Molting is the replacement of all or a substantial subset of a bird’s feathers. Birds generally molt once or twice a year; how often depends on the species. Some male birds molt into feathers called “eclipse plumage,” and although we humans also, in a way, have “eclipse plumage”—we put on special glasses and maybe even commemorative T-shirts for eclipses in the sky—eclipse plumage in birds actually has nothing to do with fleeting sun and moon phenomena. But more on that later.

Human males in eclipse “plumage” at NASA headquarters in Washington, DC

So why do birds molt? Just as human hair breaks off or gets split ends, and as the color of hair can fade a bit from exposure to sunshine or water, feathers also get damaged and start to lose their brilliance. Humans can decide they just don’t give a hoot if the ends of their hair are split, but birds can’t afford to have damaged feathers: feathers must be in good condition for flying, since accurate flying is crucial to the safety, feeding, and nesting of almost all bird species. Broken, bent, or damaged feathers cannot interlock fully to make wings function properly.

Feather sections under a microscope. Tiny filaments, called barbules, normally interlock to make a single, unified surface.
Illustration of how barbules interlock, somewhat like our Velcro®.

Although sometimes damage to a feather can stimulate the bird’s body to shed that feather and replace it earlier than at the next expected molt, in general the strategy of molting on a regular schedule works well for “tuning up” the body’s overall feather covering. (In the same way, we repair a car if it develops a sudden problem, but we also have regular tune-ups.)

The Broad-winged Hawk on the left has intact feathers; the one on the right clearly needs some replacements.

Birds also molt for two other reasons. First, as nestlings they shed the soft, warm “down” feathers of their babyhood (similarly, many humans shed their first set of baby hair, ending up bald for a while, and over time all human children shed their baby teeth). Young birds go through one or more sets of feathers before reaching full adult plumage. When juvenile birds go through several sets, they tend to do so in regular patterns that can be used to determine their ages. Thus, a Herring Gull in the first winter of its life will have different coloration than a third-winter one. Click through the gallery of photos here  to see the various plumages Herring Gulls go through on their way to adulthood (like the varying clothing styles of our young children versus our teens). You can also find a chart of Herring Gull plumages here.

Second, the males of many songbirds undergo a molt of all their feathers after the breeding season (but before migration, so that by the time they need to fly long distances, they have a new set of strong feathers). Then just before the next breeding season, they undergo a partial molt, replacing some of their dull, non-breeding, “basic” feathers with spectacular “alternate” feathers designed to attract females.

Molting Styles

Species whose feathers sustain the most damage have especially intense molting—such as Marsh Wrens, which move around a lot among sharp reeds. Marsh Wrens change ALL their feathers twice a year, whereas most birds that molt twice yearly only have a complete molt once a year, and change just some of their feathers during their second molting.

By contrast, some birds change their feathers in quite limited numbers at any one time so that they can retain optimal flying capacity at all times. This is true of the larger birds of prey: because they’re heavy, they can only afford to lose and start regrowing a small number of flight feathers at a given moment. Lose too many feathers at once, and they’ll be grounded. Their molting process is very gradual compared to that of smaller birds.

Yet another molting style is the system noted above: molting all the feathers after the breeding season, and then again partially molting just before the next breeding season. Both sexes undergo this second, partial molting, but only the males change to brilliant colors.

Marsh Wren hiding among abrasive reeds
Male Scarlet Tanagers: non-breeding on the left, breeding on the right.
Barn Swallows take 3-4 months to gradually molt all of their feathers and replace them. The bird on the left has a finished set of feathers; the one on the right looks scruffy because it’s molting.
The Loggerhead Shrike on the left looks the way we expect it to; the one on the right is molting.

Two more styles of molting bear mentioning. Some birds molt once a year into a relatively dull set of feathers after breeding, but hidden under those muted colors are brilliant hues that gradually become revealed as the outer edge of each feather rubs off through wear and tear. This is what happens with Northern Cardinals.

The male Northern Cardinal on the left is “revealing his true colors”—colors that were underneath the duller edges of the feathers he acquired during molting in late summer. The somewhat hilarious bird on right is in the process of molting.

Much more could be said about molting, but we’ll end with the style of many waterfowl, such as ducks and geese. These birds drop all their wings’ flight feathers at once—a risky strategy since they cannot fly for a few weeks and are more vulnerable to predators. Many ducks hide in quiet marshes and remote lakes during this period. The strategy seems to benefit these birds, however, because later when they need to haul their relatively heavy bodies on the long-distance treks of migration, using many many flaps of their wings, they will have a truly strong, capable, full set of flight feathers in which every single feather is new.

When they molt, the males in such species initially acquire a camouflaged appearance that makes them look a great deal like the females. This is called “eclipse plumage,” because the more typical brightness of their bodies has been eclipsed (hidden). Eclipse plumage sometimes only lasts a few months while a partial molt is bringing back the more colorful feathers. In such species, the males spend more of their time wearing fancy colors than other birds do. This can be because their courtships are quite long, beginning in autumn instead of late winter or spring as with many other birds.

Male Mallard Duck in breeding plumage
Male Mallard Duck molting
Male Mallard Duck in eclipse plumage; the slight bit of green on the cheek gives away its sex.

Christine M. Du Bois, Coordinator of the Lansdowne Bird Town and editor of Bird Beat

Image Credits:

2024 Total Solar Eclipse (NHQ202404080213) (cropped), by NASA Headquarters / NASA/Connie Moore. Public domain.

Feathers under a microscope 01, by Kateryna Martyniuk, licensed under CC BY-SA 4.0

Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget (1834) (14592598237) (cropped),
by Peter Mark Roget. Pubic domain under flickr Commons.

Broad-winged_hawk_(36537), by Rhododendrites, licensed under CC BY-SA 4.0

20240508_broad_winged_hawk_south_meadows_PD203773, by Paul Danese, licensed under CC BY-SA 4.0

Marsh_wren_MI_trip_7.18_cheboygan_DSC_0042, by lwolfartist, licensed under CC BY-2.0

Piranga_olivacea_Piranga_alinegra_Scarlet_Tanager_(6466749551), by Félix Uribe, licensed under CC BY-2.0

Scarlet_Tanager_-_Point_Pelee_-_Ontario_11052017-FJ0A4632_(28036926339), by Francesco Veronesi, licensed under CC BY-2.0

Rauchschwalbe_Hirundo_rustica (barn swallow), by Andreas Trepte, licensed under CC BY-SA 2.5

Barn_Swallow_Molting_(8712939016), by CheepShot, licensed under CC BY-2.0

Lanius_ludovicianus_Dobak (Loggerhead Shrike), by dobak, licensed under CC BY-2.0

Loggerhead_Shrike_molting, by Kevin Cole, licensed under CC BY-2.0

Northern_cardinal_in_CP_(02003) (cropped), by Rhododendrites, licensed under CC BY-SA 4.0

A_male_northern_cardinal_starting_it’s_molt, by Rwgp65, licensed under CC BY-SA 4.0

Male_mallard_duck_2, by Acarpentier, licensed under CC BY 3.0

Canard_au_bassin_Lairet_-_Parc_Cartier-Brébeuf (molting Mallard Duck), by Jstremblay, licensed under CC BY 3.0

Male_eclipse_Mallard_on_Lake_Bled, by Mihael Grmek , licensed under CC BY 3.0