Managing the abundance and diversity of breeding bird populations through manipulation of deer populations

Deer densities in forests of eastern North America are thought to have sign ificant effects on the abundance and diversity of forest birds through the role deer play in structuring forest understories. We tested the ability of deer to affect forest bird populations by monitoring the density and diver sity of vegetation and birds for 9 years at eight 4-ha sites in northern Vi rginia, four of which were fenced to exclude deer. Both the density and div ersity of understory woody plants increased following deer exclosure. The n umerical response of the shrubs to deer exclosure was significantly predict ed by the soil quality (ratio of organic carbon to nitrogen) at the sites. Bird populations as a whole increased following exclosure of deer, particul arly for ground and intermediate canopy species. The diversity of birds did not increase significantly following exclosure of deer, however, primarily because of replacement of species as understory vegetation proceeded throu gh successional processes. Changes in understory vegetation accounted for m ost of the variability seen in the abundance and diversity of bird populati ons. Populations of deer in protected areas are capable of causing signific ant shifts in the composition and abundance of bird communities. These shif ts can be reversed by increasing the density and diversity of understory ve getation, which can be brought about by reducing deer density.