Most of the dramatic seasonal changes in a bird’s appearance are the result of molt — that is, the replacement of old feathers with new.
A male Scarlet Tanager replaces the drab greenish feathers of winter with intense red in spring, and then molts back to greenish in the fall. All birds molt at least once a year (usually in late summer), and many species, like Scarlet Tanager, add a second molt in late winter that gives rise to the bright colors we enjoy in spring.
The process of molt allows birds to change their appearance dramatically, but it is expensive. Growing a full new set of feathers takes a lot of energy and resources. Consequently, many species molt just once a year. But that doesn’t mean they look the same year-round; they can take advantage of other means of changing their appearance.
Feathers are dead structures, like hair. From the moment they grow, they are subjected to sun, abrasion, and other wear and tear. The blackish pigment melanin, however, strengthens feathers, making the dark parts more resistant to wear. Birds use this fact to grow feathers that wear in specific ways.
For example, male Red-winged Blackbirds molt only in late summer, growing black feathers that have broad edges that are buff to rufous in hue. They give the fresh fall plumage a more cryptic color and pattern, and offer some protective cover to the black parts. Over the winter months, the paler brownish edges wear off, and by spring, the male’s body plumage is a uniform glossy black.
This kind of color change can be seen in many bird species. The black throat patch of the male House Sparrow, for example, is covered by whitish feather edges in fall and revealed gradually during the winter.
The brilliant red of species like Northern Cardinal and House Finch is veiled by thin grayish edges in fresh plumage in the fall and early winter.
Enjoy the delicate shades and patterns of the fresh-plumaged birds of fall, and keep an eye on them for changes over the course of the winter.
This article from David Sibley’s “ID Toolkit” column appeared in the September/October 2015 issue of BirdWatching.
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