other jays ( Langen 1999 , Clayton et al. 2000 , Clayton et al. 2001 , Emery and Clayton 2001a , Emery and Clayton 2001b ).
The complexity of behavioral patterns within this species continues to pose challenging questions for ornithologists. Along with its close relatives, the California Scrub-Jay is expected to play an ongoing role in research that investigates the ecological and evolutionary factors affecting corvids, as well as other birds.