Cavity-nesting birds decorate with snake skin to deter predators

Scientists combined new and historical data to show birds that nest in cavities -- covered nests with small openings -- are more likely to use shed snake skins in their construction than birds that build open-cup nests, and this practice helps deter predators from eating the eggs.

"What do snakes eat? They eat a lot of mice and small mammals," said Vanya Rohwer, senior researcher and lead author of the paper published in the journal The American Naturalist.

"We think that an evolutionary history of harmful interactions between small-bodied predators of birds that are often eaten by snakes should make these predators afraid of snake skin inside of a nest," Rohwer said. "It might change their decision-making process of whether or not they're going to go into a nest."

Birdwatchers have documented the use of snake skins in nests for centuries and speculated that it occurs more in cavity nests, but no one had tested this theory, said Rohwer.

To test what benefit birds might be getting out of the snake skin, the researchers explored if snake skin could reduce nest predation, reduce harmful nest ectoparasites, change microbial communities in ways that benefit birds or function as a signal of parental quality and increase the effort parents make in raising their young. Of these ideas, their results supported the nest predation hypothesis, but only in cavity nests.

For this experiment, the researchers placed two quail eggs inside more than 60 nest boxes and 80 inactive American robin nests placed around the Monkey Run Natural Area in Ithaca to simulate cavity and open-cup nests. Some nests received snake skins collected from a local snake breeder, and others did not.

Every three days for two weeks, researchers carried a ladder through Monkey Run to climb up to the nests and check on the eggs.

Trail cameras revealed that small mammals and avian nest predators visited open-cup nests, while only small mammals -- namely flying squirrels -- visited the nest boxes.

"If you were in one of those nest boxes and you had snake skin, you had a much higher chance of surviving that 14-day period," Rohwer said. "The benefits of the material are most strongly expressed in cavity nests."