BACKGROUND

Many species of forest birds have been declining over recent decades. This issue looks at whether a relationship exists between declining populations of forest birds and increasing populations of white-tailed deer. As large herbivores, white-tailed deer (Odocoileus virginianus) affect forest plant communities directly through browsing (eating) and indirectly through the cycling of nutrients and energy in food webs. These activities change characteristics of the forest habitat. Songbirds are especially sensitive to habitat changes, such as the volume (amount) and composition (types) of vegetation, because of their foraging and nesting behaviors. By changing forest vegetation, deer alter habitat for forest birds and could affect both the abundance (number) and diversity (variety) of birds.

To examine the effects of deer on forest bird populations, McShea and Rappole monitored (a) the density and diversity of vegetation, and (b) the abundance and diversity of birds at eight 4-hectare forested sites in northern Virginia. The sites were located within 25 km of Front Royal, VA in large forest tracts in either the Shenandoah National Park or the Smithsonian Institution’s Conservation and Research Center. Initially the 8 sites contained similar understory vegetation and deer densities. The researchers fenced 4 of the sites to exclude deer. They then examined changes in vegetation and bird communities that occurred between the fenced (exclosure) and non-fenced (control) sites over a 9-year period.

The purpose of the research was “to test whether deer can serve as agents of structural change in protected forests and whether manipulation of deer numbers can affect bird populations, with understory vegetation as the short term link between these two trophic levels” (McShea and Rappole 2000). McShea and Rappole found the following:

• The exclusion of deer increased the density and diversity of understory woody shrubs relative to control sites.


• Fifteen of 25 bird species examined responded positively to the increase in vegetation that resulted from deer exclusion.


• Patterns of change observed in bird populations can be grouped into three categories that are best described by the species included in Figure 2.


• Diversity of birds did not change with exclusion of deer at the geographic and time scales examined in this study.