Rosemary Jaffe

Winter kill rates and prey selection of gray wolves (Canis lupis) were studied in the upper Madison drainage of Yellowstone National Park, Wyoming. Elk (Cervus elaphus) and bison (Bison bison) formed the ungulate prey base in the study system and a single wolf pack used the study area as its winter territory. Daily ground telemetry, snow tracking, and necropsies were used to acquire data from mid-November through May, 1998—1999 and 1999—2000. During the two winters 108 wolf kills were located. Wolves preyed primarily on elk and showed the strongest selection for calves, which comprised half the diet (n=56). Thirty cow elk ranging in age from 1 to 15 (mean=9.9) and 8 bulls ranging in age from 1 to 10 (mean=5.1) were also killed. Although bison were more abundant than elk, only one adult and 13 calf kills were found. Prey switching was apparent during the first year of the study. By late March, no elk calf kills were found while cow and bull elk and bison calves were killed at an increased rate. An analytical method was developed to estimate smoothed kill rates across time using a moving window average and a weighting scheme to account for undetected kills. This technique indicated that 30% of the wolf kills were undetected. Estimated kill rates (kills/100 wolf days) nearly doubled from fall to spring each year, and were approximately twice as high the first year (11.8) as the second year (6.5) of study. Two factors that likely influenced