As anyone who has read Make Way for Ducklings knows, adult ducks shed their flight feathers—the long, stiff plumes from their wings and tails—in late spring and early summer, and they can’t fly for about a month as the replacements grow. While most ducks don’t spend that time begging for peanuts in Boston, they do have to stick to swimming and waddling to get around. Here’s an inside look at what’s going on.
The process of shedding and replacing worn feathers is called molting. All birds do it at least once over the course of a year. That can add up to a lot of feathers, from roughly 1,000 on a ruby-throated hummingbird to more than 25,000 on a tundra swan, with an average of 1,500 to 3,000.
“They lose all their feathers, but not all at once,” says Christopher W. Leahy, the Gerard A. Bertrand chair of natural history and field ornithology at Mass Audubon and the author of The Birdwatcher’s Companion to North American Birdlife. “There’s a seasonality of molting that is different with different species.”
Some birds molt their flight feathers before migration, some after, and some along the way, Leahy says. Male birds often switch from a bright breeding plumage to a duller coat that’s less conspicuous for the rest of the year. Birds molt more times in their first year than they do when they’re older, going through different plumages specific to different stages of life. And some birds, like the arctic and alpine grouse called the ptarmigan, change gradually from tundra brown to snow white, camouflaging themselves from predators as the seasons change.
Molting is just one way birds maintain their intricate coats of feathers, which are essential for flight, insulation, and display or protection. Another is preening, a near-constant activity. Feathers become dirty, wet, and matted with natural oils in the course of daily life, and the filaments of the feathers, which link together with tiny hooklike barbs, get separated.
When preening, birds use their bills to reposition the feathers, reconnect the barbs, and add oil from a gland on their backs. “They’re applying this kind of hair tonic to seal the feathers,” Leahy says. “The feathers aren’t greasy or oily, but it gives them that smooth contour and makes them waterproof.”
Birds often precede preening with a bath. Bathing helps birds rid themselves of old oils and makes it easier to apply fresh oil evenly. Some birds, including larks and house sparrows, prefer dust baths, filling their plumage with fine dry soil or sand before shaking it out. This method, too, removes extra moisture and oil, and it flushes out lice.
Birds also need to pluck off potentially deadly parasites, such as mites, that feed on feathers. You might see a robin or a phoebe deliberately sit on an anthill and let the ants crawl over its plumage, or you might see a bird crush a pack of ants and rub the fluid on its feathers; both seem to work as delousing methods. Robins and other birds are even known to find a sunny spot where they can fluff out their plumage, bringing the parasites to the surface where the birds can pick them off more easily and eat them. If you see a bird in the sun, “acting weird, with its head cocked at an angle,” Leahy says, it might be getting rid of mites.
It’s not uncommon to spot feathers on the ground in all seasons. Unless you see signs of serious disease, such as a group of dead birds nearby, Leahy says it’s fine to pick up these castoffs and examine them. If you notice a break in the feathery part and run your finger along it to smooth it together again, you’ll be reconnecting the barbs, just like a bird does when it preens.