Some like it hot: Elevated heating capacity of carotenoid-containing contour feathers compared to melanized feathers and of rural over urban feathers

Abstract

Bird feathers serve many purposes, ranging from flight and thermoregulation to camouflage and communication. Considerable attention has been paid to the insulative and heat-dissipation functions of feathers, which can depend on many factors, including the local environment and feather pigmentation. However, more studies are needed to understand the heating and cooling properties of different feathers, including in situations where certain color ornaments (e.g. melanin, carotenoid) are favored for signaling and where hot environments (e.g. urban heat-islands) may exert particularly strong and novel thermal selection pressures. We investigated heating and cooling responses of different contour feathers in male house finches (Haemorhous mexicanus) – an urban-adapted passerine bird species where males exhibit sexually selected carotenoid-pigmented plumage – in a controlled laboratory setting (i.e. standard illuminant exposure from a close distance). We used samples of carotenoid- and melanin-colored contour feathers from males captured from urban and non-urban habitats to understand the impacts of urbanization and feather pigment type on plumage thermal properties. We experimentally heated feathers for 1 min and tracked surface temperatures over this time as well as during a 1 min. cooling phase immediately after removal of the heat source. We found that, at the end of the heating phase and beginning of the cooling phase, urban feathers were cooler than non-urban feathers, and that, in non-urban areas, carotenoid-containing feathers had higher temperatures than melanized feathers. We discuss the possible morphological adaptations and implications of these thermal differences, specifically in this desert- and urban-adapted species in which males have carotenoid-based plumage color signals.