The response of avian predator populations to forest tent caterpillar (Malacosoma disstria; Lepidoptera: Lasiocampidae) outbreaks in Ontario, Canada

Abstract

Outbreaks of insect defoliators have broad ecological effects on forested ecosystems because they can cause extensive mortality in host tree populations. They also represent peaks in the density of food for specialist and generalist predators, and some species of insectivorous birds show strong responses to outbreaks of defoliators. Using over 50 years of bird counts and defoliation data, I examined the response of four species with a range of foraging specializations to outbreaks of the forest tent caterpillar, a major defoliator of deciduous trees in eastern North America. The specialist blackbilled cuckoo (Coccyzus erythropthalmus) showed strong aggregative and numerical responses to the outbreaks of forest tent caterpillar at local and regional spatial scales, respectively. In contrast, species with a lower degree of foraging specialization, the least flycatcher (Empidonax minimus), the yellow warbler (Setophaga petechia), and the black-capped chickadee (Poecile atricapillus) showed weak or null aggregational and numerical responses to the outbreaks, suggesting that they forage opportunistically on forest tent caterpillars and that this does not result in increased reproductive output. The results of this study are consistent with the idea that only species with a high degree of foraging specialization can take advantage of a food resource that fluctuates in a predictable manner, and highlights the need to consider the predator-prey dynamics when managing population outbreaks of insect defoliators.