by Sharon Oosthoek
As any female will tell you, it is important to know your potential mate’s strengths and weaknesses before committing to a relationship. Ornithologists have long thought female birds choose mates on the basis of their bright plumage – which, presumably, is a signal of a healthy male that could produce healthy offspring. A group of Ontario ornithologists, however, is casting doubt on this assumption.
“Pigments that produce colour in feathers can be used as a signal [of health] in males, but the question is, is it an honest signal?” asks University of Guelph ornithologist Ryan Norris. When Norris and colleagues from Queen’s University and the Smithsonian Institution in the United States set out to answer that question last year, they found that among American redstarts, feather colour was not an honest signal. In fact, the researchers discovered that bright plumage, rather than indicating good health, was related to what kind of insects the bird ate when it grew its feathers.
Norris and his colleagues collected single feathers from more than 200 redstarts in the tropics. These birds typically migrate to North America to breed during our summer, making their way back to the tropics afterward. Before they start the journey south, they grow new feathers. Those feathers are marked with “growth bars.” A thin bar means the bird grew its feathers quickly because it was in good health.
The researchers found no link between thin growth bars and bright feathers. But when they examined the chemical signature of the feather – thereby pinpointing where the bird was when it re-grew its feathers – they found the link they were looking for.
Knowing where the birds were when they grew new feathers told the researchers what kind of bugs the birds were eating at the time – whether the bugs had high or low levels of carotenoids (antioxidant fat-soluble molecules found in plants and animals).
A high-carotenoid diet is what gives birds brightly coloured feathers.
“The theory was that, if you’re a bird in poor health, you have to use carotenoids, which produce feather colour, for other things and therefore can’t use them in your feathers,” says Norris. If that hypothesis were true, feather colour would be a good way for females to choose only mates in good condition. “Instead,” says Norris, “in some birds, it looks like feather colour just tells you where the bird grew its feathers.”