Food habits, range use and home range of pronghorn antelope in central Montana during winter
Bayless, Stephen Roy
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A study of the food habits, range use, and home range of pronghorn antelope with emphasis on the winter period was conducted in 1966-67 on a 171,712 acre area in north-central Montana, The physiography and vegetation of the area was described. The vegetation consisted of eight types: sagebrush-grassland, grassland, shale slope, greasewood, greasewood-sage-brush, cropland, abandoned meadow, and woodland. Quantitative measurements of canopy coverage and densities of taxa were made for five of the eight types. Characteristics of the antelope population were evaluated from results of five aerial censuses in summer. There were 309 females per 100 males and 74 fawns per 100 females. Distribution of antelope and use of vegetation types was evaluated. The sagebrush-grassland type received most of the use, both in summer and winter. Most of the antelope observations in winter were in vegetation types where sagebrush was common. The average group size for antelope in winter was 23.5, with groups being largest when snow covered the ground. The average group size in summer was 9.4. Winter food habits were determined from the examination of 28 feeding sites and analysis of the contents of each of 18 rumen samples. The winter diet of antelope, determined by examination of feeding sites, consisted of 93 percent shrubs, 6 percent forbs, and a minor amount of grass. The diet determined by analysis of rumen contents consisted of 78 percent shrubs, 19 percent forbs, and a small amount of grass. Big sagebrush (Artemisia tridentata) provided the bulk of the antelope food. A statistical analysis was conducted to determine the relationship between abundance of available plant species and their significance in the diet of antelope. Big sagebrush was the only plant which was present in significantly higher proportions in the diet than in the plant community. Three dead fawns were found on the study area. The apparent cause of death was malnutrition. Their rumen contents did not vary significantly from those of other antelope collected during the winter. Poor body condition of antelope and fawn mortality was possibly related to quality of sagebrush in the diet. Winter home range size was determined for each of 16 individually marked antelope. Each of three was equipped with a radio transmitter. Marked antelope were located daily when possible, A total of 579 individual observations of marked antelope was recorded from December 10 through March 23. Eight of the marked antelope "shifted" their home range at least once in winter. A yearling female had a home range size of 5574.4 acres and a yearling male, 4l60.0 acres. Six adult females had an average home range size of 2841.4 acres; three fawn females, 2417.1 acres; and five fawn males, 1579.5 acres. Fawn females had the most variable home range size. Adult females used a larger area on the sagebrush-grassland vegetation type than on the grease-wood-sagebrush type. Both fawn males and females showed the opposite.