Three-Toed Woodpeckers Picky about Snags

Although it is well known that three-toed woodpeckers depend on the dead trees characteristic of old-growth forest, new research shows that not just any snags will do: the woodpeckers only forage on those that are dying or have died recently. This means that current management plans based on providing snags for nesting may be inadequate.

“We believe that such models are unlikely to be successful in predicting long-term habitat needs of three-toed woodpeckers,” say Louis Imbeau of the University de Quebec en Abitibi-T?miscamingue in Rouyn-Noranda, Quebec, and Andr? Desrochers of the University Laval in Sainte-Foy, Quebec, in the January issue of the Journal of Wildlife Management.

More than 60 percent of Canada’s logging operations are in old-growth stands, and many forest and wildlife managers suggest that leaving 5-10 snags per hectare is enough to conserve three-toed woodpeckers. However, this guideline is based solely on the birds’ use of snags for nesting. The woodpeckers also forage and drum on snags, and so little is known about these uses that they are not incorporated into management decisions.

Imbeau and his colleagues studied how three-toed woodpeckers use snags in the Ashuapmushuan River ecological region of Quebec. They found that the woodpeckers overwhelmingly use snags for foraging and drumming: up to 93 percent of foraging and 95 percent of drumming was on snags. Moreover, the woodpeckers prefer different types of snags for each use. Compared to other snags, those used for foraging were bigger across and less deteriorated, whereas those used for drumming had broken tops and less bark, and were less deteriorated.

It makes sense that three-toed woodpeckers forage mostly on newer snags. The birds’ primary food is bark beetles, which are abundant in dying or recently dead trees but are sparse in trees that have been dead more than a few years. “Three-toed woodpeckers will probably require a continual recruitment of recently dead snags as foraging substrates,” say the researchers. Similarly, although the purpose of the woodpeckers’ drumming is not known for certain, it may also be critical to the species’ survival. For instance, drumming may be used to mark territorial boundaries.

Imbeau and his colleagues conclude that rather than leaving a set number of snags for nesting, forest managers need to ensure a continual supply of recently dead snags for foraging. While some people have suggested actively killing trees by girdling or notching, the researchers say this should be a last resort due to high labor costs. Rather, they recommend preserving areas of old-growth forest, which will naturally supply new snags for woodpecker foraging over time.

—Robin Meadows

For more Information
Imbeau, L. and A. Desrochers. 2002. Foraging ecology and use of drumming trees by three-toed woodpeckers. Journal of Wildlife Management 66:222-231.

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