Cavity-nesting birds use snake skin to protect young from predators
When a bird adorns its nest with shed snake skin, it is not merely an aesthetic choice for home decoration. For certain avian species, this practice serves as a deterrent against predators.
Researchers have integrated both contemporary and historical data to demonstrate that birds nesting in cavities—structures with limited openings—are more inclined to incorporate shed snake skins into their nests compared to those that construct open-cup nests. This behavior is believed to help protect their eggs from predation.
"What do snakes consume? They primarily prey on mice and small mammals," stated Vanya Rohwer, a senior research associate in the Department of Ecology and Evolutionary Biology at the College of Agriculture and Life Sciences, and the principal author of "The Evolution of Using Shed Snake Skin in Bird Nests," published in the journal The American Naturalist.
"We hypothesize that an evolutionary history of detrimental interactions between small-bodied avian predators and snakes should instill a fear of snake skin within a nest," Rohwer explained. "This may influence their decision-making regarding whether to approach a nest."
Birdwatchers have observed the incorporation of snake skins in nests for centuries and have speculated that this behavior is more prevalent in cavity nests; however, this theory had not been empirically tested until now, according to Rohwer, who also serves as a curator at the Cornell University Museum of Vertebrates, located at the Lab of Ornithology.
"We aimed to investigate the rationale behind birds dedicating significant time and effort to sourcing this unusual material," he remarked.
Their research commenced with a comprehensive literature review, utilizing the Birds of the World database and academic publications that referenced snake skin and bird nests. They compiled data on the bird species that utilize snake skin and the types of nests they construct, discovering that this behavior is exclusively associated with passerines, or perching birds. Furthermore, within the passerine group, those that build cavity nests were found to be significantly more likely to use snake skins.
The researchers concentrated on North American avian species and subsequently examined historical nest record cards archived at the Western Foundation of Vertebrate Zoology. These records, which span from the late 1800s to the 1950s, were created during a time when the collection of nests and eggs was particularly popular. Early ornithologists documented details regarding nest construction, bird species, and geographical locations throughout the United States. The data obtained from these cards aligned with their previous research findings.
"The incidence of nests containing snake skin in their descriptions was approximately 6.5 times greater in cavity nests compared to those of open cup nesters," stated Rohwer. "This was quite fascinating, as it indicated that we have two entirely independent data sources conveying a remarkably similar narrative."
To investigate the potential advantages that cavity-nesting birds might derive from the presence of snake skin, the researchers examined whether it could diminish nest predation, reduce harmful ectoparasites, alter microbial communities in beneficial ways for the birds, or serve as an indicator of parental quality, thereby enhancing parental investment in offspring care. Among these hypotheses, their findings supported the idea of reduced nest predation, but exclusively in cavity nests.
For this study, the researchers placed two quail eggs in over 60 nest boxes and 80 inactive American robin nests situated in the Monkey Run Natural Area in Ithaca, simulating both cavity and open-cup nesting environments. Some nests were equipped with snake skins sourced from a local snake breeder, while others were left without.
Every three days over a two-week period, the researchers utilized a ladder to access the nests and monitor the eggs. Trail cameras indicated that small mammals and avian predators frequented the open-cup nests, whereas only small mammals, specifically flying squirrels, were observed visiting the nest boxes.
"If you were situated in one of those nest boxes with snake skin, your chances of surviving that 14-day duration were significantly higher," Rohwer remarked. "The advantages of this material are most pronounced in cavity nests."
16 January 2025
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