Woodpecker nest holes are valuable pieces of real estate that may be used by hundreds of other species over many years. Researchers are using the concept of “nest webs” to understand how this valuable resource passes from one owner to the next.
From the Spring 2025 issue of Living Bird magazine. Subscribe now.
When night falls in the forest, most birds tuck into nooks where they can feel secure—the V-shaped intersection of two tree boughs, a cluster of dense branches. Some may take to the eaves of nearby houses. Brown Creepers find a piece of shaggy bark to wedge their bodies under.
And many birds—sometimes more than half the species in a given woodland—stow themselves safely into a place we humans rarely (and barely) get to see: inside the hole of a tree.
Whether it’s a fragment of forest bordering an urban neighborhood, or an old-growth stand that’s been regenerating itself for millennia, tree cavities are a common and crucial part of habitat in every forest landscape.
When it comes to a cozy home, few places can deliver like a tree cavity, says Kevin McGowan, instructor for the Cornell Lab of Ornithology Bird Academy course The Wonderful World of Woodpeckers. McGowan says a big part of recognizing the contribution of woodpeckers to an ecosystem begins with understanding the value of a hole.
“Holes in trees make great homes for all kinds of animals,” says McGowan. “They’re off of the ground, easier to defend from predators, they stay dry when it rains. It makes them a perfect place to roost and to raise young.”
A study published in the journal Diversity and Distributions in 2017 found that nearly 20% of all bird species around the world rely on tree cavities for roosting or nesting, and a subsequent analysis found that there are some areas of the world where nearly all cavities occupied by birds are made by woodpeckers.
In North America in particular, scientists are finding that the nest holes excavated within trees offer critical safe harbors used by dozens of species of birds, mammals, reptiles, and insects—all thanks to a few VIPs : Very Important (Wood)Peckers.
Flicker Nests Have Widespread Appeal
Kathy Martin began studying woodpeckers and other cavity-nesting species in the mid-1990s, when she was a newly minted professor of forest science at the University of British Columbia. She had an idea that woodpeckers would have stories to tell about the health of forest ecosystems, and their ability to stay healthy in the face of timber harvests, at a time when logging interests were sizing up British Columbia’s vast interior forests.
Thirty years ago “there was a true environmental feeling in the industry,” Martin says. “[Timber] companies were interested in having ecologists weigh in on the management of their lands—people who would tell the real story and not just say that everything is fine.”
Woodpeckers were a logical group to study, she says: “The woodpeckers were … the ones that you would predict would be very sensitive to this kind of harvest,” including the selective logging of mature trees for the forest industry.
Over the first two years of studying the biological communities in these mixed Douglas-fir forests in western Canada, Martin and her research team recorded 32 bird species that used tree cavities, including primary excavators like woodpeckers, which can drill into a variety of tree types in different stages of decay; weaker excavators like chickadees and nuthatches, which can only excavate when the wood is already rotting; and a bevy of secondary cavity nesters, which are birds that use preexisting cavities but don’t excavate their own. The last group includes a range of species from Wood Ducks to Northern House Wrens. More broadly, Martin estimates that around 30% of all forest bird species in North America use tree cavities at some point of their life cycle, whether nesting, hiding from predators, or just finding a warm roosting place to snuggle in on a cold winter’s night.
Martin and her team also recorded cavity use by 11 species of mammals, from red squirrels to pine martens to fishers. In 1999 their study, published in The Condor, introduced the concept of a nest web—a way to describe the complex system of animals that make, enhance, and/or use tree cavities.
“It was based on the idea of the food web—you have all these species that are linked together because they are sharing the same resources,” says Martin, “and just like in food webs you have your producers and your consumers.”
In the forests Martin studies—mixed Douglas-fir, which includes pine and spruce along with aspen, poplar, and birch trees—the most important producer turned out to be Northern Flicker, which excavated nearly 50% of tree cavities observed. According to a recent estimate from the bird-monitoring alliance Partners in Flight, Northern Flicker is one of the most abundant woodpeckers in North America. PIF estimates there are about 12 million flickers across Canada, the U.S., and Mexico.
“I do think the flickers are so important because they have such a high abundance,” says Martin, “but it’s more than that.” Aside from making more holes than any other woodpeckers, Martin says they make a hole that’s big enough for many different species, but not so big that the entrance hole feels unsafe.
According to Martin’s research, flicker cavities emerged as the most important nesting resource for songbirds such as Mountain Bluebirds, raptors such as American Kestrels, and waterfowl such as Buffleheads and Hooded Mergansers. Some bird species even evict flickers from their cavity nests.
