Implications of home-range estimation in the management of red-cockaded woodpeckers in South Carolina

Abstract

I undertook a behavioral study to determine red-cockaded woodpecker (Picoides borealis) home-range size at the Savannah River Site, South Carolina, USA. In this location, because much of the timber was harvested in the late 1940s and early 1950s, the available habitat largely consisted of younger trees (e.g., less than 45 years old), not generally considered prime habitat for this species. From 1992 to 1995, I observed seven groups of red-cockaded woodpeckers to determine year-round home-range size. Most of the previous home-range studies on this species used the minimum convex polygon approach to estimate the size of the home range. I compared the minimum convex polygon and fixed kernel home-range estimation methods for each group. I found that the fixed kernel method gave consistently smaller estimates of home range than did the minimum convex polygon, a result directly related to the methodologies of the techniques. Mean home-range sizes for the 95% level were 56.9 ± 5.2 S.E. ha with the fixed kernel versus 91.9 ± 11.7 S.E. ha with the minimum convex polygon. Core area (50%) means were 4.5 ± 0.5 S.E. ha for the fixed kernel versus 16.7 ± 2.4 S.E. ha with the minimum convex polygon. It is recommended that future home-range studies use the fixed kernel estimator rather than the minimum convex polygon as it gives a more realistic and appropriate depiction of the area actually used by the birds within a given group. In estimating the number of groups that may be accommodated in a particular area, the mean home-range size as well as its shape need to be considered. Home-range size estimates at the Savannah River Site were similar to those obtained elsewhere in the species’ range. Red-cockaded woodpeckers, in spite of the prevalence of relatively young forest structure, did not increase their home-range size to compensate for the paucity of older, more mature pine habitat.

Introduction

The red-cockaded woodpecker evolved in fire-maintained pine ecosystems within the southeastern United States (Conner et al., 2001). Primarily because of widespread modification and loss of its habitat and range, its apparent rarity, and decline in local populations, the U.S. Fish and Wildlife Service declared the species to be endangered in 1970 (35 Federal Register 16047, 13 October 1970). By 1986, the population of red-cockaded woodpeckers at the Savannah River Site, South Carolina, had dwindled to four birds. At that time an intensive effort was undertaken to try to prevent the species from becoming extirpated from the site, an effort that has proven successful (Franzreb, 1997). Part of this effort entailed discovering more about the foraging behavior and home-range requirements of the bird at this location.

The majority of the Savannah River Site was harvested for timber in the late 1940s and early 1950s in the course of the sale of this land by private landowners to the Federal government. When this study began in 1992, most of the land on the site had been regenerated using several pine species. The resulting forest stands generally were less than 45 years old. Red-cockaded woodpeckers use live pine trees as their primary foraging substrate (Hooper and Lennartz, 1981, Porter and Labisky, 1986). Earlier habitat use studies in other portions of the species’ range dealt primarily with areas that contained a substantially higher proportion of older (i.e., 60–80 year-old) and larger trees (Hooper and Lennartz, 1981, DeLotelle et al., 1987, Engstrom and Sanders, 1997, Zwicker and Walters, 1999, Walters et al., 2000, Walters et al., 2002) in comparison with the tree distribution by age and size at the Savannah River Site. Would red-cockaded woodpeckers increase their home-range sizes to accommodate habitats that largely consisted of younger growth forest stands?

When this study began, the U.S. Forest Service (Region 8) standards and guidelines for the red-cockaded woodpecker specified maintaining 50 ha of foraging habitat per group (see U.S. Fish and Wildlife Service, 1985). However, the 50 ha figure was not universally accepted and, in fact, was challenged (Ligon et al., 1986, Jackson, 1986, Reed et al., 1988).

The objectives of this study were to determine year-round home-range size of red-cockaded woodpeckers at the Savannah River Site to determine if they were similar to those in areas of more mature forest stands and to derive a home-range size. In addition, I wished to compare two home-range sampling methods, the widely used minimum convex polygon method (Mohr, 1947) and the fixed kernel estimator method (Worton, 1989). There is widespread use of the minimum convex polygon approach in red-cockaded woodpecker home-range studies, but its limitations make for an interesting comparison with the recently developed fixed kernel method. Further, I wished to determine if home-range sizes at the Savannah River Site were different than seen elsewhere within the species’ range considering the availability and distribution by age and size class of pines trees for foraging. Lastly, I wanted to assess how the shapes of the home ranges differ among red-cockaded woodpecker groups in this area.