Some Birds Adorn Their Nests With Snakeskin to Scare Off Predators, New Study Finds

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Naturalists have long been puzzled to find shed snakeskin in bird nests, often tucked right under the eggs. Now, new research has finally confirmed the purpose behind the odd décor: The scaly leavings serve as predator repellent for cavity-nesting birds, scaring their enemies away and ensuring more of the eggs survive.

“It’s such a bizarre material to find in a bird nest that I think it leaves an impression on a lot of ornithologists and natural historians,” says Vanya Rohwer, a curator at the Cornell University Museum of Vertebrates in Ithaca, New York, and lead author of the study, which is published in The American Naturalist. “You see something like that, and you think, ‘How did they find that? Where did they find that? And why did they bring it to their nest?’”

Rohwer and his colleagues weren’t the first to suspect that birds might use snakeskin to deter predators. But few studies have tested the idea, and those that did have yielded mixed results. For example, in 2006 researchers found that snakeskin was highly effective at protecting Great Crested Flycatcher nests from predators, but a study from 2011 found that snakeskin made no difference to predation in Great Reed Warbler nests.

Rohwer thought the difference might lie in the type of nest: Reed warblers build cup-shaped nests in the open, whereas Great Crested Flycatchers nest in cavities. While nests in tree holes or nest boxes are protected from large predators, they are vulnerable to intruders that can fit through the opening—precisely the sorts of small, furry creatures that snakes prey on. Additionally, for more than a century, naturalists have observed snakeskin more often in cavity nests than in other types of nests. 

To begin their research, Rohwer and his colleagues systematically tested years of anecdotal observations that cavity-nesting birds use snakeskin more frequently. A rigorous analysis that included 78 species reported to use the material showed that cavity nesters were indeed far more likely to be snakeskin users. This finding was supported by a unique historical resource: egg record cards. Egg collecting was a popular hobby in the late 19th and early 20th centuries, and people would record details such as nest materials on cards that they displayed with the eggs they found. Reviewing digitized cards of 9 species that used snakeskin, the researchers found that the 5 cavity nesters of the group used snakeskin 6.5 times more often. 

“The nest record cards are fantastic,” Rohwer says. “It just feels like you’re viewing a little slice of history and using beautiful handwritten descriptions.”

The researchers also tested four proposed explanations for why cavity-nesting birds are so fond of snakeskin. They found no support for the idea that male birds bring snakeskin home as a sort of romantic gift. Another possibility was that the sheds might foster a healthier microbiome, perhaps by emitting chemicals that affect the growth of bacteria. But they found no difference in the microbiomes of different types of nests, and when they added snakeskin to Eastern Bluebird and Tree Swallow nests, it had no effect on bacteria. Using the same bluebird and swallow nests, the team also tested whether skins protect nestlings from external parasites such as lice and blowflies. Again, they found no difference.

The payoff came when Rohwer and his colleagues set up artificial nests at intervals along a trail, baited them with quail eggs, and watched to see which eggs disappeared or were destroyed. The researchers used 63 nest boxes to simulate cavity nests and 84 abandoned American Robin nests, which are shaped like open cups. They put pieces of snakeskin in half of the nests of each type. While the snakeskin did not deter predators from the robin nests, it appeared to protect eggs in nest boxes. After 14 days, only 38 percent of nest boxes without snakeskin still had all their eggs, while nearly 75 percent of nest boxes with snakeskin were untouched by predators.

Trail cameras suggested why. The robin nests were attacked by a wider range of predators, including species such as American Crows and Blue Jays that are not often eaten by snakes. In contrast, the nest boxes were only attacked by small mammals such as flying squirrels and American red squirrels.  “If you are a nest predator that’s small enough to fit into a cavity and that’s also small enough to be regularly eaten by snakes, this is probably an effective defense,” Rohwer says.

The findings help explain why snakeskin is so common and widespread as a nesting material among cavity nesters. In fact, some cavity-nesting species such as Great Crested Flycatchers and Carolina Wrens use it so often the behavior is mentioned in Audubon’s online bird guide. The prevalence of snakeskin in nests is especially impressive given that snakeskins are relatively rare in the landscape, and birds must presumably work hard to find them. 

“The results of this study suggest it is worth the time and effort to do that, because it results in lower predation rates,” says Mark Mainwaring, an ornithologist at Bangor University in the United Kingdom, who was not involved in the study. “A lot of people talk about these things, but very few people have put in the effort to go out and test it, certainly as thoroughly as this project has done.”

People used to assume birds used whatever nesting material was nearby and convenient, but research is increasingly showing that their choices are careful and strategic, says Mainwaring. For example, Blue Tits gather rare fragrant plants to protect their chicks from bacteria, and Black Kites use scraps of white plastic as signals that they are strong enough to defend their nests.

Interestingly, snakeskin isn’t the only snake-related trick birds use to scare off predators. When a person or other animal disturbs a cavity nest, the birds inside will often hiss. “This hissing sound is very similar to a snake and quickly raises all hairs on the back of your neck,” Rohwer wrote to Audubon. Similarly, Burrowing Owls make a sound like a rattlesnake’s rattle.

While Rohwer’s research makes a strong case for snakeskin as predator deterrent, he notes that there could still be birds that use sheds for other reasons. Some species with cup-shaped nests also use snakeskin routinely, often twining it around the outside. “For some of these open cup nesters that use an excessive amount of snakeskin, it seems like there’s something interesting going on,” he says. “But we don’t know what it is yet.”