URI grad student finds migrating
songbirds are picky eaters
KINGSTON, R.I. -- February 13, 2003 -- At a restaurant, most people have a clear preference for some items on the menu over others. A University of Rhode Island graduate student discovered that migrating songbirds also have food preferences. But unlike humans, birds base their food preferences in part on the fatty acid composition of the foods, something that humans cannot even detect.
In a study that may have important implications for habitat management at migratory stop-over points, Barbara Pierce found that migrating songbirds select foods that provide them with specific fatty acids.
Pierce caught red-eyed vireos on Block Island during fall migration and kept them in captivity for several months, providing them with a choice of foods. The birds consistently preferred the diet with the highest percentage of unsaturated fats.
"These results emphasize the importance of ensuring that migratory stop-over sites have exactly what the birds need to complete their migration," said Pierce, who grew up in Coventry, Conn. and now lives in Middletown, Conn. "If the right food isnt available, it may make it more difficult for them to reach their final destination."
While Pierce stressed the importance of protecting habitat at key migration sites, she indicated that it is equally important that the quality of the habitat be protected as well.
"A site might have a lot of fruit available for the birds to feed on, but it might not be the right fruit," she said. "If the only fruit available is something thats hard to digest, or if the nutrients they require arent available, then the fruit is doing them no good."
According to Pierce, who is working in collaboration with URI Assistant Professor Scott McWilliams, during the breeding season (late spring and summer) most songbirds feed on protein-rich insects. But during migration, when increased energy is needed to sustain their long migratory flights, they consume mostly berries and other abundant fruits.
Different fruits contain different fatty acids, and those fatty acids contribute to a birds total fat composition. The fat of wild birds is composed of a 60/40 ratio of unsaturated to saturated fats, which Pierce said is primarily the result of their diet. When tested in a "fly wheel" to measure their metabolic rate, birds with the 60/40 fat ratio had much more energy to sustain flight than birds with a higher percentage of unsaturated fats.
This result seems counterintuitive since, when Pierce gave the birds a choice of foods containing different fatty acids, the birds consistently chose the food containing more unsaturated fats. Why do the birds prefer unsaturated fats when they get more energy from a more balanced ratio of fats?
"This seeming contradiction probably tells us something important about the habitat," she said. "They must be selectively looking for those fatty acids when theyre feeding during migration, maybe because those fatty acids are hard to find."
The bottom line, according to Pierce, is that a birds diet determines its fat composition, and its fat composition determines whether it has enough energy to complete its migration. "If the availability of unsaturated fats are limited in the wild, then we need to be careful to make sure that those fats can be found at key migratory sites," she said. "Or else the birds may be unable to complete the trip."