“[Flickers] are kind of wimps,” Martin says. “They don’t defend their cavities very well,” which means their nest holes are often commandeered by more aggressive birds like kestrels and starlings.
Importantly, only around 10% of the cavities Martin and her team found were so-called natural cavities—that is, cavities not started by an excavator, such as a hole in a tree at a broken bough caused by wind or ice, then rotted out by bacteria or fungi.
But, Martin says, “a lot of these natural decay cavities that form in older trees and snags are crappy cavities … not as secure, not deep enough” for safe bird nests.
In other parts of the world, natural cavities that form without the aid of woodpeckers play a much larger role in forests. A study published in Biodiversity and Conservation in 2017 found that in temperate rainforests in Chile, 75% of nests used by secondary cavity nesters—birds such as Blue-and-White Swallow and the Patagonian Sierra Finch that don’t drill their own cavities—were located in holes caused by tree decay. The other 25% of nests were in holes excavated by primary-cavity producers such as Magellanic Woodpecker and White-throated Treerunner, a nuthatch-like species.
That pattern holds true for most places outside of North America where nest webs have been studied. According to an analysis published in Frontiers in Ecology and Evolution in 2011, avian excavators (mostly woodpeckers) produced around 77% of nesting cavities in North America, but only an average of 26% across Europe and South America, and 0% in Australasia (where woodpeckers do not occur).
Kristina Cockle, lead author of the study, explains the discrepancy is in part due to the difference in tree structures around the world. She says that in much of the world, forests are dominated by broadleaf trees with huge branches: “When a large branch breaks off … it has good potential to create an entrance to the already-decayed heartwood inside the tree.”
In the areas of North America and northern Europe where these kinds of studies have been done, however, “trees tend to have a single main stem, with small branches that self-prune,” she says. When weather events knock branches off this type of tree, it doesn’t tend to expose the heartwood and the tree can quickly heal the wound—which means fewer opportunities for the formation of sizable natural tree cavities.
A Front-Row Seat
Virginia Tech researcher Jeff Walters has had a front-row seat to watching Very Important Woodpeckers in a forest ecosystem for more than three decades.
Specifically he’s gotten familiar with the haunts and habits of the Red-cockaded Woodpecker, an iconic bird protected under the Endangered Species Act that makes its living in the longleaf pine forests that dot the landscape from Virginia to Texas. As important as woodpeckers are in the forests of western Canada, the woodpeckers of the longleaf pine forests are absolutely vital, according to Walters’s research. In a study published in The Condor in 2008, Walters and Virginia Tech PhD student Lori Blanc found that almost every single tree cavity used by birds and other creatures within a longleaf pine nest-web community in northern Florida originated with a woodpecker—432 out of 433 cavities.
In conifer systems, Walters says, there is “more dependence on excavated cavities from woodpeckers compared to hardwood or tropical forests. Pine trees just don’t produce a lot of … natural holes on their own.” Pines also produce resin that may help protect their wood from infection following injury.
Additionally, he says that longleaf pine systems were clear-cut around the turn of the 20th century, so there are a limited number of older, decaying trees and tree snags—places that have soft spots where woodpeckers usually drill holes. Luckily the Red-cockaded Woodpecker has a secret superpower that enables it to provide cavities in a way that most other woodpeckers can’t; they drill into live trees.
“Red-cockaded Woodpeckers add an additional resource beyond the holes in dead pines, which is often where most of the action is in nest webs in conifer forests,” says Walters.
Drilling into a live tree is anything but easy. Most woodpeckers in North America can finish a nest cavity in a matter of weeks, or maybe a couple months. The average Red-cockaded Woodpecker, on the other hand, takes years, sometimes up to a decade or more, to excavate a cavity in a live tree. That’s because chiseling away at live wood is difficult, due to the dense sapwood layer under the bark that allows the tree to transport nutrients from the roots to the leaves.
Red-cockadeds prefer trees where the sapwood is healthy for a single reason, says Walters: “Resin wells.”
“If you ever see a picture of a Red-cockaded Woodpecker cavity that’s being actively used, you’ll see all this sap all around it,” he says. The sappy coating around the nest holes creates a barrier of protection against would-be predators. “Snakes can’t get through the sap.”
Walters’s research also shows that flickers again play an important role in longleaf pine nest webs, but often they rely on Red-cockaded Woodpeckers to get things started. Red-cockaded Woodpeckers and Northern Flickers excavated the most cavities (about 50%) that were used by other cavity-nesting birds, and flickers were also the primary enlargers of holes started by red-cockadeds that eventually housed some of the largest cavity-dwellers in longleaf pine forests—American Kestrels and Eastern Screech-Owls.
