Foods Birds Eat Influence Feather Colour, Finds U of G Prof

December 20, 2006 - News Release

For a songbird called the American redstart, its feather colour may not be determined by its physical condition, as many scientists believed, but by the food it eats while it grows new feathers, a University of Guelph integrative biology professor has found.

By collecting single feathers from more than 200 different American redstart songbirds, Ryan Norris and his colleagues from Queen’s University and the Smithsonian Institution were able to determine where the birds had moulted, what condition the birds were in and the brightness of their feathers. These three pieces of information allowed Norris to test competing hypotheses of whether feather colour is dependent on condition or diet. His research is published in the current issue of Biology Letters.

“What we found was, the condition wasn’t related to the feather colour,” he said. “We found the amount or type of carotenoids (antioxidant fat-soluble molecules found in plants and animals) a bird gets from the food it eats offers the best explanation for variation in feather colour.”

This information provides important clues as to how species can develop, said Norris. “If the males from two populations look different because they eat different insects, then female birds may choose to mate only with males from their population, and over a longer time scale, it could actually lead to differentiation between populations.”

He sampled American redstart songbirds in the Caribbean and Central America. The migrating birds spend their summers anywhere between British Columbia and Louisiana. After breeding in the north, they replace their feathers and then fly south for the winter.

When feathers are being grown, they carry unique hydrogen isotope values, depending on the bird’s latitude at the time. These values arise from variations in precipitation and are then transferred all the way up the food chain from plants to insects to animals. Looking at the ratio of heavy and light hydrogen isotopes in the feathers allowed Norris to estimate where the birds moulted in North America. “The isotope value of where the bird grew its feathers doesn’t change because, once the feather’s grown, the chemical composition in that feather is fixed,” he said. It’s a lot easier to track where a bird has been by analyzing isotopes than by putting a band on it and trying to find it again, he added.

Interestingly, the researchers found a relationship between the colour or brightness of the feather and where the bird had spent its summer in North America. “Males that moulted at southerly latitudes were more likely to grow yellowish feathers, whereas males that moulted further north tended to have more orange-red feathers,” said Norris. “Redstarts moulting at lower latitudes may consume insects with lower carotenoid concentrations.”

The faint growth bars that appear on each feather also allowed Norris to estimate what condition the bird was in when it grew its feathers. “Every 24 hours, a bird lays down a light and a dark growth bar as it regrows its feathers,” he said. The width of those bars is related to how fast the feathers grew. If they grow quickly, the growth bars will be thin, which means the bird is in good physical condition. If the bars are wide, the bird grew them slowly because it was in poor condition.

Unlike the link between isotope values and feather colour, Norris found no relationship between the birds’ condition and feather colour. “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,” he said. If that hypothesis were true, feather colour could be an “honest signal” for females to choose only mates in good condition. “Instead, in some birds, it looks like feather colour might not be an ‘honest signal.’ It just tells you where the bird grew its feathers.”

Ryan Norris
Department of Integrative Biology

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