Cavity-Nesting Birds Use Shed Snake Skin in Nest Construction to Deter Predators

Many species of birds use shed snake skin in nest construction, but this behavior is poorly understood. In new research, ornithologists at Cornell University used comparative and experimental approaches to suggest that the evolution of this behavior is mediated by nest morphology and predator communities.

They used a series of experiments and comparisons to test four hypotheses whereby snake skin could award fitness benefits (nest predation, nest microbiotas, nest ectoparasites, social signaling) and found support for the predation hypothesis.

“What do snakes eat? They eat a lot of mice and small mammals,” said Dr. Vanya Rohwer, lead author on the study.

“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.”

“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.”

“We were trying to address why birds are investing all this time and effort in finding this bizarre material.”

First, Dr. Rohwer and colleagues reviewed the literature and found that 78 species from 22 families have been reported to use shed snake skin in nest construction.

All but one of these species were passerines and, using comparative analyses, the researchers found that this behavior was disproportionately observed in cavity-nesting species.

Second, they examined a subsample of North American species, all of which were reported to use snake skin in nest construction, to see whether the proportion of nests with snake skin differs between cavity- and open cup-nesting species.

The analysis suggested that the proportion of nests with snake skin was roughly 6.5 times higher in cavity- than in open cup-nesting species.

“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,” Dr. 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 scientists 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 authors 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, the team 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,” Dr. Rohwer said.

“The benefits of the material are most strongly expressed in cavity nests.”

The team’s results appear in The American Naturalist.

Citation:

Vanya G. Rohwer et al. The Evolution of Using Shed Snake Skin in Bird Nests. The American Naturalist, published online December 17, 2024; doi: 10.1086/733208

This article was first published by Sci-News on 22 January 2025. Lead Image: The great-crested flycatcher (Myiarchus crinitus) is notorious for using shed snake skin in its nest construction. Image credit: Barbara Taylor / Macaulay Library.

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