Common raven
Common raven
While watching a northern raven circling overhead the other day, I noticed some discrepancies in its body feathering. There were notable gaps and notches in the wings and tail feathers. Why was this so? This raven was clearly going through stages of seasonal feather molt.

We often hear the phrase “light as a feather” and, for birds, feathers are a necessity of life that provides them with mobility, mating prowess and protection from the elements. Made of keratin protein, feathers are analogous to our human hair and fingernails. Once fully formed, feathers become “dead” structures that must be regularly replaced through an orderly molt process triggered by seasonal hormonal changes. Damaged feathers can’t heal themselves and must be replaced. If an entire feather is lost, however, that feather will begin growing back immediately. That’s why people clip the wing feathers of captive birds rather than plucking them out.

Depending on the given species, the molt cycle varies to some degree for small and larger birds. Many smaller birds (that acquire adult plumage in a single year) replace all feathers once — and may also replace certain feather groups a second time later on. Chickadees, flycatchers, jays, vireos and swallows are examples of the once-per-year phenomenon.

Buntings, tanagers and warblers renew all of their feathers after the nesting season and then assume a duller-looking “winter plumage” for part of the year. Before the next breeding season, spring warblers undergo a partial molt of body feathers to achieve the males’ colorful breeding plumages.

A few species undergo two complete molts per year for practical reasons. Consider marsh wrens that rub perpetually against abrasive cattails, gradually deteriorating their outer feathers. Bobolinks face a different challenge — intercontinental distances. Twice a year, bobolinks cover several thousand miles between northern U.S. and Canadian nest sites and wintering grounds near the tip of South America. This staggering journey requires an intact set of durable, freshly grown flight feathers. By mid-August, Maine’s male bobolinks have already traded their contrasting black-and- cream spring wardrobe for a brown and streaky appearance.

Huge birds such as eagles don’t grow a complete set of flight feathers every year. It takes about five years for eagles to attain full adult plumage, and feather production consumes a lot of energy. Instead, eagles replenish individual primaries and secondaries in a sequential manner over multiple years. Also, the timing of molt mustn’t interfere with nesting duties. Conveniently enough, food resources for nestlings are readily available within the span of the nesting period.

Waterfowl have their own unique molting scenario, losing all primary feathers simultaneously. Upon entering “eclipse plumage,” they become basically flightless for periods of 20 to 40 days. Species such as wood ducks retreat to secluded wooded swamps where they can feed and hide from predators. Molting geese graze on land but don’t venture too far from water at those times.

Beyond the molting of feathers, other factors may contribute to temporary changes to a bird’s appearance. These involve inevitable feather wear and fading of plumage due to UV radiation from the sun. Darker feathers containing melanin are most resistant to wear. Lice and bacteria can also create feather losses.

I once experienced a serendipitous example of feather molt in real time when a male harrier (a.k.a. Grey Ghost) and I crossed paths along an open stretch of windy blueberry barren. As the graceful raptor cut across the wind to alter course, a blade-like filament ejected from its right wing. I stood stock still as a black-tipped primary feather spiraled downward and landed directly at my feet.