A bald-headed cardinal arrives at a feeder. The bird had lost his usually recognizable crest due to molting. Phil Coale/Associated Press

The nesting season has come to a close for many of our Maine birds. That means that many of our migratory breeding birds will be departing soon for their wintering grounds. Some have already left. Our swallows and many of our flycatchers are among the earliest migrants. We’ll have to wait until 2022 to see barn swallows and cliff swallows.

Warblers, vireos and thrushes will be streaming south in September. The sparrow migration will pick up speed in October.

Migration, even for short distances, is an arduous task that demands a high expenditure of energy. Two other events in a bird’s life entail equally high energy costs: nesting and molting. The costs of each activity are so high that no bird can do two of them at once.

There are about 11,000 species of birds in the world so exceptions always arise when one tries generalize about birds. That is certainly true of molting; exceptions to general patterns abound. Nevertheless, we can make some general observations on molting.

We can start by asking why birds molt. Feathers are remarkably tough and strong structures, particularly when one considers how light they are. However, feathers do degrade over time.

The flight feathers on the wings deform on every downward power stroke of a flying bird. This deformation eventually causes the feathers to wear down, particularly near the tips.

Driving rain, contact with vegetation in flight and sand blown by the wind abrade feathers. Birds also suffer significant feather damage from feather mites and some bacteria.

Worn feathers do not form a smooth, aerodynamic surface on a bird, increasing turbulence as it flies. The turbulence makes flying much less efficient. Eroded feathers do not trap air as readily as fresh feathers, reducing the insulating properties.

Generally, birds undergo one complete molt every year when every contour feather on their body is replaced. For a bird like a chipping sparrow, that entails replacing about 2,000 feathers. A swan will have to replace over 25,000. No wonder molting is so expensive.

The usual pattern is for the complete molt to occur after nesting but before any migration is begun. This sequence of nesting to molting to migration has obvious advantages. After the breeding season, food is generally abundant enough to allow a post-breeding bird to find enough energy to fuel its molt. Then, it is ready to migrate on fresh, efficient flight feathers.

A typical bird will have a second partial molt near the end of the winter. Some of the body feathers will be replaced, transforming, for example, a drab and olive male scarlet tanager into a stunning red bird. However, the flight feathers on the wings and tail are generally not replaced. So, the northward migration must be done on worn feathers.

Some birds do undergo two complete molts a year. Species that migrate very long distances and species that live in abrasive habitats (thorn scrub or coarse grass) replace all their feathers twice a year.

It’s easy to see evidence of molting in the flight feathers of a flying bird. The flight feathers are usually replaced in a sequence so that only a few feathers are missing at any time. The innermost primary feathers and the outermost secondary feathers are molted first. In a molting bird in flight, you can see gaps or shorter feathers showing the current stage of wing molt.

Geese, swans and ducks – as well as loons – opt for the fast track during their flight feather molt. All of the primary feathers are shed at once. Until the feathers regrow, these birds are flightless. The birds find sheltered wetlands with enough food to allow them to hide and feed as their primaries grow.

It’s fun now to start watching warblers as males turn from their gaudy breeding plumage to more muted colors.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu