Why do male goldfinches change color?

Perhaps you've been noticing some odd-looking finches in your backyard lately. They're blotchy, not quite olive but not quite yellow. Those are male American Goldfinches, getting ready for this summer's mating season.

Male goldfinch in process of molting

All birds molt, but male American Goldfinches do it in style. In summer, they "change their clothes" to a bright lemon yellow, with a strikingly black cap, black tail feathers and bars of black and white on the wings. Male goldfinches are one of the brightest birds in the backyard, with some people mistakenly calling them "canaries."

In fall, the males start changing to wintertime olive. After six to eight weeks, they're almost indistinguishable from the drab-colored females, with the exception that the females look a bit grayer. Some goldfinches will migrate south but most remain through the winter. It's just that they're not so noticeable. This has the side benefit of keeping the birds safer from predators in the bleak winter foliage.

In the spring, when the males begin molting again, they gradually grow an entirely new set of feathers, one at a time. At the early stage of this process, the bird is part olive and part yellow. They may seem a bit ragged looking. Goldfinches do not have the ability to change colors, at least not in the same way as a chameleon changes its colors with its surroundings. The change in color is due to hormonal changes affecting carotenoid pigments. The hormones are triggered by the male's reproductive cycle. The brighter the male during breeding season in June, the greater they'll impress a female and mate.

Male goldfinch in summer

Some researchers also believe that the hormonal changes also contribute to the increase in daylight or changes in the bird's diet. 

By the way, the change in color isn't limited to the feathers. The bills of the both sexes also change color, from a dark bill in winter to a vivid yellow-orange in summer. After the dark-billed young fledge in late summer or early fall, the bills of adults again become darker from melatonin deposition, possibly because of hormonal changes.

American Goldfinches are among the last birds to nest in North America. Nesting occurs around early July when many of the birds' favorite plants have gone to seed. That enables the parents to find ample food for their youngster. The nests are built in maple, oak or other deciduous trees or in pines as far as 30 feet up. They prefer collecting plant down, such as thistle and milkweed, to provide the soft, downy fibers that the birds prefer.