Carotenoid-based plumage colour saturation increases with temperature in Australian passerines

Aim: Birds are often coloured yellow, olive, orange or red by carotenoids, which are plant pigments that are ingested with food. Here we test whether there are consistent patterns of intraspecific geographic variation in male carotenoid-based plumage coloration linked to latitude, temperature, precipitation and primary productivity. It has been suggested that patterns of geographic variation should be different for yellow, compared to the metabolically derived red carotenoid-based coloration, but this has not been tested. Location: Australia. Taxa: Forty-nine randomly selected species of Australian passerine birds (Passeriformes), 30 with yellow and 19 with red carotenoid-based plumage coloration. Methods: Carotenoid-based plumage patches of male museum specimens were measured using reflectance spectrometry, and spectra analysed using models of avian colour vision. For each species, we used linear models to determine whether geographic variation in carotenoid-based colour correlated with latitude, temperature, precipitation or primary productivity. To determine whether effects are consistent across species, we used phylogenetic meta-analyses. Results: Although spatial environmental effects on carotenoid-based plumage coloration varied across species, overall, species tended to have more saturated carotenoid-based coloration at lower latitudes, and warmer regions. These effects applied mainly to resident species (as opposed to migratory or nomadic) and were stronger for red compared to yellow carotenoid-based colours. We found no consistent effects of precipitation or primary productivity. Main conclusions: Geographic variation in male carotenoid-based plumage coloration of Australian passerines is unlikely to be caused by variation in plant productivity (the ultimate source of carotenoids), and may be better explained by latitudinal gradients in the intensity of sexual selection or predation risk. Alternatively, temperature effects on red carotenoid-based coloration may be driven by the hypothesized link between carotenoid and cellular metabolism. Latitudinal and temperature effects have the potential to generate perceivable differences in coloration across realistic spatial scales, contributing to divergence in visual signals.