Why Do Nuthatches Coat Their Nest Entrances With Sap?

From the Spring 2026 issue of Living Bird magazine. Subscribe now.

Since the 1940s, researchers and birders have noticed that Red-breasted Nuthatches have an odd habit: After excavating their nest cavities, these nuthatches gather resin from pine and other conifer trees and use their bills (and, in at least one instance, a piece of bark) to spread it around the entrances to their nest cavities.

But why? It’s easy for scientists to invent a plausible explanation for why a bird might do something like this, but it’s much trickier to actually prove why a behavior is useful. However, ornithologists in Arizona used a clever set of experiments to do just that. The study, published in December 2024 in the journal Functional Ecology, posits that Red-breasted Nuthatches are using sticky conifer sap as a sort of No Trespassing sign to ward off pests, predators, and other birds that might want to invade their space.

Red-breasted Nuthatches breed throughout much of northern and western North America, excavating cavities in dead and dying trees in which to lay their eggs. They nest in a range of tree species, but regardless of what sort of tree their nest cavity is in, they often seek out the sticky resin produced by conifers, bring it back in their bills, and plaster it around their front doors.

Over the years researchers have come up with several possible hypotheses to explain why they’d go to this trouble. Perhaps it’s just a meaningless leftover behavior, inherited from nuthatch ancestors that used mud as plaster to narrow the entrances to their nests and keep out larger birds and animals. On the other hand, the stickiness or chemical properties of the conifer resin itself could deter unwanted visitors—such as predators like squirrels that might feast on nuthatch eggs, or competitors like wrens that might kick nuthatch families out and use the cavities for themselves.

A possible answer backed by actual observational data was finally published in the December 2024 study, the result of a series of graduate student side projects over multiple decades. In the late 1990s, Cameron Ghalambor (now a professor at the Norwegian University of Science and Technology in Trondheim) was studying nuthatches in the mountains of Arizona for his PhD dissertation at the University of Montana. Ghalambor spent countless hours watching nuthatches at their nests, but finished his PhD with research about trade-offs in nuthatch incubation strategies and moved on to research about fish without ever getting his data on nuthatch resin-gathering published.

Decades later, James Mouton arrived at the same field site in the Mogollon Rim in Arizona as a field technician. Ghalambor’s old side project was still being discussed among researchers at the site. In 2014, Mouton and others decided to revisit Ghalambor’s experiments, as Mouton started his own research on cavity-nesting birds as a University of Montana graduate student.

Ghalambor’s nuthatch research more than 30 years ago included a small experiment to try to get at what was behind the resin-gathering behavior. The basic study design was simple, so Mouton and his colleagues picked it up and carried it forward in the 21st century, starting with setting up nest boxes.

“We can move them around, we can put them where we want them, and we can have total control over the situation,” Mouton says. Half of the nest boxes received a coating of resin around their entrances (applied with wooden popsicle sticks, since the researchers lacked bills), and half did not. To lure in potential nest predators, they also placed balls of cat food and peanut butter in some of the boxes as bait.

Then, the researchers observed what happened next. In the days that followed, squirrels were less likely to chew at the entrances of boxes with resin and enter to eat the cat food bait within, and Northern House Wrens were less likely to begin building nests in boxes with resin. The pine-scented goo deterred both predators and competitors. Additionally, Mouton and colleagues placed this resin use in the context of the broader nuthatch evolutionary tree, and discovered that although Red-breasted Nuthatch is distantly related to a set of species that plasters its nest entrances in mud, its closest relatives are not known to engage in any similar behaviors. That means the chance that this was an inherited behavior was slim.

Mouton says that the resin-spreading behavior likely represents a previously unrecognized example of a bird co-opting a plant’s defenses for its own use; resin helps conifers repel pests and seal wounds, and it’s easy for birds to borrow these benefits for themselves. He also says this type of relationship among birds and plants may be more common than currently recognized. 

“It’s a really compelling experiment because it’s so simple,” says Mouton, who made sure that Ghalambor was credited as a coauthor when the research was finally published. “[It] really gets after the function of the sap in particular.”

Virginia Tech’s Jeffrey Walters, an expert in bird behavioral ecology who was not involved with the research, praises Mouton’s work as an interesting look at an understudied topic. 

“I think it’s a really good paper,” Walters says. “I like the design of the experiments, the results are clear and easy to interpret, and they have really good hypotheses.”

Walters is known for his own research on Red-cockaded Woodpeckers, which have a strikingly similar habit—excavating resin wells in their nesting and roosting trees to deter snakes from climbing the trunks. Both Mouton and Walters are curious about the chemical properties of conifer resin and whether there might be more to its defensive powers than stickiness alone.

“We tried to get a couple of chemical ecologists to look into this, and never could,” says Walters, noting that he thinks that resin protects Red-cockaded Woodpeckers from parasites as well. “Resin seems to work against a lot of different things. … Is it some kind of chemical defense, or is it just the physical stickiness that other things want to avoid? I think that would be an interesting question to figure out.”

Mouton, who is now an assistant professor at The Ohio State University, says that the project was fun, and that the research gave him a new appreciation for what he considers to be nuthatch intelligence. The observational data included a nuthatch seen adding even more resin to its nest entrance after an encounter with a squirrel. One thing he won’t miss, however, is carting heavy wooden nest boxes through the forest.

That part “wasn’t not fun,” he says diplomatically. “But it was arduous.”