Long-Term Trends in Habitat Selection by Kentucky Warblers

Abstract

-Microhabitat selection of the Kentucky Warbler (Oporornis formosus), a forestinterior Neotropical migrant, was examined using a geographical information system. The distribution of territorial male Kentucky Warblers was mapped within a 460-ha study area in northwestern Virginia from 1979 through 1992. Each annual distribution of territories was compared, using logistic regression, to a random distribution of territories with respect to habitat features codified within a vector-based geographical information system. Kentucky Warblers selected forested areas with cove hardwoods and avoided oak/hickory overstory. The warblers also were found more often in areas that contained streams and in areas with low white-tailed deer (Odocoileus virginianus) densities. Since 1979 there have been significant shifts in Kentucky Warbler use of habitat at this site, resulting in fewer territories within oak/hickory and edge habitats. Received 22 November 1993, accepted 31 August 1994. KENTUCKY WARBLERS (Oporornis formosus) are insectivorous, ground-nesting birds that breed within the interior forests of the northeastern United States and winter from southern Mexico to South America. This species is one of many wood-warblers that has shown an apparent decline over the last two decades (Robbins et al. 1989) and, therefore, is of particular interest to land managers and conservationists. In studies using multivariate analysis to describe forest bird communities, Kentucky Warblers were found to be associated with large, mature forests that possess a dense understory (Lynch and Whigham 1984, Robbins et al. 1989). As with most wood-warbler species, however, little is known of specific microhabitat requirements beyond the valuable, but anecdotal, reports from early naturalists (Chapman 1907, DeGaris 1936, Bent 1953) and annual surveys that detected population declines (Robbins et al. 1989). Declines within some unmanaged forests have been postulated to be due to an increased density of white-tailed deer (Odocoileus virginianus; Boone and Dowell 1986, Robbins 1991) through reduced density of understory shrubs. Geographical information systems (GIS) have been used successfully to model habitat requirements of Sage Grouse (Centrocercus urophasianus; Homer et al. 1993) and Wild Turkey (Meleagris gallopavo; Donovan et al. 1987). These models, however, identified landscape-scale features that were associated with flocks or populations of birds and, thus, may be inappropriate for analysis of habitat use by individual birds within a forest block. Given that Kentucky Warblers inhabit mature, deciduous forests, it may be possible to determine how they are distributed within this landscape-scale habitat using a GIS. Since 1979 extensive surveys for Kentucky Warblers have been conducted at the National Zoological Park's Conservation and Research Center (CRC). This site is a small (1,200-ha), protected area in the Blue Ridge Mountains of Virginia, 5 km south-southeast of Front Royal, Virginia. These annual surveys can be compared to vegetation and habitat attribute maps recently produced for the study area with a GIS. We examine: whether landscape level features and habitat measures derived from aerial photographs, when organized within a GIS, are suitable for quantifying habitat requirements of small forest birds; whether shifts in habitat use are detectable; and what factors might precipitate habitat shifts.