Molting Molds Flying Bird Size Limit

A study in the journal Public Library of Science Biology finds that it's the rate at which flying birds can replace their flight feathers that determines how big they can grow and still get off the ground. Karen Hopkin reports.

[The following is an exact transcript of this podcast.]

If you were a bird, how big could you be? Well, an ostrich can weigh 400 pounds—but it can’t get off the ground. So what if you want to fly? University of Washington scientists say it depends on how fast you can replace your flight feathers when you molt. The report is in the journal Public Library of Science Biology.

Feathers don’t last forever. So birds periodically shed their plumage and then sprout replacements. Small birds molt once or twice a year, and they replace their 9 or 10 primary flight feathers a few at a time over the course of a few weeks.

But bigger birds need bigger feathers. And that’s where the problem comes in. As birds increase in size, the rate at which they grow their feathers doesn’t keep up with their longer feather length. So if big birds tried to do like little birds, they’d spend the whole year molting—leaving little time for finding food or mates. [“Hey, whaddya doing?” “Still molting.”]

Some large birds, like honking big geese and 30-pound swans, get around the problem by forgoing flight while they get all their molting over with in one fell swoop. In other words, these birds of a feather defrock together.

—Karen Hopkin