Many bird species make displays to attract the opposite sex—elaborate plumage, songs, etc. We tend to think of these displays in terms of sexual selection, i.e. the peacock with the biggest, prettiest tail feathers will attract the most females and reproduce. But most species including grebes, gulls and finches form pair bonds and stick with each other after mating to help raise their young. They also continue to display to each other. But why would go to the trouble of showing off once they already have a mate?
Trevor Price, professor of biology in the Department of Ecology and Evolution, wondered the same thing. Why would birds continue to display to each other after mating? The answer seems to be a kind of quid pro quo for the good of their offspring. When both birds in a pair bond display to each other, they both make a greater investment in their young.
In a study recently published in the Proceedings of the Royal Society, Price and his colleagues from the University of North Carolina and Imperial College London showed that mutual displays by both parents evolve because the offspring are much better off when both parents contribute to their care. In other words, the parents shake a tail feather for each because it’s good for the brood.
“We’ve just gotten hung up on this idea that displays are there to attract females,” said Price. “The idea is that a male and a female together do much better than anyone else.”
Using population genetic models, he and his colleagues determined that mutual displays between birds are especially likely to evolve when the investment in offspring by both parents produces a benefit greater than twice that of one parent, or when the sum is greater than the parts, so to speak.
It’s tempting to make an analogy to human love and say that the birds take care of each other and their young out of mutual affection, and this theory may show how this sort of thing happens. The displays trigger a biological response in the other bird that is beneficial to offspring. For example, a female might display a large patch of yellow feathers on her breast. This causes the male to have more testosterone in his blood and defend the nest more aggressively.
The irony, Price said, is that the birds would probably be better off if they didn’t display to each other at all and just put all their energy into raising their young. The bird expends energy to produce these displays, and showy, colorful plumage could attract predators.
“The best thing they could do is just not display and put all their energy into raising eggs. But they’re just stuck with it, because if they stop displaying, neither of them will put enough energy into the eggs,” said Price. “So they’re locked into this system. It’s a great example where Darwin’s principle is not for the good of the species, but for the good of the individuals that made these displays.”
Servedio MR, Price TD, & Lande R (2013). Evolution of displays within the pair bond. Proceedings. Biological sciences / The Royal Society, 280 (1757) PMID: 23427172