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.