If you walk along a shore beside a channel where salmon run in late summer, you may find the trees full of shabby, tattered eagles and the ground below them littered with shed feathers. Or paddle up into Wachusett Inlet in Glacier Bay at that time of year and find the water surface covered with shed feathers of Canada geese. It’s the molting season for lots of birds, when many of our birds typically get rid of old, worn feathers and replace them with new ones.
Feathers were a marvelous invention; in the far distant past, some reptilian ancestors of the birds began to develop them (for reasons still debated). However they first arose, they have proved useful in many ways: insulation, rain-coat, sunburn protection, camouflage, sexual attraction, social display, and flight.
Feathers are made of keratin, a protein that provides light-weight strength, flexibility, colorfastness, and some durability. Feather keratins are very different from those that make our fingernails and hair, or those in cow horns and deer hooves. They are more similar to those of reptiles, reflecting the evolution of birds from dinosaurian stock.
All feathers develop in permanent follicles in the skin—a little dimple supplied with nerves, muscles, and blood vessels. The center of each follicle is living tissue around which the growing feather develops. Developing feathers are continually supplied with nutrients carried by the blood, so if you yank them out or cut them too close to the body, they will bleed. This is very different from mammalian hair, which comes from a different kind of follicle; it’s just pushed out from the living root as a strand of dead cells—that’s why hair doesn’t bleed or hurt when it’s cut. Once a feather is fully developed, the blood supply is cut off and the mature quill is hollow.
Every follicle has the capacity to produce a feather during the whole life of the bird. But feather development can be turned on or shut down according to the season. So, for example, some songbirds produce colorful plumage for the spring breeding season but shed those colors for camouflage when that season is over. Or the molt may be interrupted during migration and resumed on the wintering ground. A given follicle can produce different kinds or colors of feathers during the life of a bird: downy feathers for a nestling, then juvenile feathers for a fledgling, then adult feathers; or brilliantly colored adult feathers for the breeding season but camouflage patterns for the nonbreeding season. All of that is under the control of a set of special signaling genes that tell cells when and how to grow (these are known as ‘hedgehog genes’, because their malfunction makes a fly larva look like a bristly hedgehog. Hedgehog genes occur in virtually all animals, however).
Unlike mammalian hair and fur, every feather can be moved individually, fluffed up or flattened out according to need. Chilly temperatures? Fluff up the body feathers to increase insulation. Or raise a crest to assert social position. Or puff out the throat feathers to show off an attractive color. Or make minute adjustments to the plumage in flight to improve aerodynamic efficiency. Furthermore, a single broken feather can be replaced, when needed.
Birds carry a lot of feathers around. A small songbird might have two to four thousand feathers, while a big bird such as a tundra swan may have around twenty-five thousand feathers. Despite the light weight of each feather, altogether the plumage commonly weighs two to three times the weight of the bird’s bones.
Although feathers are very strong for their weight, they are subject to wear and tear. Reportedly, white feathers are especially delicate, lacking certain protective pigments. Feather edges fray, the quill may break (in a fight or while escaping from a predator, for instance), or feather-eating parasites may weaken whole patches of feathers. Contact with vegetation or simply lots of flying can cause abrasion. Worn feathers are not very good for insulation or flight or showing off or anything else, so they have to be replaced periodically—annually, at least, and in some cases more often.
However, shedding and replacing all those feathers is an expensive business. Some of the costs come from the loss of feathers; insulation is poor, for instance, so there is an increased metabolic (energy) cost for staying warm. Loss of flight feathers on the wing makes flying less efficient; in some cases, all the flight feathers are lost, leaving the bird flightless for a few days or weeks, during which it is more vulnerable to predators. Other costs are incurred by the synthesis of new feather structure; this requires protein and energy. Energy consumption during molt can be as much as fifty-eight percent higher than basal metabolism. Also, the synthesis of keratin protein requires certain sulfur-containing amino acids, which may necessitate selective foraging to obtain this special nutrition.
Some species of bird pay the costs of molting after paying all the sometimes-high expenses of the nesting season. But some birds molt and breed at the same time—doubly expensive! For example, birds that nest in the High Arctic generally molt at least some feathers while nesting, in order to get both things accomplished before it’s time to migrate south (another big expense).
It’s expensive, being a bird! Birds typically have higher basal metabolic rates than humans do and maintain a higher body temperature. Add in the costs of reproduction and molting and migration—and you clearly get a high cost of living!