Three major woodland caribou habitats are: open larch or black spruce bogs (the major source of arboreal lichens), intermediate to mature jack pine rock ridge forests (the major source of ground lichens) and rock ridge-shored lakes (major travel, loafing and feeding areas at the beginning of the spring thaw). During early winter, the caribou feed intensively on arboreal lichens in open bogs under windless, thin snow cover conditions but, if the reverse conditions exist, intensive feeding shifts to ground lichens found on ridge areas. During the remainder of the snow period, major feeding is on intermediate to mature jack pine ridges where the snow cover is softer due to the lack of wind crusts and thinner due to qali formation. Major utilization of lakes occurs only during periods of thick snow cover when the nival conditions on lakes are more conducive to loafing and travel than adjacent forest types. The woodland caribou threshold of sensitivity to nival conditions is approximately 65 cm. The hardness threshold is approximately 80g/cm2 for jack pine ridge areas, 400 g/cm2 for open bog areas and 700 g/cm2 on lakes. The density threshold is approximately 0.20 to 0.36 for jack pine ridge areas, 0.18 to 0.24 for bog areas and 0.25 to O.33 for lakes. These thresholds vary with the thickness of the snow cover in the three habitats and height of hard, dense layers above the substrate. A minimum of 183 woodland caribou inhabited the extensive study area during the study period. This population was comprised of five groups that ranged in size from 8 to 55 individuals. No overlap in their winter ranges was evident. In a winter of thin snow cover, the bands making up the resident groups are smaller and feed more extensively over their winter range. Conversely, in a winter of thick snow cover, there is a greater aggregation of individuals into larger bands which feed intensively in small areas of their winter range. Association between whitetail deer and woodland caribou is almost non-existent. Association between whitetail deer and moose is high only during periods of thin snow cover when the two species inhabit the same habitat type. Association between moose and woodland caribou is less than what would be expected by chance and this lack of association is primarily due to ecological segregation. Moose appear to be restricted little in this portion of their winter range though they are generally observed on high ground or ridge areas during the onset of the winter period. When the bogs and swamps are frozen, they again inhabit a melange of habitat types and during late winter, are frequently found in areas which harbored deer in the early winter months. In the East Lake Winnipeg snow regime, average snow cover thicknesses have little effect on moose activity; any shift in activity normally does not occur until large areas exhibit snow cover thicknesses in excess of 70 cm. Whitetail deer are influenced most by the nival environment and, while inhabiting mixed deciduous-coniferous forests during the major portion of the winter, they are restricted to areas offering thin, soft snow conditions during January and February. Of the three ungulate species in the study area, whitetail deer are first to exhibit a response to the nival conditions and react to snow cover thicknesses in excess of 25 cm by moving from normal summer range to areas with more favorable snow conditions.