Size matters. The bearded vulture, at left, takes 3 months to grow its longest primary feather; the bluethroat's takes less than three weeks.

(left, African white-backed vulture) Marek Szczepanek; (right, a Bluethroat) Richard Bartz

Why Big Bird Isn't Bigger

Birds that fly can get only so big. Above a certain weight, they just can't manage the muscle power to take off. But another factor also comes into play when large birds fly, according to a new study. They take much longer to regrow their feathers than smaller birds do.

Flying birds top out at about 15 kg, a size reached by some swans, pelicans, and several other species. These big birds have larger wings, larger flight feathers, and different ways of molting, an observation that led researchers at the University of Missouri, St. Louis, to suspect that feathers might limit bird size.

While birds are regrowing the long primary feathers that make up the wing's surface, they don't fly as well. So the team scanned the literature for measurements of feather growth rates for individual species and also measured feathers of 1774 museum specimens. The researchers report today in PLoS Biology that although feathers of bigger birds do grow faster, the growth rate doesn't increase as quickly as the mass of the bird. So big birds take disproportionately longer to regrow their feathers.

That means big birds have to evolve different molting patterns to deal with their slowpoke feathers. Most birds that weigh more than 3 kg or so make their flight feathers last longer by molting them only once every 2 or 3 years, and some speed up the molt by losing a couple of feathers per wing at a time. Some big water birds, such as ducks and swans, molt all their feathers at once; they can still find food by swimming. The authors speculate that the largest known flying bird ever, a 70-kg raptor that lived in South America 6 million years ago, may have molted all its flight feathers at once and lived off its protein reserves while they grew back.

Big birds have to get around on incomplete wings while they're molting--which means that they may have more trouble finding food or avoiding predators. That could explain why there aren't any more 70-kg raptors flying around, the researchers say.

It would be interesting to see how flying on worn-out feathers, like those that the bird replaces by molting, affects fitness, says evolutionary biologist Folmer Bokma of Umeå University in Sweden, who was not affiliated with this study.

Bret Tobalske, a comparative biomechanist at the University of Montana, Missoula, praises the approach of the study. "It's just remarkably new in terms of the insight it gives about body size of birds," he says.