Migratory birds are lighter coloured.

Migratory birds undertake long and challenging journeys that have selected for a suite of adaptations from sensory mechanisms that facilitate orientation to extreme feats of endurance that push physiological limits. Recent work on two distantly related species revealed that migrating individuals increase their flight altitude dramatically during the day compared to at night. These studies suggested that the phenomenon is driven by thermoregulation: the ascent to cooler heights during the day may offset heat generated by absorption of solar radiation. If thermoregulation is an important selective force on migratory species, migrants should have evolved lighter, more reflective plumage to avoid overheating. Here we show, across the entire avian radiation, that migratory species are indeed lighter coloured.