You buy new clothes. Birds grow their own. The donning of fresh plumage is called molt, with new feathers replacing old.
The feathers we see are dead once completely grown. They break and show wear. They can’t be repaired, so they’re regularly replaced — through molt — on a schedule that varies greatly from species to species.
Molt is obvious on some birds. American goldfinches replace all feathers following the breeding season, a complete molt in the fall. Males go from bright to drab.
They undergo a partial molt as spring approaches, producing new contour feathers, those that define shape. These feathers also carry the bright colors of the breeding season.
Our chickadees molt, but you’d never know it. Molt brings no change in appearance for chickadees, or for our woodpeckers, jays and nuthatches, among others. It freshens plumage, replacing damaged or worn feathers. (If a feather is lost it is replaced immediately.)
House finches are yardbirds that do molt, the brighter red on males announcing spring.
First-year bald eagles are brown birds. They molt into plumages known as basic I, basic II, basic III, basic IV, finally reaching definitive plumage in the fifth year.
Definitive gives them the white head and tail most of us use to identify that bird. As bald eagles mature the white appears on a patchwork basis.
Generally speaking, the more white the older the young bird.
Warblers, as beginning birders learn, are easier (relative term) to identify in spring, when in alternate plumage. They molt after breeding to the less colorful basic plumage that creates identification problems in the fall.
I’ve long thought that the colorful plumage we enjoy spring and summer should be called basic. It’s certainly basic to our ID efforts. But it’s termed alternate, because basic is what these migrants, tropical in origin, wear returning to their homeland for the winter.
A red-winged blackbird molts once a year, in late summer. New contour feathers show brown or buff edging. This will wear away by spring, giving us the shiny black bird with bright shoulder patches.
Feathers appear on a bird’s body in what are called tracts, providing organized placement. The feathers don’t pop up in a haphazard fashion. There is bare skin between tracts. Feathers lie flat to provide complete coverage.
The feathers grow encased in a sheath. “The Handbook of Bird Biology” from the Cornell Lab of Ornithology, my source for much of this material, describes sheaths as mailing tube look-alikes.
The sheath covering wears away, allowing the feather vanes to slowly expand. The feathers are packaged that way to allow them to grow densely without crowding.
The replacement feather pushes out the old feather. This can explain the occasional feather you find on the ground. Feathers, by the way, are made of the protein keratin, found as well in hair, nails, horns, claws, hooves and the outer layer of skin.
Most birds molt in an orderly process, never losing so many feathers at one time that the bird is endangered in any way.
Waterbirds, however — ducks, geese, loons, grebes, swans — do lose all flight feathers in a rapid molt. This is because the ratio of wing area to body mass requires a full set of wing feathers for flight. Losing them piecemeal would make flight difficult.
You probably won’t see waterfowl in this condition. They find safe, secluded places for this molt, like the middle of a quiet lake.
Molt is an energy-expensive process. Birds don’t molt during breeding season with its intense energy demands. Migration is similar. Some birds molt before migration, some after, with others splitting molt into before and after. It depends on the species.
Blue jays and cardinals molt, but also sometimes go bald. According to Cornell, why is a mystery. (No, they are not all males.)
Lifelong birder Jim Williams can be reached at firstname.lastname@example.org. Join his conversation about birds at startribune.com/wingnut.