In 1889, the naturalist Allan Octavian Hume wrote that he was puzzled by macabre decorations he observed in many birds’ nests: strips of dried snakeskin.
“Are birds superstitious, I wonder? Do they believe in charms?” he wrote in “The Nests and Eggs of Indian Birds.” If not, why were so many birds using pieces of snakeskin to adorn their nests? Hume and several of his contemporaries had a hypothesis: The snakeskin scared away predators.
A new study suggests that they were onto something: After analyzing century-old records of birds’ nests and observing over 140 nests with and without snakeskin, researchers reported last month in The American Naturalist that in some types of nests, the presence of snakeskin greatly reduced the risk that predators would take the eggs.
All reptiles shed patches of dead skin as they grow, but snakes shed skins off their entire bodies in one big piece. However, finding a snakeskin in the wild can be tricky, said Vanya Rohwer, a curator at the Cornell University Museum of Vertebrates and an author of the study. This scarcity of snakeskin makes it all the more remarkable that so many birds use it in their nests.
“How in the world are they finding it? And why do they invest all that time to bring it back to their nests?” Dr. Rohwer said.
Dr. Rohwer delved into digitized historical records of birds’ nesting behaviors, including handwritten observations of nests from over a century ago. He and his colleagues determined that cavity nesters — birds that build their nests in holes in structures like trees and cliffs — are six and a half times more likely to incorporate snakeskin into their nests than species that create more “classic” cup-shaped nests.
Based on this information, Dr. Rohwer devised a series of experiments to “try and understand what the benefits of shed snakeskin might be.” He surveyed the microbes and larger parasites in nests with and without snakeskin to see if the skin warded off critters like fleas and mites or reduced the nest’s harmful microbes. There didn’t seem to be any correlations. “The other idea that we looked at was a nest predation idea,” Dr. Rohwer said.
In a forested area called Monkey Run near the Cornell campus in Ithaca, N.Y., Dr. Rohwer put quail eggs into 65 nest boxes, which are like cavity nests, and 80 empty robin’s nests, which are open-cup nests. He added snakeskin to half of the nests and spent the next several weeks checking on them with a ladder. “My wife nicknamed me Ladder Man,” he said.
During his stint as Ladder Man, Dr. Rohwer found something surprising: While the snakeskin didn’t seem to make much difference to the safety of the eggs in the open-cup nests, the scaly scraps made a big difference in the nest boxes.
“The cavity nests with snakeskin had a much higher chance of surviving a 14-day incubation period compared to a cavity nest without snakeskin,” he said. In particular, snakeskin seemed to deter small mammals like mice, which are known to take eggs.
This discovery suggested additional questions. “What is scary about snakeskin? Is it the smell of snakeskin? Is it the sight of snakeskin to these small mammals?” Dr. Rohwer said.
Ross Crates, an ecologist at the Australian National University who was not involved in the study, noted that other research had shown that some birds hissed to ward off predators to their nests. “Pretending that there’s a snake in the nest in some way is beneficial for these small, hole-nesting birds that have less capacity to actually physically defend the nest against larger, mostly mammalian predators,” Dr. Crates suggested.
Dr. Rohwer said that nests had been overlooked in scientific research, in part because they can be difficult to observe and study. Digitized data sets like the ones he and his colleagues used are helping researchers to make discoveries. “We’re only now just starting to really look at some of the unique materials in bird nests,” he said.