Northern Prairie Wildlife Research Center
Fig. 5. Seasonal distribution of verified complaints in Roseau and Kittson Counties compared with all other counties in the wolf range, 1979-86.
The peak in cattle losses in May corresponded to the availability of newborn calves that month; most cattle killed were young calves. The high potential for calf losses in spring demonstrated the potential effect of turn-out date (related to snowmelt) on the cattle loss figures for a given year. A sharp decline in depredations on cattle from May to June, compared with July and August (Figs. 4a and 4b), could have been related to a surge in predation on deer fawns in early summer. Fawns, which are born in June, become the major food of wolves. (Frenzel 1974; Van Ballenberghe et al. 1975; Fritts and Mech 1981; Fuller 1989). Maturing calves are less vulnerable after May or June, which suggests a reason for the overall decline in cattle losses after that time. We noted that calves killed in latesummer were often lateborn individuals born on pasture.
The seasonality of cattle losses in Minnesota differed from those in Alberta and British Columbia, where calf losses peaked in late summer (Dorrance 1982; Gunson 1983; Tompa 1983a). The difference between the areas might result from wolves being closer to cattle in spring and early summer in Minnesota.
No reason is clear for the peak in sheep losses in July and August, as sheep were available on pastures before then. Possibly increasing food demand by pups in midsummer and establishment of rendezvous sites near pastures were factors. In Alberta, sheep were killed during July-October, with no significant variation throughout the pasture season (Gunson 1983). Similarly, no reason is known for the distinct peak in turkey losses in August and September, as they are available on range well before that time. Losses decrease rapidly after September when turkeys are shipped to market.
We recorded several instances in which wolf rendezvous sites were established near turkey flocks in August and September. Each instance was characterized by heavy killing, and pups may have participated because they were known to have frequented the depredation sites. In such instances, wolf packs showed optimum foraging strategy (Schoener 1971) by exploiting an abundant, easy-to-capture, and highly localized food resource. This behavior especially served the high energy requirements of the pups (Mech 1970), which are about 4.5 months old by early September. During periods when wolves killed turkeys on successive days, their entire diet probably consisted of domestic prey. However, in most depredation situations, the biomass obtained from domestic prey was far too small to sustain a pack except for a limited period.
Seasonality of losses in Minnesota were similar to those in British Columbia except that a secondary peak occurs in midwinter there (Tompa 1983a). In British Columbia, wolves follow their prey in winter into areas of lower elevation where most farms are located; this action could account for the winter losses. No such seasonal movement of wolves occurs in relatively flat Minnesota, although some deer yards are near farms and residences, thus increasing wolf activity there in winter.