Influence of Forest Structure and Diseases on Nest-Site Selection by Red-Breasted Nuthatches

Abstract

Empirical studies of the effects of forestry on cavity-nesting birds largely neglect weak cavity excavators and instead focus on large, strong cavity excavators such as woodpeckers (Picidae). Weak cavity nesters tend to be smaller and may be less limited by the availability of large trees but more dependent on suitably softened wood. In this paper, we describe availability, use, and preference for nesting habitat of a small cavity-nesting bird, the red-breasted nuthatch (Sitta canadensis), in mature coniferous forest in southeastern British Columbia. We describe nesting habitat at 3 spatial scales: (1) the level of forest stands proposed for logging (cutblocks), which ranged from 8.1 to 18.4 ha in size; (2) the level of 0.04-ha patches of forest centered on nest trees (nest plots); and (3) the level of individual nest trees. In addition to considering basic structural habitat features, we analyzed the relation between nesting use and several factors related to forest health, including Armillaria root disease (Armillaria ostoyae), stemwood decay, mountain pine beetle (Dendroctonus ponderosae) infestation, and dwarf mistletoe (Arceuthobium spp.). Red-breasted nuthatches nested at higher densities on cutblocks with more dead trees (snags) and higher levels of root disease. Within nest plots, we also detected a preference for nesting in Armillaria-infected trees. Cavity nesting of nuthatches was influenced by both forest structure and the disease agents that contribute to it. We recommend that standing diseased and dead trees be retained in patches during logging operations. Retention of such trees in distinct patches more likely satisfies requirements for worker safety and for limiting the spread of diseases than does retention of single dead and diseased trees throughout cutblocks. Wildlife values not only should be incorporated into timber harvesting regulations but also should be considered when planning activities designed to improve forest health for timber production.