Adaptive Geographical Variation in Bill Shape of Scrub Jays (Aphelocoma coerulescens)

Abstract

The hypothesis of an adaptive basis to geographical variation in scrub jay bill shapes was tested using morphometric and phylogenetic approaches. Bill shapes of scrub jays include a short, hooked form and a long, pointed form, at least in females. The variation is closely associated with habitat use across the species1 range: populations living in oak woodlands have short, hooked bills, whereas populations living in pinyon-juniper woodlands have long, pointed bills. The short, hooked form is apparently the primitive (pleisiomorphic) state in the species. The pointed form probably has been derived twice and lost once during the phylogenetic history of the species, each derivation being associated with invasion of pinyon-juniper woodlands and the loss with invasion of oak woodlands. Each bill form is most efficient at exploiting the foods present in the habitat with which it is associated, according to behavioral observations and functional arguments. Hence, the hypothesis that the variation is adaptive is strongly supported. A historical geographical scenario for the adaptive evolution of bill shape in scrub jays is presented.