Climate underpins continent-wide patterns of carotenoid-based feather colour consistent with Gloger's observations

Animal coloration has long been predicted to vary across geographic and climatic gradients in accordance with a long-standing ecogeographical rule. But further to his widely supported predictions that melanin pigmentation increases towards the Equator, Gloger observed that reds and yellows are more vivid in warm regions and thus more prevalent at lower latitudes, a prediction supported further by Görnitz, who suggested these colours would be more intense in areas with higher rainfall. Yet, studies of the associations between geography or climate and carotenoid-based plumage coloration to test these observations at a continental scale are scarce. Here, we investigated the extent to which yellow and red feather colour varies according to these hypotheses in Pogoniulus tinkerbirds with distributions across sub-Saharan Africa. We tested first for associations of feather colour with geography along latitudinal or elevational gradients, and then for associations with the climatic factors of rainfall and temperature that may underpin colour variation on continental scales. We find evidence consistent with Gloger and Görnitz's observations that more saturated colours and warmer hues at lower latitudes were primarily attributed to a relationship of underpart colour with temperature and rainfall. By contrast, forecrown colour, a trait previously associated with sexual selection, had a more complex association with geography and climate, with red forecrown hue associated with rainfall, but red and yellow intensity showing contrasting patterns with latitude. We highlight the complex nature of carotenoid-based plumage coloration, suggesting that although environmental factors affect the abundance of carotenoid availability, plumage coloration is also influenced by other selective pressures.