Editor’s Note (Aug. 2016): This species is the result of a recent taxonomic split. Maps, rich media, and Introduction, Appearance, and Systematics articles have been updated. Other articles are being edited and may reflect content from the original Western Scrub-Jay account.
The California Scrub-Jay is a familiar jay that occurs in coastal 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), the latter occupying inland states of the American Southwest, extending into southern Mexico (Chesser et al. 2016).
Beyond differences in their geographic ranges, California Scrub-Jays generally occupy dry scrub and oak woodland habitats, whereas Woodhouse’s Scrub-Jays occur in scrub and woodland, especially piñon-juniper; both species have adapted well to suburban landscapes. 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. It has a heavier bill that is more hooked than that of the Woodhouse’s Scrub-Jay ( Pitelka 1951d ), a difference that may be an adaptation to food resources in their different habitats ( Peterson 1993 , Bardwell et al. 2001 ). Further, the two species exhibit differences in behavior and vocalizations ( Dunn and Garrett 2001 , ). 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 (A. woodhouseii) (Chesser et al. 2016). 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).
Across their geographic range, the California Scrub-Jay exhibits much variation in physical appearance (e.g., Pitelka 1951d , Pitelka 1961b , Pitelka 1951d , Pitelka 1961b , Peterson 1990b , Peterson 1993 , Bardwell et al. 2001 ). Individuals show variation in bill morphology that reflects differences in diet. Along the Pacific Coast, birds in northern portions of the range also are generally larger than their southern counterparts, with the smallest California Scrub-Jays living on the Baja Peninsula.
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 morphological and behavioral evolution, the California Scrub-Jay is a model species 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 ). Although less expert at storing and recovering food items than the Clark's Nutcracker (Nucifraga columbiana), 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 morphological and 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.