White-tailed Deer Browsing and the Conservation of Forest Songbirds and Understory Vegetation: A Natural Experiment within the Apostle Islands National Lakeshore

Ostrowski, Diana

A thesis submitted to the faculy of the graduate school of the Uiniversity of Minnisota. In the Western Great Lakes region, white-tailed deer (Odocoileus virginianus) densities are high, while ground-nesting songbirds are declining. By degrading habitat, overabundant browsing likely contributes to songbird declines. To quantify deer browsing influence on avian habitat, species abundance and diversity, a natural experiment was designed within the Apostle Islands National Lakeshore. Because multiple islands contain populations of the coniferous shrub Canada yew (Taxus canadensis), which can structurally dominate a forest understory and is selectively browsed by deer, another objective was to investigate the influence of yew on avian communities. Breeding bird and vegetation surveys were conducted across habitats with various combinations of deer abundance and yew density. Understory nesting species responded most strongly to browsing; dominant species declined, while less dominant species, and consequently diversity, increased with deer browsing. Yew density significantly impacted the bird community when deer were present, but had weak effects when deer were absent.