Cryptic plumage signaling in Aphelocoma Scrub-Jays

Abstract

Recent studies of avian vision and plumage coloration have revealed a surprising degree of cryptic sexual dimorphism, with many examples of male–female differences in UV reflectance that are invisible to humans. We examined the potential for male–female and adult–subadult differences in plumage coloration in the genus Aphelocoma. This group of jays comprises 10 phylogenetic species, which are found across southern and central North America and include cooperatively breeding species, as well as species that form socially monogamous pair-bonds typical of most species of birds. Our goal was to determine whether male–female and adult–subadult differences in plumage coloration were more common in species with complex social systems (i.e., cooperative breeders). We collected a series of reflectance measurements from hundreds of museum specimens and analyzed the results using a model of an avian visual system. We found that age- and sex-related differences were not more frequent in species that practice cooperative breeding. Hence, plumage signaling relating sex and age may not be strongly associated with complex social systems. Rather, the relative lack of a stable and familiar social environment, as well as other selective pressures and constraints (e.g., habitat use and plumage complexity), may have favored a greater degree of age- and sex-related differences in plumage coloration in jays that practice simple biparental care.