Climate, crypsis and Gloger's rule in a large family of tropical passerine birds (Furnariidae)

Glogers rule predicts endothermic animals should have darker colors under warm and rainy climates, but empirical studies have typically found that animals tend to be darker under cool and rainy climates. Moreover, Glogers rule has rarely been tested jointly with the prediction that animals occupying dark habitats should have darker colors to enhance crypsis. We aimed to disentangle the effects of climate and light environments (habitat type) as correlates of plumage brightness in a large Neotropical passerine family. We found that cooler and rainier climates are associated with darker plumage, even after accounting for habitat types, and that darker habitats are associated with darker plumage, even after accounting climate. There was an important interaction between precipitation and temperature, whereby the negative effect of temperature on brightness becomes stronger under cooler temperatures. Climate and light environments have separate but complementary effects in driving macroevolutionary patterns of plumage color variation in birds.