4 November 2015 1:00 pm
Bird species in which one male can mate with many females tend to have more colorful males. But the promiscuity has an even stronger effect on females, making them drabber. That’s one of the more surprising conclusions in a new study of more than half of all living species of birds, which also reveals that a bird’s size and breeding location has a strong influence on the extravagance of its plumage.
“This paper is one of the most ambitious comparative studies ever conducted,“ says Geoffrey Hill, an ornithologist at Auburn University in Alabama, who was not involved with the work. But Richard Prum, an ornithologist at Yale University, says the paper is flawed because the team relied on pictures of birds in a book rather than observing them in the wild. “You couldn't study animal pheromones with scratch-and-sniff recreations."
Most scientists believe that bright colors signal good health or a great immune system. But why are some bird species more colorful than others? That’s been tough to resolve because it is hard to quantify how colorful a plumage is, says Bart Kempenaers, an ornithologist at the Max Planck Institute for Ornithology in Seewiesen, Germany. “How do you compare bright red with bright blue or yellow? That is the problem we had to solve."
Kempenaers and his colleagues tried a new approach: scanning pictures. The scientists focused on passerine birds, a group that makes up more than half of all known bird species and that is sometimes known as perching birds for their arrangement of toes—three pointing forward, one back. The researchers scanned illustrations in the Handbook of the Birds of the World, the only book covering every known living bird species, and then used a computer program to quantify how colorful each bird’s plumage is.
The tricky part was getting just one number that they could compare across species. For each bird, the scientists looked at six different patches of feathers (nape, crown, forehead, throat, upper breast, lower breast) and then identified the 1% of birds that were closest in color in the same patch. The more males, the higher the score for that patch. The researchers then calculated the average of the six patch scores for each bird. In essence, the scientists measured how "malelike" a bird appeared. But because male birds, in general, tend to be more colorful, that measure also works as a measure of how colorful a bird’s plumage is.