The passerine birds of the genus Aphelocoma include the scrub jays and their relatives. They are New World jays found in Mexico, western Central America and the western United States, with an outlying population in Florida. This genus belongs to the group of New World (or "blue") jays–possibly a distinct subfamily–which is not closely related to other jays, magpies or treepies. They live in open pine-oak forests, chaparral, and mixed evergreen forests.
Seven species of Aphelocoma are generally recognized currently. They are believed to have evolved in the Pleistocene, and the Florida species is known to have been recognizably distinct and present in its current range for at least 2 million years. The inland, coastal, and Santa Cruz island populations of the (former) western scrub jay are now considered three distinct species, namely Woodhouse's, the California and the island scrub jays. Two different populations of the Mexican jay might similarly represent two distinct species.
mtDNA NADH dehydrogenase subunit 2 sequence data is unable to properly resolve the relationships of the species. Judging from New World jay biogeography, the unicolored or Mexican jays might represent the most basal lineage; morphology would tentatively lean towards the latter which retains more of the group's color patterns, while the available molecular data allows no robust conclusions whatsoever. In any case, the data of Rice et al. (2003) suggests – albeit also with very low confidence – that the Mexican jay comprises two clades which might constitute two different species. However, far too few individuals have been sampled to say anything definite on that matter, except that the lineages – if they indeed exist – do not correspond to the geographical pattern of intraspecific variation (see species article for more).
The western scrub jay is now made up of three species. These would be separated by the Great Basin, with the Pacific coastal lineage (California scrub jay) and the island scrub jay, as well as the inland lineage (Woodhouse's scrub jay), with the Florida scrub jay being a sister species. This treatment fails to address the problem of birds from inland southern Mexico. Nonetheless, it is actually because the molecular diversity pattern is so badly resolved that it supports the view that rapid Late Pliocene radiation of the North American scrub jays led to the present diversity. Studies on the evolutionary history of Aphelocoma jays suggests that all New World jays originated in North America or Mesoamerica.
Aphelocoma jays are slightly larger than the blue jay and differ in having a longer tail, slightly shorter, more rounded wings, and no crest on the head. The top of the head, nape, and sides of the head are a rich deep blue. Some species have a white stripe above the eye and dark ear coverts. The breast is also white or grey-white and the back is a grey-brown contrasting with the bright blue tail and wings in most species. One species, the unicolored jay, is blue all over, superficially similar to the pinyon jay from much further north. The bill, legs, and feet are black.