EFFECTS OF GYPSY MOTH DEFOLIATION ON FOREST BIRDS: AN ASSESSMENT USING BREEDING BIRD CENSUS DATA

Abstract

Although the gypsy moth (Lymantria dispar) is a significant insect pest of eastern deciduous forests in the United States, relatively little is known about its effects on forest bird communities. We used six Breeding Bird Census sites from Connecticut, Pennsylvania, and Virginia to assess changes in bird species richness and individual species density in the years surrounding a gypsy moth outbreak. Individual species' responses were variable among states, and only a few species showed consistent responses to outbreaks across sites. Yellow-billed Cuckoos (Coccyzus americanus) and Black-billed Cuckoos (C. erythropthalmus) appeared two years prior to an outbreak and then disappeared immediately after an outbreak on four of the sites and increased in numbers on another site. Indigo Buntings (Passerina cyanea), which are usually associated with open habitat, increased temporarily after outbreaks and then returned to pre-outbreak densities within 5 yrs after the outbreak. At the community or guild level, there was a significant reduction in species associated with closed-canopy forests during the outbreak year(s) compared with the average of all other years (before and after the outbreak). There were no other general responses by the avian communities to the outbreaks, including associations with habitat preference, foraging guild, or nesting substrate. This study suggests that the effects of gypsy moth defoliation on the avian community are likely to be short-term (assuming that tree mortality is not severe) and spatially variable. The minimal nature of these effects also suggests that compared with pesticide options for gypsy moth control, allowing the gypsy moth to defoliate, when feasible, is preferable when managing for forest birds.