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COPYRIGHT 2005 Wilson Ornithological Society
ABSTRACT.--As human disturbance of natural landscapes increases, so does the need for information on declining, threatened, and potentially threatened native species. Proposed listing of the Mountain Plover (Charadrius montanus) as threatened under the U.S. Endangered Species Act in 1999 was found unwarranted in 2003, but this species remains of special concern to management agencies and conservation groups. Whereas large concentrations of breeding Mountain Plovers occur in Montana and Colorado, estimates of the numbers of Mountain Plovers in Wyoming have ranged from only 500 to 1,500 individuals and are based largely on conjecture. In 2002, we visited all known breeding locales in the state to define areas of concentrated sightings in the Laramie, Shirley, Washakie, Great Divide, and Big Horn basins. In 2003, we used distance sampling to estimate breeding bird densities in these five areas. We pooled these estimates and applied the resulting density to a minimum occupied range for the Mountain Plover based on the documented sightings and a previously derived home-range size of 56.6 ha [+ or -] 21.5 (SD) to generate a minimum population estimate for the state. Average Mountain Plover density was 4.47 [+ or -] 0.55 (SE) birds/[km.sup.2]. We calculated a minimum population estimate of 3,393 birds for Wyoming. The Mountain Plover population breeding in Wyoming appears to contribute substantially to a revised continental population estimate of 11,000 to 14,000 birds. Our approach may have applications to quantifying minimum population status of other uncommon species or species of special conservation concern using current database records, such as those compiled in Natural Heritage Programs at the state level. Received 28 January 2004, accepted 10 December 2004.
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The Mountain Plover (Charadrius montanus) is one of 12 avian species endemic to the grasslands of North America (Mengel 1970). Plovers nest on the shortgrass prairie and shrub-steppe of the western Great Plains and Colorado Plateau, especially in areas used historically by large assemblages of herbivores, such as prairie dogs (Cynomys spp.), bison (Bison bison), and pronghorns (Antilocapra americana; Knopf 1996a). The species winters from north-central California to Arizona, Texas, and northern Mexico.
Once numerous in Colorado and Wyoming and common in western Kansas, South Dakota, and Nebraska, Mountain Plovers began to decline throughout their range early in the 20th century (Laun 1957). They have continued to do so over the past 30 years at a rate approximating 3% per year (Knopf 1996a). As a result, the species' continental breeding range has been significantly reduced. Today the majority of the Mountain Plover's breeding range is restricted to east-central Montana (Skaar 2003), the tablelands of Wyoming (Oakleaf et al. 1992), and eastern Colorado (Andrews and Righter 1992, Kingery 1998). The North American population was recently estimated at 8,000 to 10,000 birds (Knopf 1996a).
In response to evidence of the species' widespread decline, in 1999 the U.S. Fish and Wildlife Service (USFWS) proposed listing the Mountain Plover as threatened under the U.S. Endangered Species Act (ESA) (U.S. Fish and Wildlife Service 1999). The USFWS recently determined that threats to Mountain Plovers and their habitat are not likely to endanger the species in the foreseeable future; thus, the proposed listing of the bird was withdrawn (U.S. Fish and Wildlife Service 2003). Regardless, the Mountain Plover remains as a species of special concern to wildlife and land managers throughout its range.
Although significant breeding populations occur in Montana and Colorado, there is evidence that Wyoming may provide habitats for many breeding Mountain Plovers as well. Survey efforts for plovers in Wyoming, especially in the wake of the recent ESA proposal, have revealed pockets of breeding birds throughout the state, particularly in south-central and eastern Wyoming. The contribution of Mountain Plovers in Wyoming to the continental breeding population is poorly understood, as no reliable statewide population estimate exists. Rough estimates, based largely on conjecture, have ranged from 500 to 1,500 individuals (FLK).
Wunder et al. (2003) recently estimated the size of a similarly undefined population of Mountain Plovers in South Park, (Park County), Colorado. Distance sampling was used to estimate density of breeding plovers, from which a population estimate was extrapolated based on an estimate of occupied habitat. Plovers in South Park occurred at an average density of 7.9 [+ or -] 0.9 (SE) birds/[km.sup.2] across sampled portions of >80,000 ha of potential habitat. Wunder et al. (2003) concluded that South Park, with an estimated population of >2,300 Mountain Plovers, contributes 1520% of breeding plovers to the continental population.
Following the success of Wunder et al. (2003) in generating a population estimate in South Park, Colorado, we used distance sampling to generate density estimates of...
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