by Whit Gibbons

March 28, 2010

Birds are noticeably more active in warmer weather, leading to the following questions.

Q: Goldfinches around our yard are changing color. I know that the males will soon become bright yellow. Does the color of the feathers change, or do the birds grow new feathers?

A: Many species of birds take on breeding plumage much brighter than their drab fall and winter look. The answer to the question about feather colors is virtually the same for all of them, although the timing of the process is variable among species. The short answer is that the color of a feather is set until the feather is lost. Feathers are lost and replaced through molting, and for some species, like goldfinches, the new feathers that appear at the beginning of the breeding season are a brighter color. This prenuptial molt leads to the beautiful yellow plumage with the contrasting black wing bars displayed by male goldfinches in the summer. Sometime after nesting is complete, goldfinches molt again, replacing the colorful feathers with the more basic colors we see the rest of the year.

Other birds that take on a new appearance with breeding plumage after a prenuptial molt are male summer tanagers, which become bright red; indigo buntings, which become bright blue; and various warblers, which become more brightly colored. Once a feather is in place for any of them, it stays the same color until molting occurs. Injured feathers can be replaced singly, but most finches, tanagers, and warblers have a prebreeding molt that involves almost all of the feathers that have color.

Q: I have heard of a bird that puts a snake skin in its nest. Is this true, and if so, what is the purpose?

A: The great crested flycatcher is noted for placing a shed snake skin in its nest and leaving it there until the young have fledged. Other birds, including tufted titmice and blue grosbeaks, have been reported to do the same on occasion. Flycatchers, which nest in tree holes (or bird boxes), will often drape a snake skin on the outside of the nest cavity, as well as weaving others into the nest itself.

The phenomenon was reported in the scientific literature as early as the late 1800s, and ornithologists speculated that the snake skin is a deterrent to would-be egg predators that might themselves become prey to snakes. But establishing which predators might be deterred and whether the strategy really works has been difficult. Nonetheless, Elizabeth Medlin and Tom Risch of Arkansas State University conducted a convincing study that identifies a predatory culprit and suggests that the use of a snake skin as a greeting card is an effective deterrent.

Two species of animals that encompass most of the geographic nesting range of the great crested flycatcher are rat snakes and flying squirrels. Flying squirrels will eat bird eggs, and rat snakes will prey on flying squirrels. A rat snake would also eat a bird egg, but flying squirrels are probably a greater threat because of their voracious mammalian appetites and greater agility in getting from one nest cavity to another. Medlin and Risch performed a field experiment by putting up 60 nest boxes and placing quail eggs in each nest, as well as a simulated egg of modeling clay. The eggs served as indicators of whether the nest had been preyed upon. For some predators the clay egg registered tooth marks. The researchers placed a snake skin in the nest in 20 of the boxes; they placed one in the nest and one outside the nest in 20 more; 20 nest boxes had no snake skin.

The results were straightforward enough to support the contention that snake skins deter flying squirrels from entering a bird nest and eating the eggs. None of the 40 nests with snake skins were attacked. Of the nests without snake skins, 20 percent had eggs eaten by flying squirrels. Despite the nationwide interest in bird-watching that has persisted for well over a century, many ecological mysteries about birds remain to be solved.

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