Ecology of pronghorns on the Piñon Canyon Maneuver Site, Colorado

Abstract

During January 1983 - July 1985 the population dynamics, movements, and habitat use of pronghorns (Antilocapra americana) were studied on the 1040 km² Piñon Canyon Maneuver Site, in southeastern Colorado, prior to initiation of military training. Forty-seven adults and 32 fawns were radio marked; 247 adults were color collared or ear tagged only. Using aerial strip and quadrat surveys, I estimated a summer population for 1983, 1984, and 1985 of 694, 690, and 657 respectively. Adult female pregnancy rate was approximately 98%, mean litter size for females > 1.5 years was 1.9, and fetal and fawn sex ratios were not different from 1:1. Fawn mortality was 89% in 1983 and 80% in 1984. Adult mortality was 20% in 1983 and 28% in 1984. Annual rate of increase (λ) over the 2 year study was .806. Home range size of pronghorns was extremely variable within and between sexes and among seasons. Winter ranges were 2-fold and 5-fold larger than summer range for females and males, respectively. Sixty-five percent of all weekly movements were < 3 km. Females preferred open grassland in all seasons, and cholla grassland in winter. Yucca grassland was preferred in spring and fall, and cholla/open grass edges were highly preferred in all seasons. Males preferred open grassland in all seasons but winter, and preferred cholla grassland in all seasons but spring, cholla/open grass edges were also preferred in all seasons. Fawns used open grassland and cholla/open grass edges more than expected, while using woodland and shrub habitats less than expected.