Woodpecker Nest Survival, Density, and a Pine Beetle Outbreak

Mountain pine beetle (Dendroctonus ponderosae) outbreaks in western North American coniferous forests are increasing in size and severity. An understanding of wildlife population responses to pine beetle outbreaks is needed to inform habitat conservation strategies. We monitored 355 nests of 5 woodpecker species during 2 sampling periods, before (2003–2006) and after (2009–2014) the peak of a pine beetle outbreak in dry mixed conifer forest of Montana, USA. Three of 5 woodpecker species represented the beetle-foraging group: American three-toed (Picoides dorsalis), hairy (Dryobates villosus), and downy (D. pubescens) woodpeckers. The other 2 species studied were northern flicker (Colaptes auratus), a foraging and habitat generalist, and red-naped sapsucker (Sphyrapicus nuchalis), a sap forager and bark-gleaning insectivore. We analyzed daily survival rate of nests in relation to pine beetle outbreak (445,000 ha) severity and timing, along with covariates unrelated to the outbreak (temp, nest height, and nest tree diameter). Our results provided stronger evidence for relationships between woodpecker nest survival and the non-outbreak variables than those associated with outbreaks. Our results indicated limited support for nest survival relationships with beetle severity (annual and cumulative pine tree mortality at 0.81-ha and 314-ha scales). Nevertheless, we observed a significant increase in densities of hatched nests for beetle-foraging woodpeckers following the outbreak. Our results suggest that woodpeckers, particularly beetle foragers, respond numerically to pine beetle outbreaks through increased nesting densities more so than functionally via nest survival.