All snakes, most lizards and some politicians shed their skin.
Actually, all humans shed their skin, including the political subspecies. When considering how to discuss feather molting in birds, I first looked up to see what we do. We shed about 8 pounds of skin per year. The outer layer of skin is called the epidermis, and it’s composed of dead skin cells primarily made up of keratin. Keratin is the same substance that forms horns and hooves, claws and fingernails, porcupine quills and turtle shells. It’s tough stuff.
The type of keratin in feathers is even tougher. It’s got to be for birds to beat those wings against the sky. Humans continuously shed dead skin as it wears out and flakes off. Birds molt, shedding feathers at particular times of year and in specific patterns. That’s where things get complicated.
The fundamental reason to molt is to replace worn feathers. But there are additional reasons depending on the species. For instance, most songbirds rely on color to attract mates. It pays to be bright and beautiful in the spring. But once breeding season is over, it’s a liability. Predators spot you more easily.
This is especially true if you winter in the tropics, where there are many more critters that think you look tasty. Some brightly colored birds are acceptably safe in Maine’s summer but unacceptably threatened in Panama’s winter. When they molt into their fall plumages, they’re likely to go drab. It’s even true if Maine is the bird’s winter paradise. Snow buntings are dirt colored when breeding up north. They molt into a whiter color before wintering on Maine’s snow-covered fields. Sanderlings are browner in the subarctic summer. They turn whiter in winter to better match the color of our sandy beaches.
Color change means a bird molts twice per year, turning bright for spring and dull for fall. Warblers, tanagers and buntings employ this strategy. Birds that don’t change color can get away with molting once per year. Chickadees, jays, swallows, flycatchers, thrushes, hawks and hummingbirds molt annually.
Birds that live in harsher environments, brushing against vegetation a lot, wear out their feathers faster. Grassland species such as bobolinks and wetland species such as marsh wrens replace their feathers semi-annually.
Some birds grow into their adult plumage gradually. Bald eagles and herring gulls take four years to fully mature. With each annual molt, their new feathers take on more of the adult characteristics, making it possible to determine the age of immature birds. Youngsters benefit from their immature coloration. Territorial adults may be less likely to perceive them as a threat and drive them off.
The timing of the molt matters, too. Replacing feathers takes a lot of energy. Many birds do it after they’re done raising the kids. They get fresh feathers for the long flight ahead and to change color for the tropics. Many Maine birds are molting right now. Some birds molt just before breeding season, when food supplies are adequate and the stress of child-rearing is not yet upon them. They start the rigors of nesting season with fresh feathers.
Similar species may have different strategies. Most Maine-breeding flycatchers molt here, then fly south for the winter. The least flycatcher leaves earlier than others and molts upon arrival. It suffers the disadvantage of migrating on worn wings but grabs the best feeding territories before the other flycatchers get there.
Molting is a dangerous time. Every missing feather reduces a bird’s ability to eat, flee and stay warm. Raptors replace their flight feathers a few at a time so they won’t starve while waiting for new feathers. They also molt symmetrically, losing similar feathers on each side so their flight won’t become unbalanced. Molting is slow, and it can take up to two years to replace all feathers. Waterfowl can feed without flying, so they tend to molt all at once, putting up with a flightless period in order to get through the process more quickly.
Common eiders take it to an extreme. Once mating is over, males collect in big rafts offshore and begin molting together, remaining flightless in their bachelor flocks for a month. Females stay close to their foraging areas all summer and delay molting until the kids are on their own. If you’re wondering why you’re seeing only females right now, that’s why.
Researching this topic gives me pause. The Internet says that most household dust is exfoliated human skin. I’m going to go vacuum now.
Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at duchesne@midmaine.com.