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.