Climate underpins continent-wide patterns of carotenoid-based feather color consistent with Gloger’s observations

Animal coloration has long been predicted to vary across geographic and
climatic gradients in accordance with a longstanding 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 colors 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 color
vary according to these hypotheses in Pogoniulus tinkerbirds with
distributions across sub-Saharan Africa. We tested first for associations
of feather color with geography along latitudinal or elevational
gradients; and then for associations with the climatic factors of rainfall
and temperature that may underpin color variation on continental scales.
We find evidence consistent with Gloger and Görnitz’s observations; that
more saturated colors and warmer hues at lower latitudes were primarily
attributed to a relationship of underpart color with temperature and
rainfall. By contrast, forecrown color, a trait previously associated with
sexual selection, had a more complex association with geography and
climate, with red forecrown hue associated with rainfall, but 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.