When the right tree can be hard to find

SOMEWHERE in Washington State’s eastern Cascade Range a bird perches motionless against a burned tree. It’s an American three-toed woodpecker (Picoides dorsalis), sporting black back feathers, a white mustache stripe, and a yellow cap. About the size of a large banana, the bird has evenly-spaced white flecks on the sides of its chest and on its primary and secondary feathers—reminiscent of a checkered pashmina.

The bird was extracting beetle larvae from another burnt tree when a larger, louder Black-backed woodpecker (P. articus) drove it away. But here, on this burnt tree, it drums. This one is mine. Stay away.

Soon it will pick a spot on the tree, say 30 feet above the forest floor, and bore rapidly into the wood with its chisel-like bill until it excavates a deep pocket. The bird and its mate will roost and nest in this safe place until its favorite food runs out, perhaps in four or five years. By then, another forest burn might provide a new supply of wood-boring insects.

In the woodpeckers’ absence, other birds or small mammals like owls, squirrels, and bats move in. The new tenants disperse seeds and spores that help new plants and fungi grow. When the dead, burnt trees—also called snags—fall to the ground, wood-living bees and wasps take up residence. Fungi and lichen flourish on the fallen tree’s gnarly surface, and once in a marten hides behind the log.

“Woodpeckers are ecosystem engineers,” says Teresa Lorenz, a research wildlife biologist with the Pacific Northwest Research Station. “Many small animals compete for their vacated nests because they are so protected from the elements and other predators.”

Ecosystem engineers are organisms that significantly modify, maintain, or destroy a habitat. In the woodpeckers’ case, the species help build and maintain a habitat for other species by excavating nests in live and dead trees. When forest managers find many different woodpecker species in a forest, it often signals the presence of a thriving and diverse community of plants and animals.

Because of the woodpeckers’ crucial role in maintaining healthy forests, researchers like Lorenz want to understand how these cavity-excavating birds choose trees to nest in. When forest managers know what these birds like, they can take steps to ensure that struggling forests have these kinds of trees.

More than fifty years ago, studies suggested that woodpeckers picked trees solely based on external tree- or habitat-level factors, like the size of the tree, the tree species, or what the vegetation cover is like near the nest. Other studies pointed to wood decay, but used only visual indicators to assign a snag decay class as a proxy for measuring this. Lorenz felt there was a missing piece: few studies actually measured wood inside the tree that cavity-excavating birds had chosen to nest in.

Matsuoka’s Method

The idea for the study came to Lorenz and her colleagues in the summer of 2011, when they were investigating how far woodpecker nestlings wandered from their nests soon after they fledge. Using hole saws, they drilled into trees to get to the nestlings, to band them. “We noticed an enormous variation in the thickness of the wooden plug,” Lorenz says. “Sometimes the nestling would be just inside the bark of the tree. Other times we had to drill through four inches of wood. We had all these hypotheses as to what was going on.”

A colleague suggested that the variation could be due to the properties of the wood in the tree. Intrigued, they checked the literature. What if wood hardness had something to do with the birds’ preference when looking for trees to nest in? Lorenz and her colleagues did not find many studies that addressed this question. “We were surprised that nobody had considered that this wood inside the tree was influencing where the birds were nesting,” Lorenz says. So, she decided to investigate the question for her Ph.D. dissertation at the University of Idaho.

The problem was there weren’t many reliable methods for measuring wood hardness in a forest setting. “But we found an obscure article in the Japanese Journal of Ornithology and this guy had figured out an ingenious way to measure the hardness of wood inside a tree, using a cordless drill, a foot-long drill bit, and an increment borer, without destroying the nest or harming the tree,” Lorenz says.

An increment borer is a tool used by foresters and other researchers to extract a section of wood tissue so they can count a tree’s annual rings and determine its age. Japanese researcher Shigeru Matsuoka’s method involves inserting this tool into a pre-drilled hole above the nest cavity opening. The torque required to spin the increment borer would represent wood hardness. With the most important aspect of her field methods squared away, Lorenz set out with colleague Philip C. Fischer to gather field data in the eastern Cascade Range in Washington State.

Lorenz had selected six species of cavity excavating birds for the study. She picked three strong excavators: the Three-toed woodpecker, the Black-backed woodpecker, and the Hairy woodpecker. She also picked a weaker guild of excavators, namely the Northern flicker (Colaptes auratus), the White-headed woodpecker (Picoides albolarvatus), and Williamson’s Sapsucker (Sphyrapicus thyroideus).

They searched for nests in all major forest types from March to July in 2011 to 2013, hiking through Ponderosa pine, Douglas fir, and western hemlock in lower elevations and through higher elevation forests of grand fir, subalpine fir, and western larch. Lorenz and Fischer located adult birds by broadcasting playbacks of their calls and drumming. “Then, we followed the adults until they led us to their nest cavities,” Lorenz says. They marked the locations of occupied nests on portable GPS units. If they noted fresh woodchips on the ground surrounding the nests, they marked those as current-year excavations. They found nests in forest patches ranging from unburnt to severely burned.

