Thanks to conservation efforts, U.S. deer densities are the highest they have been in 100 years. But the downside is that too many deer may be bad for forest birds, according to new research in the August issue of Conservation Biology. This is the first study showing that deer can drive at-risk bird species out of forests. The link is that deer determine which types of plants grow in the understory, which in turn determine the types of birds that live there.
“Large hooved mammals are landscape engineers, and deer are the dominant herbivores in the eastern United States,” say William McShea and John Rappole of the Smithsonian Institution’s Conservation and Research Center in Front Royal, Virginia, who collaborated on this study.
In 1991, McShea and Rappole fenced white-tailed deer out of eight 10-acre plots in protected forests in both the Conservation and Research Center and Shenandoah National Park. They assessed the abundance and types of understory vegetation in the plots in 1994 and 1997 and estimated the abundance and types of breeding birds in the plots by mist-netting yearly from 1990 through 1998.
The results showed that excluding deer shifted the understory vegetation from grasses to forbs (non-grass herbaceous plants) to Rubus species (the blackberry family) to woody shrubs. While bird diversity was constant through these shifts, the relative abundance of the 25 species that commonly nested in these forests varied considerably: 10 species increased and four declined.
Those that declined were resident birds (such as the tufted titmouse, blue jay, and northern cardinal) that are stable or increasing in the U.S. Those that increased were migratory species (such as the hooded warbler and ovenbird) that are more at risk according to factors that include range-wide abundance and population trends.
While it is clear that managing deer can benefit at-risk bird species in eastern forests, McShea and Rappole recommend basing management on the density of understory vegetation rather than on the density of the deer themselves. This is because deer at a given density can result in different vegetation densities, depending on such factors as soil wetness and richness. Specifically, the researchers suggest monitoring orchids and lilies, which are common throughout the eastern U.S. and sensitive to changes in deer densities.
McShea, W.J. and J.H. Rappole. 2000. Managing the abundance and diversity of breeding bird populations through manipulation of deer populations. Conservation Biology 14(4):1161-1170.