Credit: Left: Frode Jacobsen; Right: Glenn Bartley/BIA/Minden Pictures/Corbis

Some female warblers lost their bright colours just as the birds were evolving to become migratory, suggesting that this behavioural change spurred the evolution of sex differences in plumage colour.

To find out why female songbirds are often as colourful as the males in tropical species but less colourful in northern ones, Troy Murphy at Trinity University in San Antonio, Texas, and his colleagues studied 108 species of wood warblers (Setophaga tigrina; female pictured left, male pictured right). Migratory species tend to live farther north, and the authors found that the longer the bird's migration, the more distinct the sexes look. In multiple species, these sex differences evolved at around the same time as the birds first began migrating.

The findings suggest that sex differences in colour are driven by the needs of females. Non-migratory females often defend their territories using bright colours to signal fighting ability. But females that migrate rarely act in this way, and bright colours could make them more visible to predators during their migration.

Proc. R. Soc. B 282, 20150375 (2015)