Some birds use snake skins in their nests to deter predators

When birds incorporate snake skins into their nests, it’s not just for decoration – it’s a calculated strategy to ward off predators. 

A recent study led by researchers from Cornell University reveals that cavity-nesting birds are significantly more likely to use shed snake skins in their nests than birds that build open-cup nests. 

This behavior, found exclusively among perching birds (passerines), seems to deter predators like small mammals, giving cavity nesters a better chance of protecting their eggs.

The research, published in The American Naturalist, combines historical and contemporary data to investigate this unique nesting behavior and explore its benefits.

Why do birds use snake skins?

Predators of cavity-nesting birds often include small mammals that are also prey for snakes. By placing shed snake skins in their nests, these birds may exploit the natural fear these predators have of snakes.

“What do snakes eat? They eat a lot of mice and small mammals,” explained lead author Vanya Rohwer, a senior research associate at Cornell University. 

“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.”

This instinctive fear could cause predators to avoid nests lined with snake skins, effectively reducing the likelihood of predation. 

The researchers suggest that this behavior may have evolved as a specific defense mechanism to enhance the survival of eggs and chicks in cavity nests.

Tracing the behavior across species

Although birdwatchers have documented the use of snake skins in nests for centuries, this study is the first to systematically examine the behavior. 

Rohwer and his team began by conducting a comprehensive literature review, including data from Birds of the World, academic papers, and historical nest records housed at the Western Foundation of Vertebrate Zoology.

The findings revealed that the use of snake skins is exclusive to passerines, a group of birds that includes nearly half of all known bird species. Within this group, cavity-nesting birds were significantly more likely to include snake skins in their nests than birds that build open-cup nests.

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

To validate these findings, the researchers analyzed historical nest records spanning from the late 1800s to the 1950s. These records, meticulously kept by early birdwatchers, documented nest construction, species, and locations across the United States. 

The data aligned with the team’s initial findings: cavity nests were 6.5 times more likely to contain snake skins than open-cup nests.

“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,” Rohwer said.

Testing the snake skin hypothesis

To determine the benefits of using snake skins, the researchers conducted an experiment at the Monkey Run Natural Area in Ithaca, New York. They set up over 60 nest boxes to simulate cavity nests and 80 inactive American robin nests to represent open-cup nests. 

Half of the nests in each category were lined with snake skins collected from a local snake breeder, while the other half were left unlined.

Each nest was stocked with two quail eggs, and the team monitored them for two weeks, checking every three days. Trail cameras captured predator activity around the nests, providing crucial insights into the effectiveness of snake skins as a deterrent.

The results were clear: snake skins significantly reduced predation in cavity nests. Predators, including small mammals like flying squirrels, avoided nest boxes containing snake skins, giving those nests a higher chance of survival during the 14-day period.

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

Interestingly, snake skins did not provide the same level of protection for open-cup nests, which were visited by a wider range of predators, including birds. 

This suggests that the enclosed structure of cavity nests enhances the protective effect of snake skins by limiting predator access and amplifying the deterrent signal.

Exploring additional benefits of snake skins 

The researchers also considered other possible benefits of using snake skins, such as reducing nest parasites, altering microbial communities in the nest, or signaling parental quality. However, the results primarily supported the predation deterrence hypothesis, particularly for cavity nests.

The findings highlight the adaptability of birds in using their environment to protect their offspring. By selecting and incorporating such an unusual material, cavity-nesting birds demonstrate a sophisticated survival strategy that likely evolved to address specific ecological challenges.

Shedding light on evolutionary strategies

This study highlights the ingenuity of birds in adapting to their environments. The use of snake skins by cavity-nesting passerines represents a clever defense mechanism, reducing the risk of predation and increasing the likelihood of reproductive success.

While the behavior is most beneficial for cavity nests, it also highlights the complex interplay between predator-prey dynamics and nesting strategies. As Rohwer notes, this behavior reflects the resourcefulness of birds and their ability to exploit predator instincts to their advantage.

The research raises fascinating questions about the evolution of nesting behaviors in birds. The use of snake skins is not just a random quirk but a deliberate and effective survival tactic. The fact that cavity-nesting passerines are more likely to engage in this behavior suggests a deep evolutionary link between nest type and predator defense strategies.

By combining historical data with modern experiments, this study offers new insights into how birds adapt their nesting practices to enhance the safety of their young.

The research also highlights the importance of studying the intricate relationships between animals and their environments, reminding us of the innovative strategies that have evolved over millions of years.

Whether it’s the choice of nesting material or the structure of the nest itself, birds continue to surprise and inspire with their ingenuity in the face of life’s challenges.

Image Credit: Barbara Taylor/Macaulay Library

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