Rethinking Gloger’s Rule: climate, light environments and color in a large family of tropical birds (Furnariidae)

Ecogeographic rules provide a framework within which to test evolutionary hypotheses of adaptation. Gloger’s rule predicts endothermic animals should have darker colors in warm and rainy climates. This rule also predicts animals should be redder in warm and dry climates, the so-called “complex Gloger’s rule.” Empirical studies frequently demonstrate that animals are darker in cool and wet rather than warm and wet climates. Further, sensory ecology predicts that, to enhance crypsis, animals should be darker in darker light environments. We aimed to disentangle the effects of climate and light environments on plumage brightness and redness in the large Neotropical passerine family Furnariidae. Birds in cooler and rainier climates had darker plumage, even after controlling for habitat type. Birds in darker habitats had darker plumage, even after controlling for climate. The effects of temperature and brightness interact so that the negative effect of precipitation on brightness
is stronge...