Effects of Experimentally Reduced Prey Abundance on the Breeding Ecology of the Red-Eyed Vireo

Given the demonstrated importance of caterpillars in the breeding-season diet of many neotropical-nearctic migratory forest songbirds, a large-scale manipulative experiment was conducted to examine how variation in caterpillar abundance influenced the breeding ecology of the Red-eyed Vireo (Vireo olivaceus). The Red-eyed Vireo is a canopy-forager that consumes and feeds its young a large proportion of caterpillars during the breeding season. Caterpillar abundance was experimentally reduced in May of 1997 and 1998 on nine replicate 30-ha study plots (three treated, six untreated) in the Monongahela National Forest, West Virginia, through the application of Bacillus thuringiensis according to gypsy moth (Lymantria dispar) management protocols. Each annual application significantly reduced the abundance of Lepidoptera larvae on the Bacillus-treated plots relative to the nontreated plots for five and six weeks after spraying, respectively. This time period coincided with the nest initiation, incubation, and nestling stages of the Red-eyed Vireo. However, there was minimal evidence that this reduction in Lepidoptera larvae had a concurrent negative effect on the Red-eyed Vireos' ability to successfully rear nestlings. There were no differences in clutch size, hatching success, nestling mortality, overall nest success, or annual adult survival between the treated and untreated plots in any year of the study. It did appear, however, that Red-eyed Vireos waited 3-5 d longer to initiate nests in years when caterpillar abundance was low, due to either natural or experimental causes. Because of a relatively short breeding season, this delay could reduce the seasonal productivity of this species by 0.15-0.25 young per female per year. Even though the effects of Bacillus-induced caterpillar reduction on Red-eyed Vireo reproduction were minimal, we urge caution when considering the application of Bacillus over larger spatial scales, repeatedly in the same area, or in locations of endangered species where even a modest reduction in seasonal productivity could be detrimental.