All snakes, most lizards and some politicians shed their skins.
Actually, all humans shed their skin, including the political subspecies. In fact, we shed about 30,000-40,000 skin cells per minute, totaling over eight pounds per year. Our skin completely renews itself every 28 days.
Replacing our outer covering is straightforward and routine for humans. However, this isn’t the case for birds.
Replacing feathers is complicated, varying widely by species and sex. Skin and feathers are both made of keratin, but the keratin in feathers is thicker and tougher. It has to be. Besides beating their wings against the air, birds also brush up against stuff. Feathers suffer a great deal of wear.
It takes a lot of energy to replace feathers, raise chicks and migrate. Birds can only do one of these things at a time. Since the timing of migration and nesting varies by species, each species has its own molting strategy.
Some bird species molt in summer before migrating, so they’ll have brand-new feathers for the long journey. Others molt after migration, so they can spend the winter foraging with fresh plumage.
Similar species may have different strategies. Most Maine-breeding flycatchers molt here, then fly south for the winter. The smallest of the bunch — the least flycatcher — leaves earlier than others, and molts upon arrival in Central America. It suffers the disadvantage of migrating on worn wings, but beats larger flycatchers to the best feeding territories.
Some birds change color when molting, altering their plumage to match their habitat. Maine’s scarlet tanager is arguably the reddest color in nature. But the male turns olive-green before heading for Panama, to better elude the jungle predators.
Red knots and red phalaropes are shorebirds named for their brilliant breeding plumages. However, there’s hardly a hint of red in their winter feathers.
Sanderlings are shorebirds with seasonally changing plumage — dirt-colored on their tundra breeding grounds, sandy beach white in Maine in winter.
Snow buntings turn white like snow as winter approaches, then molt to brown for camouflage on subarctic tundra in summer.This change is especially clever. Light-pigmented feathers wear out faster than dark ones. When snow buntings molt before winter, brown feathers grow underneath the white. As the white feathers degrade over time, their plumage transitions to brown without having to molt again.
Warblers change color twice a year, molting to match where they’re migrating. Birds that don’t change color — chickadees, jays, swallows, flycatchers, thrushes, hawks and hummingbirds, for instance — molt only once annually.
Birds living in harsh environments wear out their feathers faster, and molt more often. Male bobolinks return from their winter grounds in Argentina dressed in a dashing, albeit upside-down, tuxedo. But in late summer, before they head for the equator, they change into a plumage you’d barely recognize.
Fledgling songbirds often leave the nest disguised in streaky juvenile plumage. They need camouflage until they get wise to the world. Within a few weeks, they’ll start molting into a more adult plumage.
Some birds take years to grow into their adult plumage. Bald eagles take over four years. With each annual molt, their new feathers take on more of the adult coloration, making it possible to determine the age of immature birds. Their age-related color-coding probably helps sort out their social interactions.
Raptors require speed and agility. Too many missing feathers can reduce hunting success. Accordingly, hawks replace their flight feathers just a few at a time, and it can take up to two years to regrow all of them.
Ducks, conversely, can feed without flying. Most duck species replace all their feathers over a brief period, rendering them temporarily flightless.
Common eiders take it to an extreme. Once mating is over, males collect in big rafts offshore and begin molting en masse, remaining flightless and clustered in their bachelor flocks for a month. You might have wondered why you’re seeing only females and immature eiders right now. Females stay close to shore, and delay molting until the kids are out on their own.
Crows amuse me. The kids run their parents ragged throughout early summer. Once the fledglings are old enough to forage for themselves, the parents molt. You’ll see family groups of four or five crows just about everywhere right now. The adults are the ones missing flight feathers. The kids aren’t molting — their feathers are brand new.
Fun fact: In the time it took to write this column, I lost 2,700,000 skin cells. No wonder I’m tired.