08/27/2026
Great crested flycatchers line their nests with shed snakeskin, and birdwatchers have been finding it inside their cavities and writing it down since at least 1893. A nest card from that year described a crested flycatcher nest lined with snake skin as usual in a hole in an oak tree. As usual. For over a century, naturalists assumed the birds were attracted to the texture or the shine. Some proposed they simply liked long flat translucent objects. In December 2024, Cornell researchers proved the snakeskin is a security system. Nests with it had zero predation over the test period. Nests without it had 20 percent.
Vanya Rohwer, senior research associate in Cornell's Department of Ecology and Evolutionary Biology and curator at the Cornell University Museum of Vertebrates, led the study published in The American Naturalist. His team combined three approaches: historical nest records dating back to the late 1800s, comparative analysis of which bird species use snakeskin and which do not, and a field experiment at the Monkey Run Natural Area near Ithaca, New York, that tested whether the skin actually deterred predators or just looked interesting.
The historical records established the pattern. Cavity-nesting birds, species that build inside tree holes, nest boxes, and enclosed spaces with small entrances, were far more likely to incorporate shed snakeskin than birds that build open-cup nests in branches or on the ground. The great crested flycatcher was the most consistent user, coiling entire shed skins inside the cavity and sometimes draping a second skin visibly outside the entrance so anything approaching the hole would see it before deciding to enter.
The experiment tested whether the skin worked. Rohwer's team placed two quail eggs inside more than 60 nest boxes and 80 inactive American robin nests across the study area. Some received shed snakeskin. Some did not. Trail cameras recorded every visitor.
The cameras revealed why cavity nests benefit and open nests do not. The robin nests sitting in the open were attacked by a wide range of predators including American crows and blue jays, large birds that are not eaten by snakes and have no reason to fear snakeskin. The nest boxes were attacked exclusively by small mammals, primarily southern flying squirrels and American red squirrels. Small mammals are eaten by snakes. A flying squirrel approaching a dark cavity, reaching the entrance, and encountering the scent or sight of snakeskin inside is a flying squirrel making a decision about whether to enter a space that smells like something that kills flying squirrels.
Rohwer described the logic directly: if you are a nest predator that is small enough to fit into a cavity and also small enough to be regularly eaten by snakes, this is probably an effective defense. The snakeskin does not fool the squirrel into thinking a live snake is inside. It introduces doubt. A cavity that smells like snake is a cavity that might contain one. The squirrel's survival depends on never being wrong about that assessment. Leaving is cheaper than checking.
The geographic overlap reinforced the interpretation. The southern flying squirrel's range across eastern North America matches the great crested flycatcher's range with striking precision. Where the squirrel lives, the flycatcher uses snakeskin. Where the squirrel does not, the behavior is less common. The bird's defense evolved in direct response to the predator it was designed to deter.
Image is for illustration purposes only
Source: Rohwer et al. (2024), The American Naturalist / Cornell University / Audubon / Phys.org / Sci.News.
