A red rumped swallow perches on a thin branch.

November 4, 2015

Female birds can’t shake their colourful feathers, and other lessons from studying 6,000 species

The evolution of male songbirds as the colourful consorts of drab female partners is more complicated than long thought, says a McMaster researcher on a team that looked at nearly 6,000 species for a massive study published in the journal Nature.

Conventional wisdom has held that in many species, male birds were more colourful because they were competing for female attention, and that female birds were less colourful because they needed camouflage while guarding their nests.

Now advances in computing power and new methods that compare colouration in different species have allowed the researchers on the study to look at every species of passerine, the perching songbirds that make up about 60 per cent of the world’s 10,000 species of birds.

The study offers impressive new evidence that supports some old theories while setting others to rest, explains Cody Dey, an author of the Nature paper who was completing his PhD in Biology at McMaster at the time of the research.

The question of plumage colour is significant, Dey, explains, because birds, especially males, give up so much to look so good, creating an evolutionary mystery that asks exactly what they gain in return for rendering themselves more vulnerable to predation.

“These are the questions that have been asked since the start of ecology,” he says.

Among the ideas the research supports is that females would have evolved to be even more different than males than they already are, except that every female inherits the genetic material of a colorful male – an ancestry that is impossible to shake.

“If colourful males do better, they’re going to produce colourful daughters as well, even though it’s not necessarily advantageous for the daughters,” Dey says.

At the same, time, the broad study of plumage patterns also suggests that female songbirds have evolved their own colouration to gain advantages in their particular climates and surroundings, adding weight to the idea that females are just as important in shaping the colours of their species as males.

The study takes a wide view of the world’s songbirds, putting empirical evidence behind the observation that tropical birds are more colourful than their cousins from temperate climates, probably because they do not migrate.

Tropical birds have the luxury of permanent homes and longer lives and raising fewer offspring, but they pay for it by having to defend permanent territories, and their bright feathers may help them assert control over their home turf.

In non-tropical migratory species, mating relationships are typically limited to one season, diminishing the need for colour, and allowing physiological resources to be used for other efficiencies.