Selective Use of Black-Tailed Prairie Dog Towns by Mountain Plovers

Abstract

--Habitat use by Mountain Plovers (Charadrius montanus) was studied in north-central Montana during 1978 and 1979. Mountain Plovers were found to selectively inhabit black-tailed prairie dog (Cynomys ludovicianus) towns. Horizontal visibility and bare ground were significantly greater inside prairie dog towns used by plovers than adjacent areas. Total plant cover and grass cover were significantly lower inside prairie dog towns than on adjacent areas. Most towns on the study area were associated with an area that was intensively grazed by cattle. Plovers used only the active towns larger than 3 ha located on level upland sites (n = 16 out of 35). The ecology and behavior of the Mountain Plover (Charadrius montanus) have been studied in southeastern Wyoming and northeastern Colorado (Laun 1957; Baldwin 1971; Graul 1973a, b, 1974, 1975; Graul and Webster 1976). None of these reports mention Mountain Plovers using prairie dog towns. Cameron (1907) found these plovers to be associated with prairie dog towns in southeastern Montana. Black-tailed prairie dog (Cynomys ludovicianus) towns occupy level sites, are intensively grazed by cattle, and have short vegetation. Prairie dog towns represent a distinctive habitat in this region and also are used by Homed Larks (Eremophila alpestris), Killdeer (Charadrius vociferus), and Burrowing Owls (Athene cunicularia). The purpose of our study was to investigate the association of Mountain Plovers with prairie dog towns, and to describe related habitat characteristics. STUDY AREA AND METHODS Our study area, comprising 560 sq km on the north side of the Missouri River, was 100 km southwest of Malta in Phillips County, Montana on the Charles M. Russell National Wildlife Refuge (CMR; Fig. 1). It is within the timbered breaks of the river and is characterized by alternating deep-cut coulees and steepsided ridges. Major ridgetops are broad and flat, becoming progressively wider with increasing distance from the river. Ponderosa pine (Pinus ponderosa) and Rocky Mountain juniper (Juniperus scopulorum) dominate slopes with northerly exposures. The dominant plant species on the level ridgetops are western wheatgrass (Agropyron smithii), blue grama (Bouteloua gracillis), prickly pear (Opuntia polycantha), fringed sagewort (Artemisia frigida), and big sagebrush A. tridentata). Knowles (1975) described fully the vegetation of this area. The mean annual precipitation and temperature are 35 cm and 6.6?C. To obtain distributional data on Mountain Plovers, we established a system of 18 vehicular transects throughout the study area (Fig. 1). Nine of the transects, totalling 19.2 km, were in prairie dog towns, and nine others, totalling 18.5 km, were outside of towns. The route was run 10 times, generally in the morning, at approximately one-week intervals from I June to 8 August 1979 using a three-wheeled all-terrain motorcycle. Cruising speed along the transects was 15 km/h; we stopped only to record observations of Mountain Plovers visible from the vehicle. A chi-square test of homogeneity was used to test the hypothesis that numbers of plovers within and outside of towns were homogeneous. Horizontal visibility within and adjacent to six prairie dog towns used by Mountain Plovers was measured with a coverboard 2 m high, I dm wide and divided in 1 dm intervals. One transect was placed inside and one adjacent to each of the six towns. Each visibility transect consisted of two parallel, 150-m lines spaced 60 m apart. Six stakes were placed along each line at 30-m intervals. Coverboard sightings were taken from each stake along one line perpendicular to the corresponding stake on the opposite line, and diagonally to the closest stake left and/or right of the corresponding stake (stake 1, line A to stake 2, line B, etc). This scheme allowed 16 measurements per transect. Eye level for the sightings was at 1 m and visibility was estimated to the nearest 5%. A Mann-Whitney U-test was used to test