Now is the time of year when I start looking closely at American goldfinches. During the winter, females and males are difficult to tell apart. The dark on the wings is black in males and dark brown in females. The wing bars, particularly the upper one, are a bit more yellow in males. In a month, telling males from females will be easy. The bright yellow body and the black cap on the forehead leave no doubt that such a bird is a male in his summer finery.

The transformation occurs through the process of molting, the replacement of older worn feathers pushed out by newly formed feathers from below. Molting is essential because feathers, marvelously light and strong, do wear down. These feathers must be replaced as they abrade or flight would be difficult and insulation poor.

With more than 11,000 species of birds in the world, generalizations about molting are hard to make. Most birds do molt their contour feathers (wing feathers, tail feathers and the large body feathers) twice a year. A bird usually molts all of its contour feathers in a sequenced fashion in the fall, leading to its basic plumage. In the spring, another molt occurs leading to the alternate plumage, the plumage of the breeding season. Our American goldfinches will soon be molting into their alternate plumage. The spring molt is often with the head, body and sometimes tail feathers replaced.

The distinction between basic and alternate plumage can be dramatic as in the warblers, tanagers and rose-breasted grosbeaks. The two plumages are similar in other birds such as gulls, sparrows, wrens, chickadees and nuthatches. Those birds with similar alternate and basic plumages still undergo two molts a year, despite their seemingly unchanging appearances.

Molting requires a significant amount of energy. The only activities in a bird’s life that rival the energetic cost of molting are nesting and migration. Each activity pushes a bird to its limit. No bird can do two of these three activities at once. A typical pattern for a migratory bird is molt into alternate plumage (often a partial molt), migrate north, nest, molt into basic plumage and migrate south.

Here are a couple of examples that demonstrate the energetic demands of molting. Some peregrine falcons breed on the Arctic tundra. The Arctic summer is not long enough to allow the falcons to nest and then molt into basic plumage. After nesting, the falcons begin a molt, replacing some of their flight feathers. They then migrate to their wintering quarters, forced south by the deteriorating weather. Once in their winter quarters, they resume their molt.

Yellow-breasted buntings have a broad breeding distribution in Europe and Asia, stretching from the Arctic tundra to central China. They winter in Southeast Asia and India. Like the Arctic peregrine falcons, the brief Arctic summer does not allow enough time for the buntings to nest and then molt before migration. Those high latitude birds migrate immediately after nesting to the lower Yangtze area of China. In that moderate climate, the birds undergo a complete molt and then continue their migration south on fresh feathers. Buntings nesting in the southern part of the breeding range have plenty of time to nest and then molt before they embark on their southward migration.

Two of our local bird species transform themselves from basic to alternate plumage without molting.

The dorsal black coloration of breeding snow buntings is actually present in basic-plumaged birds.

As the winter proceeds, the back feathers of a snow bunting erode, exposing the black coloration along the middle portion of each feather. The black is hidden in the winter by the shingle-like arrangement of overlapping feathers. The white spangles on winter Eurasian starlings are eroded in the same way, leading to the black alternate plumage. Ornithologists call this phenomenon molt by wear.

Herb Wilson teaches ornithology and other biology courses at Colby College and perches in South China. He welcomes reader comments and questions at:

whwilson@colby.edu