Woodpeckers and other cavity-excavating birds worldwide are keystone species.

These birds excavate their nests out of solid wood, and because their nests are often well protected against predators and the environment, other species use and compete for their old, vacant nests.

The presence of cavity-excavating birds in forests has far-reaching effects on species richness and ecosystem health.

Given the species' importance, Teresa Lorenz, a research wildlife biologist with the U.S. Forest Service Pacific Northwest Research Station, wanted to find out why cavity-excavating birds do not use many trees seemingly suitable for nesting. This puzzle has eluded researchers for decades. Lorenz and her colleagues also wanted to know what role wood hardness plays in the birds' nest site selection.

The researchers found that across 818 snags in Yakima, Kittitas and Chelan Counties in Washington's eastern Cascade Range, trees not used by birds had wood five times harder than trees that were. Such trees could not be used by birds simply because their wood was too hard for the birds to excavate. Within burns used by at-risk woodpeckers, 86 to 96 percent of seemingly suitable trees contained unsuitably hard wood. Wood hardness limits nest site availability for these declining species

Researchers found no reliable visual cues to distinguish between suitable and unsuitable trees. Currently, the most effective management solution is to provide large numbers of snags, which can be difficult without the aid of fire. In dry forests, a mixed-severity fire that kills trees is an important but underappreciated strategy for providing enough snags for cavity-dependent species. Low-severity prescribed fires may not provide enough snags for these species.

Suitable snags are limited, such that snag availability drives landscape-level habitat-selection by some species. For example, white-headed woodpeckers selected severely burned patches for nesting, which was initially puzzling because this species does not characteristically forage in burn.

Providing snags that woodpeckers can excavate is important for forest ecosystem health in the Pacific Northwest, where More than 50 species use woodpecker-excavated cavities for nesting or roosting. Mixed-severity prescribed-fire may be useful in creating breeding habitat for the white-headed woodpecker, a species traditionally associated with old-growth forests.

This article was excerpted from a longer article from the U.S. Forest Service, which is available at: https://www.fs.fed.us/pnw/sciencef/scifi199.pdf.