There’s another, less visible, group of players in the longleaf pine nest web. Michelle Jusino began studying Red-cockaded Woodpeckers under Jeff Walters as a PhD student in the early 2010s and quickly became interested in a different denizen of tree cavities: fungi.
“These birds have long been thought to have an association with one particular fungus [called Porodaedalea pini] because that fungus causes heartrot,” says Jusino. But she says that the scientific knowledge of that association was based on anecdotal observation. “Sometimes when we see woodpecker holes, we see this fungus on the tree.”
So for her dissertation, Jusino designed a study to find out exactly what fungi are present in woodpecker cavities, and how they get there. She and her team of researchers drilled fresh holes into living longleaf pine trees and put galvanized steel screens over some of the holes to keep out Red-cockaded Woodpeckers. The other holes were left open for red-cockadeds to access.
Her results, published in Proceedings of the Royal Society B in 2016, found that both sets of holes contained fungal communities that weren’t present at first, but that those communities were very different from each other. The holes accessed by Red-cockaded Woodpeckers developed clusters of dozens of different fungi that were very similar to the fungal communities found in natural red-cockaded excavations.
“These birds are helping facilitate these fungal colonizations, first by making holes, and then by transporting [the fungus] from one tree to another,” says Jusino. “What that study didn’t prove was whether that fungus is really helping the bird in return.”
While Red-cockaded Woodpeckers overwhelmingly prefer live trees with healthy sapwood, Jusino says they seem to have a preference for trees where the heartwood is beginning to decay. But the question remains: How are these woodpeckers identifying such trees when there is usually no evidence on the outside? Jusino isn’t sure if red-cockadeds are sensing the rotting heartwood and accompanying fungi through smell, feel, or some other cues—or if heartwood rot may be accelerated due to the fungus that woodpeckers transport to the tree.
“It’s still kind of a wide-open question,” she says. “We suggested two hypotheses: one is tree selection by the birds, that the birds are somehow selecting the trees based on the presence of fungus, and the other is that birds are facilitating the movement of this fungus from tree to tree.”
Now a researcher at the Center for Mycology Research, part of the U.S. Forest Service, Jusino says she’s designing a new study that aims to answer that question. She’s planning to start the cultures this summer, though thanks to the slow excavation habits of Red-cockaded Woodpeckers and slow growth of fungus it will take several years to get results.
Forests of the Future
As a stream of published research adds to the understanding of woodpeckers as home builders in North American forests, an emerging area of research is looking ahead to the crucial role woodpeckers will play in forests of the future.
Wildfires and Woodpeckers
Climate change and wildfires are drastically altering forest landscapes in the American West. According to Andrew Stillman, a postdoctoral fellow at the Cornell Lab of Ornithology who studies how bird populations respond to fires, woodpeckers are a key to helping forest communities recover after a fire.
“Wildfires often create a pulse of dead or dying trees, followed by a pulse of insects that come to take advantage of that dead wood,” says Stillman. “The woodpeckers are following the insects into the burned areas.”
Stillman says that when woodpeckers swarm into a burn area full of dead trees, they go right to work excavating cavities: “And so the woodpeckers themselves then provide a pulse of nest sites for a lot of other species … like bluebirds, which eat a lot of berries and start spreading seeds around these recently burned places.”
North Carolina State PhD candidate Lauren Pharr sees some of the same dynamics at play in her studies on Red-cockaded Woodpeckers in the fire-prone longleaf pine forests of the Southeast, and she thinks the woodpeckers are providing more than nesting homes—they’re providing shelters during extreme climate events.
During Pharr’s four years of studying red-cockaded nesting sites, she has noted an increase in severe rain events in the North Carolina sandhills, and several hurricanes in the Florida panhandle. She says Red-cockaded Woodpecker nest holes are helping a menagerie of other cavity-dwelling species, from bluebirds and nuthatches to other woodpeckers like red-headed and red-bellied, to snakes, small mammals, even flying squirrels.
“That is the most fun thing out there, climbing a tree and then a flying squirrel comes out, and it’s so cute to see him fly,” Pharr says. “It’s so spectacular.”
She says those cavities are important for all kinds of animals in the face of climate change: “[Red-cockaded Woodpeckers] are mitigating stress for those other species. So if they need a place to go … in places with extreme weather events or anything like that, those cavities help.”
Pharr’s advice for forest managers who want to optimize habitat for birds: Start with making sure a forest’s woodpeckers are taken care of.
“Red-cockaded Woodpecker itself is a really big indicator species, and the management that we do with Red-cockaded Woodpeckers also is very beneficial to other species in that ecosystem,” Pharr says. “So it’s a win-win.”
