Ecogeographical and climatic predictors of geographical variation in plumage and morphology in rufous-capped warblers, Basileuterus rufifrons

Many broadly distributed species exhibit geographic variation across their range. Intraspecific variation across geographical ranges can allow us to test ecological and sexual selection hypotheses that may help explain this variation. In this study, we quantified geographical variation in morphology and plumage colouration of rufous-capped warblers (Basileuterus rufifrons), a broadly distributed Neotropical passerine bird ranging from northern Mexico to northern South America. We measured morphological characteristics and plumage colouration of 473 museum specimens covering the entirety of the species’ range, and compared these with geographic location and climate data obtained from the WorldClim data base and the University of Idaho Climatology Lab. If variation in body size, measured by wing length, follows Bergman’s rule, we expect it to increase with latitude; whereas if body size follows Allen’s rule, we expect extremities to shorten at higher latitudes. If plumage colouration follows Gloger’s rule, we expect plumage colouration to become darker in more humid climates, whereas if it is limited by the availability of nutrients, we expect brighter plumage colour in habitats with greater primary productivity. If sexual selection drives divergence in male and female body size and colouration, we expect birds in northern populations to exhibit greater sexual dimorphism and dichromatism and to exhibit more contrast against the vegetation background than southern populations, where male and female sex roles are expect to converge in more tropical areas. Data analyses are still in progress. Our research of geographical variation in rufous-capped warblers will help us better understand the relative importance of ecogeographical rules and the influence on sexual selection geographic variation, and may help us predict how animals could respond to future changes in environmental conditions