When a bird drapes its nest with snake skin, it isn't just making an interesting home décor choice. For some birds, it keeps predators at bay.
Researchers 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.
Researchers combed through a collection of historical nest cards to find observations about birds that use shed snake skin in their nests. This card from 1893 reports finding a crested flycatcher nest lined with snake skin "as usual" in a hole in an oak tree.
"What do snakes eat? They eat a lot of mice and small mammals," said Vanya Rohwer, senior research associate in the Department of Ecology and Evolutionary Biology in the College of Agriculture and Life Sciences and lead author of "The Evolution of Using Shed Snake Skin in Bird Nests," published Dec. 17 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, who is also a curator at the Cornell University Museum of Vertebrates, housed at the Lab of Ornithology.
"We were trying to address why birds are investing all this time and effort in finding this bizarre material," he said.
Their research started with a literature review, searching the Birds of the World database and for academic papers that mentioned snake skin and bird nests. They collated data on which bird species use snake skin and their nest type and found that this behavior is only associated with passerines, or perching birds. And within the passerine order, birds that built cavity nests rather than open-cup nests were significantly more likely to use snake skins.
Focusing on North American birds, the researchers then reviewed historical nest record cards housed at the Western Foundation of Vertebrate Zoology. These cards date back to the late 1800s through the 1950s when nest and egg collection were in vogue. The early birdwatchers recorded observations of nest construction, bird species and location across the U.S.
Data collected from these cards correlated with their earlier findings.
"The proportion of nests that had snake skin in the nest description was about 6.5 times higher in cavity nests compared to open cup nesters," Rohwer said. "So that was really, really neat, and that suggested to us that we have these two totally independent lines of data that are telling a very similar story."
To test what benefit cavity-nesting 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."
The study's co-authors include Jennifer Houtz, Ph.D. '23; Maren Vitousek, associate professor in the Department of Ecology and Evolutionary Biology; Robyn Bailey, extension support specialist in the Cornell Lab of Ornithology; and Eliot Miller, visiting scientist in the Cornell Lab of Ornithology.
