Ecogeographical patterns in owl plumage colouration: climate and vegetation cover predict global colour variation

Aim: Ecogeographical rules link animal colours, especially those produced by melanin pigments, with variation in environmental conditions over wide geographical scales. In particular, Gloger's rule, coined in two versions for endothermic animals, suggests that tegument darkness would increase at high temperature, as well as in highly humid environments. On the other hand, the thermal melanism hypothesis, predicts that darker colourations should be more frequent in colder areas given their thermoregulation benefits.

Location: Global

Time period: Contemporary

Major taxa studied: Strigiformes

Methods: Here, we provide a global comparative test of these contrasting expectations in all extant owls (n = 198 species), a group of nocturnal birds displaying huge variability in the degree of melanin-based plumage colouration and environmental specialization. Combining analyses at both species and assemblage level, we assessed the climatic and environmental variables explaining variation in plumage lightness and redness across broad geographical gradients.

Results: Darker and redder owl phenotypes are more likely found near the equator. Species and assemblage level analyses reveal that owls have more frequently darker and redder plumages in warmer regions. In addition, owl species living in more vegetated areas are darker, and owl assemblages show darker colours in wetter areas.

Main conclusions: Global patterns of colour variation in owls do not fit expectations from the thermal melanism hypothesis but supports Gloger´s rule. Our findings also stresses that several alternative selective forces may explain climatic effects on colouration over large geographical scales. Experimental work is urged to uncover the possible mechanisms behind the detected associations between owl colour and environmental variables.