Science Corner
Cheryl Lowe, NOSC Board Member

A Rectangular Hole in a Tree?

On a winter walk last month, some friends and I found several rather large rectangular holes carved into tree trunks along the trail. We’d been told that Pileated Woodpeckers make these distinctive holes, but we wanted to know more. Why this rectangular shape? Is it caused by the structure of the wood, a distinctive excavating technique or something else? Why don’t other woodpeckers also make square holes?

Is it the Tree?

Pileated Woodpeckers focus on the size and condition of a tree more than which species it might be. They prefer larger trees in earlier stages of decay, usually snags or trees with broken tops. In Washington, that might include western hemlock, Pacific silver fir, western red cedar or others, depending on what’s available. Their favorite foods are carpenter ants, which are most likely found in large tree trunks in early stages of decay as the ants excavate a network of tunnels or galleries up and down these trees to connect one larval chamber to another.

In one Olympic forest study, scientists found up to 71% of a Pileated Woodpecker’s diet (determined by analyzing their scat) was composed of carpenter ants. Other food sources include damp-wood termites, round-headed wood-boring beetle larvae and thatch ants. The percentage of each species found in the scat varied seasonally, however. Scientists speculate that during nesting season, food demands are higher, so although they prefer carpenter ants, they will eat other available high-energy foods they find.

Foraging Techniques

Pileated Woodpeckers are the largest North American woodpecker (16-19 inches long). Our next largest resident woodpecker, the Hairy Woodpecker, is half that size. Pileated Woodpeckers are found from the mountains of central California north through British Columbia, across Canada to Nova Scotia, and throughout most of the eastern US. The Washington range encompasses the eastern and western forested areas of the state.

With their long neck and strong chisel-shaped beak, they exert a lot of force when excavating, often add a twisting motion to pry off wood chunks on trees in early to moderate stages of decay. Having done just enough wood carving to know that following the grain makes for easier carving, I can easily imagine that the chips would break off along the grain, enhanced by the woodpecker’s inclination to follow carpenter ant tunnels vertically in search of food. Our other woodpecker species are weaker excavators, focusing on gleaning, pecking and prying off bark chips to uncover insects.

Some studies report Pileated Woodpeckers also forage on downed logs, but we seldom see that here. Downed logs in our coastal forests are probably too wet and cool to support carpenter ant colonies, although carpenter ants can be found in larger downed wood in drier forest habitats such as eastern WA and OR.

Not Always a Rectangle

Pileated Woodpeckers are year-round forest residents—foraging, nesting and roosting in different types of trees.

Their nesting cavities are marked by round to oval rather than rectangular holes in trees that are in the earliest stages of heartwood decay, with relatively sound wood. They excavate a new nest hole each year, perhaps to ensure the wood is solid enough for raising a new family.

Roosting cavities, on the other hand, are mostly located in hollow trees (more advanced stages of decay). The openings for these roosts are usually oval or oblong in shape, the cavities larger than nesting cavities, and often with multiple openings to provide alternate escape routes. Individual Pileated Woodpeckers may use 6-7 different roost trees during the course of a year. In a local study, large old western red cedar trees were particularly important as roost sites, perhaps in part because cedars are resistant to fire and disease, and thus persist for longer periods of time.

Woodpeckers and Salmon

So, what does all this have to do with salmon and salmon habitats? Woodpeckers and salmon in our region often share the same riparian habitats and both are keystone species—salmon providing marine-derived nutrients to the surrounding forests after they die, which supports the trees and insects that woodpeckers rely on. Woodpeckers in turn create cavities that are later used by other cavity nesters such as small owls; larger cavity-nesting birds such as wood ducks, buffleheads, mergansers and goldeneyes; squirrels; bats; martens and raccoons-- part of the expanding ripple effect of salmon in our watersheds.

Next time you see a rectangular hole in a snag, look around. It’s another reminder that there is so much more to be seen.

References

Science Findings (Pacific Northwest Research Station) 2003. “Coming Home to Roost: The Pileated Woodpecker as Ecosystem Engineeer” https://www.fs.usda.gov/pnw/sciencef/scifi57.pdf

US Forest Service Research Article 2007. “Foraging Ecology of Pileated Woodpeckers in Coastal Forests of Washington” https://www.fs.usda.gov/pnw/pubs/journals/pnw_2007_raley001.pdf

WA Dept of Fish and Wildlife, Species and Habitats website: https://wdfw.wa.gov/species-habitats/species/dryocopus-pileatus#resources

Image 1: https://devilslakewisconsin.com/2023/04/10/pileated-woodpeckers-natures-tree-carvers/

Image 2: https://wdfw.wa.gov/species-habitats/species/dryocopus-pileatus

Image 3: https://avianreport.com/woodpecker-bill/

Image 4: https://birdfact.com/articles/pileated-woodpecker-nesting

Image 5: https://piercecd.org/466/Western-Redcedar