Why some birds add snake skin to their nests: The reason is brilliant

If you've ever seen a bird's nest lined with what looks like a snake's shed skin, you're not imagining things. A study published in February 2025 edition of The American Naturalist found that cavity-nesting birds use snake sheds to deter would-be predators from eating their eggs and nestlings.

The behavior's been documented for centuries. Birdwatchers have been noting it since at least the 1800s. But until recently, it was treated more like a curiosity than something worth understanding. Vanya Rohwer, the study's lead author and curator of birds and mammals at Cornell University, realized there must be some ecological motivation behind the behavior. So he and his team decided to find out why birds were actually investing all that time and effort into finding something as strange as a shed snakeskin.

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Here's the simple logic: snakes eat a lot of mice and small mammals. Those same small mammals are the predators that threaten bird eggs and nestlings. So the theory is that having a snakeskin in your nest might scare away the small mammals that would otherwise ransack it. According to Rohwer, birds with cavity nests think that an evolutionary history of harmful interactions between these small predators and snakes should make those predators afraid of snake skin. In other words, the skin signals danger to the very creatures that threaten the birds' offspring.
It's not foolproof. But it might be enough to make a mouse or squirrel think twice.

A study reviewed the literature and found that 78 species from 22 families have been reported to use shed snake skin in nest construction, with all but one being passerines—or perching birds. But here's where it gets interesting. Not all birds use this strategy equally. The proportion of nests with snake skin is roughly 6.5 times higher in cavity-nesting species than in open cup-nesting species.

This matters because it shows birds aren't just randomly grabbing interesting materials. They're making calculated choices based on their specific vulnerabilities. A cavity nest—those enclosed nests with small openings—faces different predator pressures than an open nest exposed on a branch. For cavity nesters, the snakeskin apparently works. For birds building open nests, it doesn't move the needle as much.

Researchers tested four different hypotheses about what benefits snake skin might provide—including reducing nest parasites, altering microbial communities, and social signaling—and found the strongest support for the predation hypothesis. The snakeskin works primarily as a predator deterrent, not for some other reason scientists might have guessed.

The broader takeaway is something Rohwer himself pointed out: birds aren't just grabbing random nesting materials. He stressed that many questions about the use of snakeskin in bird nests remain unanswered, and this study demonstrates the need for further research into how and why birds choose their nesting materials. What we're seeing is sophisticated animal behavior—birds solving specific problems with specific solutions. They just happen to be problems most of us never even noticed they were having.