Lorenz and Fischer covered a lot of ground. Each of their ten study sites were 1,483 to 7,413 acres in size. “You get in really good shape,” Lorenz says.

Soft pockets

Each carrying about 30 pounds of equipment, Lorenz and Fischer returned in the summer to make their measurements. After ensuring the nests had been vacated by the birds, the researchers would scale a tree on climbing ladders to measure wood hardness. They drilled 5 centimeters above the nests, because Matsuoka’s study suggested this spot would be representative of the excavated wood.

“In the beginning, we had assumed at that wood at the nests would be entirely soft, but that wasn’t the case,” Lorenz says. “There was some variability when we first started drilling into the trees. Some had hard surfaces. But when you get into the tree a bit, that’s when you find these soft pockets. Other times, the soft pockets were close to the surface of the tree, and other times the tree was soft from the get go.”

To determine if a tree’s external features is a reliable indicator of interior wood hardness, the researchers measured wood hardness in random trees within different decay classes.

Then the researchers measured vegetation features that were hypothesized to influence cavity-excavating birds’ nest site selection in past studies, such as diameter at breast height of the nest tree; nest and tree height; orientation of the nest cavity entrance; proportion of the ground covered by shrubs; tree and snag density at the nest sites; and tree species. For nests found in snags, they examined the remaining bark, tree growth form, and other features to determine the tree’s species.

“It was a pretty brutal field season,” Lorenz says. “We worked from 6 a.m. to 4 p.m. And it got really hot in these sites.”

Lorenz and Fischer also used remote sensing data. To estimated pre-fire canopy cover at nest sites found in snags, they used Gradient Nearest Neighbor Classified Landsat satellite imagery captured between two and eight years prior to each fire.

Lorenz was concerned about seeing an effect in nature, measuring it, but finding a lot of variability. “What if we went to another tree and that pattern didn’t hold?” Lorenz says. “But in this study, we were able to measure so many trees that we were confident that the effect we were seeing was real. And it has major management implications.”

Drawn to snags

Lorenz and her colleagues found that cavity-excavating birds preferred to nest in trees with significantly softer interior wood. They found that across 818 snags, trees not used by birds had wood five times harder than trees that were. These trees could not be used by birds simply because of the birds’ physical limitations in pecking ability.

They found that at-risk species, namely the Black-backed and the White-headed woodpeckers, were nesting within burns that contained 86 to 96 percent of trees with unsuitably hard wood. This suggests that past studies that did not measure wood hardness counted many sites available to cavity-excavating birds when they were unsuitable. “By not accounting for wood hardness, managers may be overestimating the amount of suitable habitat for cavity-excavating bird species, some of which are at risk,” Lorenz says.

In their study plots, the researchers did not find reliable visual cues to distinguish between suitable and unsuitable trees. Snag decay class was a poor indicator of internal wood properties—and this was not the first study to demonstrate that fact, though it was the first study to do so in ornithology (past studies had been done by foresters, and published in forestry journals).

“Currently, the best management solution we can recommend is to provide large numbers of snags for the birds, which can be difficult without fire,” Lorenz says. According to the researchers’ calculations, if one of every 20 snags (approximately 4 percent) has suitable wood, and there are five to seven species of woodpeckers nesting in a given patch, approximately 100 snags may be needed each year for nesting sites alone. This does not account for other nuances, like the fact that most species are territorial and will not tolerate close neighbors while nesting, or the fact that species like the Black-backed woodpecker need more foraging options. Overall, more snags are needed than other studies have previously recommended.

Based on their results, Lorenz and her colleagues see the critical role that mixed-severity fires play in providing enough snags for cavity-dependent species. Low-severity prescribed fires often do not kill trees and create snags for the birds. In addition to benefiting the cavity nesters, a mixed severity prescribed burn could also aid species that, historically, did not prefer to nest in snags. During their field work, Lorenz and Fischer observed that White-headed woodpeckers, a species traditionally associated with old-growth forest, used prescribed burns presumably because of shortages in suitable nest-sites.

“I think humans find low severity fires a more palatable idea. Whereas unfortunately or fortunately, these birds are all attracted to high severity burns,” Lorenz says. “The devastating fires that we sometimes have in the west almost always attract these species of birds in relatively large numbers.”

Many studies have shown that a severely burned forest is a natural part of western forest ecosystems. Snags from these fires attract insects that love to burrow beneath charcoal bark. And where there are insects, there are birds that love eating these insects.

Lorenz and her colleagues stress that providing snags that woodpeckers can excavate is crucial for forest ecosystem health in the Pacific Northwest, where more than 50 wildlife species use woodpecker-excavated cavities for nesting or roosting.

“This study was just the beginning of what I hope will be decades of research,” Lorenz says. Picking up on what she learned in this study, Lorenz is now collaborating with Forest Service colleague Michelle Jusino to investigate a kind of fungi that follows forest fires. “There is some mechanism by which the death of a tree somehow enables a species of fungi to decay the wood,” she says. The research is bound to take her back into the forests of the Pacific Northwest, where woodpeckers follow forest fires in a timeless cycle of change.

Originally published in Science Findings. Download the publication here.