Insect outbreaks increase populations and facilitate reproduction in a cavity-dependent songbird, the Mountain Chickadee Poecile gambeli

Abstract

Large-scale insect outbreaks, typical of temperate ecosystems, may increase food availability and influence species interactions in insectivorous bird communities. We assessed how population densities of a secondary cavity nester, the Mountain Chickadee Poecile gambeli, varied with densities of avian cavity excavators, potential competitors and nest predators, during two large-scale insect outbreaks of Western Spruce Budworm Choristoneura occidentalis and Mountain Pine Beetle Dendroctonus ponderosae. At the regional level, Mountain Chickadee densities doubled with increases in budworm and beetle availability, then declined with decreases in beetle availability, despite high budworm supply. At the site level, densities of excavators were the best predictor of site-level variation in Chickadee densities in the following year. During and after the beetle outbreak, Mountain Chickadees used more cavities excavated by Red-breasted Nuthatches Sitta canadensis and Downy Woodpeckers Picoides pubescens, both of which are bark insectivores that increased in densities concomitantly with the beetle outbreak and whose foraging activities may have facilitated the Mountain Chickadees' access to bark beetles. Thus, Mountain Chickadees showed a numerical response to the food pulse at the regional level, but a functional response to the pulse of nesting cavities at the site level. Plasticity in resource selection and heterospecific attraction may allow Mountain Chickadees to respond to resource pulses in highly variable environments. © 2012 British Ornithologists' Union.