Impact of Tent Caterpillar Defoliation on the Reproductive Success of Black-Capped Chickadees

Abstract

Outbreaks of folivorous Lepidoptera caterpillars are common in temperate forests (Ives and Wong 1988, Butterworth 1990). The high fat content, low chitin content, and large size of these larvae make them an important component of the diet of many temperate birds (Betts 1955, Robinson and Holmes 1982, Holmes and Schultz 1988, Sample et. al. 1993). Outbreaks of most species should increase available food for breeding birds, improving their reproductive success. Some caterpillars, however, have morphological or chemical defenses that make them unpalatable to avian predators (Heinrich and Collins 1983). During outbreaks, these species have the potential to reduce the abundance of palatable caterpillars through severe defoliation. Previous studies have not considered the effects of outbreaks of unpalatable Lepidoptera species on the breeding biology of birds. This investigation determines the effects of forest tent caterpillar (Malacosoma disstria) outbreaks on the reproductive success of Black-capped Chickadees (Parus atricapillus) near Athabasca, Alberta, Canada. The tent caterpillar is a major defoliator of trembling aspen (Populus tremuloides) in Alberta, undergoing three to five year population outbreaks every five to ten years (Ives and Wong 1988, Butterworth 1990). Egg masses and very early instar larvae can be consumed by avian predators. Thus, for resident bird species, an outbreak of tent caterpillars could increase food supply in winter and early spring prior to and just after hatch of egg masses. Late instar tent caterpillars, however, are equipped with spiny hairs and bristles and are unpalatable to most avian predators (Smith 1991). Thus, outbreaks of tent caterpillars that lead to defoliation during the brood-rearing period may decrease reproductive success for chickadees. Tent caterpillar outbreaks and complete canopy tree defoliation occurred in our study area during 1986 and 1987, while few or no caterpillars and little or no defoliation were evident during 1989-1992. We tested two predictions. First, in years with tent caterpillar outbreaks, initiation of breeding would be earlier and clutch sizes larger in response to extra food when compared to years with no outbreaks. Second, tent caterpillar defoliation would reduce the reproductive success and subsequent survival of female Black-capped Chicka ees through food limitation during brood-rearing.