A small bird flitted from twig to twig within a tangle of oak and cedar, avidly watched by a small group of bird-watchers. The bird acted like a warbler but we needed to catch a good look through binoculars to be certain of its identification. This was several weeks ago, and when we finally had it in our sights, confusion reigned for a moment or two.

The bird was distinctive, with a large white ring around each eye and brilliant chartreuse feathers on head and back. Then suddenly we knew what it was, a bird whose name fits it for only half of the year, a chestnut-sided warbler.

Whatever long-ago naturalist named it must have discovered this bird in the spring, because by late summer there's nothing chestnut about it. A warbler with a bright yellow head and a rusty wash along each flank (giving it its name) becomes a chartreuse and white warbler in August.

Many birds wear bright and vibrant feathers during the breeding season to advertise their health and vitality to potential mates (and to potential rivals). For many female songbirds, a male's bright plumage suggests that he'll be a good provider for their brood. After breeding season, however, feathers that draw attention may hinder a migratory bird's survival. Chestnut-sided warblers head down to Cuba, Belize and Venezuela in the fall, and will have a better chance of avoiding predators for the next six months if they blend in with the vegetation.

An energy drain

Birds' calendars are marked by three major annual events: molting, breeding and migration. Each of these in turn consumes nearly all of a bird's energy, so songbirds wait to produce an entirely new feather coat until they've finished raising the next generation but before heading out on migration.

With so many millions of birds out there, each molting so many thousands of feathers, it's a wonder we're not raking them up like fallen leaves. Of course, it doesn't work like that; birds lose a few feathers at a time and their bodies replace them before several more drop off to be replaced. As their molt progresses, I may find a blue jay feather under the bird feeder, or a neat black feather with white dots that fell from a downy woodpecker's wing. A full molt takes several weeks and a bird may either emerge looking just as it did before, as is the case of robins and chickadees, or somewhat different, as happens with many warblers.

By late fall, energy-rich fruits, nuts, berries, insects and seeds are abundant, good fuel for changing out all their feathers from head to tail. These new feathers will last for many months, before normal wear and tear, parasites and the elements take their toll, requiring another full molt.

That chestnut-sided warbler, along with most of its cousins and many other songbirds, will molt some eye-catching feathers next spring, before heading north.

Exceptions to the rule

Goldfinches have an unusual molt — they molt two complete feather coats each year, going through a full molt just before breeding season, and then repeating the process in the fall. Spring's goldfinches become that familiar bright yellow, while fall goldfinches show no gold at all, instead ending up as brown and taupe birds, convincing some that goldfinches leave our area for the winter.

Another way of achieving similar ends: Cardinal feathers change between fall and spring, but they achieve breeding plumage without undergoing a molt. Their new back feathers in autumn have a grayish cast, muting that standout red a bit as the landscape turns white. This might help them escape the notice of predators, and the darker color adds strength to the feathers. The gray gradually wears off during the winter, producing bright red males for courtship and breeding in spring.

"What is bird-watching but feather-watching," asks Thor Hanson, author of "Feathers," a fascinating look at birds and its unique outer covering. Feathers, after all, in their infinite variety of colors, patterns and shapes, define what a bird is.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at valwrites@comcast.net