Individual study: Exclusion of deer through the use of fenced exclosures increases woody plant density and diversity and populations of ground and mid-canopy bird species, northern Virginia, USA
McShea W.J. & Rappole J.H. (2000) Managing the abundance and diversity of breeding bird populations through manipulation of deer populations. Conservation Biology, 14, 1161-1170
Circumstantial evidence points towards the fact that bird populations may be sensitive to deer densities through the role that they play structuring the forest understorey. In this study, the ability of deer (Cervidae) to influence forest bird populations was assessed by monitoring the density and diversity of vegetation and birds within eight comparative sites in Virginia (USA), four fenced to exclude deer, was investigated over 9 years.
Study sites and deer densities: Eight sites were located in forest in either the Shenandoah National Park (SNP) or the Smithsonian Institution's Conservation and Research Center (CRC) near Front Royal, Virginia, USA. These plots contained mature deciduous tree species with understory shrubs including flowering dogwood Cornus florida, spicebush Lindera benzoin and redbud Cercis canadensis. Plots within sites were 4 ha in size (200 × 200 m or 160 × 250 m) and separated by at least 1 km.
Vegetation density: In 1990, three 24 × 24 m quadrats were established at each site and woody plants> 1 m in height and <4 cm in diameter were identified and counted, surveys were repeated in 1994 and 1997. Vegetation density was also estimated each July with using a 'coverboard' technique; the number of woody stems was found to be correlated with the average coverboard value for each site.
Bird populations: Breeding bird populations were estimated through mist-netting censuse from 1990 to 1998 undertaken between 30 May and 30 June. All captured birds were identified, sexed, assessed for reproductive condition, given a uniquely numbered U.S. Fish and Wildlife Service band (ring), and released. Only the first capture of each individual was counted during each breeding season.
Species were designated as guild members based on life-history characteristics (distance of migration and foraging height) and nesting characteristics (<2 m above ground,> 2 m above ground but below the canopy, and in the canopy).
In the deer exclosures the density and diversity of understorey woody plants increased, after an initial increase in the herbaceous ground flora, over the 9 year study period. Bird populations as a whole also increased over time within the exclosures, particularly ground and intermediate canopy species. However, bird diversity did not increase significantly as species tended to be lost and replaced by others as the understorey vegetation developed: species that prefer an open understorey e.g. chipping sparrow Spizella passerina) declined first, with those preferring a denser herbaceous ground cover (e.g. indigo bunting Passerina cyanea) increasing but then declining with replacement of the understorey by taller woody vegetation. Finally, species that prefer a dense, woody understorey (e.g. ovenbird Seiurus aurocapilla) gradually increased as vegetation succession progressed.
The authors conclude that in these areas at least, deer were capable of influencing composition and abundance of bird communities through their grazing and browsing activities.
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