You may have heard a bird watcher refer to a “confusing fall warbler,” and wonder what that meant. In simple terms, it means that male warblers in the fall look very different than they do in the spring, and are confusing to bird watchers who are trying to identify them. Warblers, and many other migratory songbirds, change their plumages in late summer from brilliant spring breeding garbs to more subtle, less conspicuous attire for winter, giving them a plainer look and making them more difficult to identify.
A good example of this is the male blackpoll warbler, which has a black cap in spring, but in fall, the black cap is gone, and the head feathers are gray like the rest of its body. The black-throated green warbler has a black throat in spring, but in fall, the black throat is gone, and in its place a white throat with a hint on some darker stripes. Female warblers are also less brightly marked, but the difference is not as noticeable as in males, because females are rarely as brightly colored, making them less conspicuous while tending nests.
Protective coloration is the likely reason why males become less colorful in fall. Survival in winter in subtle feathers becomes more important than attracting mates with bright feathers in summer.
All birds molt at the end of the breeding season, but in general, only those songbirds that migrate experience changes in their coloration. Most resident birds replace their spring and summer plumages with similar but denser winter coverings.
-- George H. Harrison