by Whit Gibbons

May 19, 2013

Q: My husband was excited recently because he witnessed a robin building a nest by recycling a shed skin from a snake that lives at the foundation of our house. We're not sure exactly what kind of snake. He's three feet long and darkish but is kind of shy and goes back in the hole when we get close. We both were surprised that a bird would have anything to do with any part of a snake, living or not. It was amazing how much skin the bird stuffed in its beak before flying off to the nest, where it incorporated the shed skin in with twigs and dead grass. Any thoughts on why a bird would do such a thing? We live in Clarksville, Tenn.

A: Great question and even greater observation--I am not aware of robins joining the ranks of birds using snake skins during nest building to scare off predators, which is the explanation for the phenomenon, but several bird species are known to do so. The great crested flycatcher places a shed snake skin in its nest and leaves it there until the young have fledged. 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 part of it into the nest itself.

The following is the response I gave a few years ago to a similar question about birds using snake skins in their nests. Ornithologists at Arkansas State University speculated that the snake skin is a deterrent to would-be egg predators that might themselves become prey to snakes. The scientists 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. Rat snakes would eat a bird egg, but flying squirrels are probably a greater threat. The investigators put up 60 nest boxes and placed quail and simulated eggs of modeling clay in each. They placed one or more snake skins in the nest in 40 of the boxes; 20 nest boxes had no snake skin.

The results supported 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. Flying squirrels ate eggs in 20 percent of the nests without snake skins.

Brett DeGregorio, a doctoral student from the University of Illinois who conducts research on snake predation on birds at the Savannah River Ecology Laboratory, has observed what he calls "a sad twist" on bird behavior that attempts to use snake skins to ward off predators. He says that "birds, particularly Blue Grosbeaks, on the Savannah River Site have certainly adopted this strategy [but] we see most of our grosbeaks incorporating cellophane wrappers, flagging tape, or other bits of litter into their nests. Perhaps they are fooled by these clear bits of debris into thinking they are snake skins." One of the goals of Brett's research is to determine if "blue grosbeaks that successfully incorporate snake sheds in their nests are more likely to avoid predation" of their nests. Or will a piece of cellophane fool a nest predator?

Despite the ongoing nationwide interest in bird-watching, many ecological mysteries about birds remain to be solved. Your observation about the robin adds to our knowledge of which birds use snake skins. And Brett's research may provide an example of how human litter is negatively affecting the natural world. By the way, your snake is probably a rat snake, which in your part of the country would be very dark or even black in color. They love to eat flying squirrels.

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