Partial migration by large ungulates: characteristics of seasonal moose Alces alces ranges in northern Sweden

We studied seasonal migration of individually radio-collared moose Alces alces in a partially migrant population in northern Sweden. First, to investigate habitat selection at the level of individuals, we examined use vs availability of habitat characteristics within seasonal ranges. Second, to investigate what habitat characteristics migrants use to select these seasonal ranges, we contrasted the characteristics of summer and winter ranges of individual migrants. Third, to investigate why some moose migrate whereas others do not, we contrasted the snow conditions and the composition of the vegetation in the seasonal ranges of 36 migrant and 30 resident moose. At the level of habitat selection within a range, moose selected areas which had less mire, clear-cut and field habitats, as well as less snow. At the level of the entire range, migrants and residents had rather similar range compositions and differed only in migrants having less field habitats in their winter ranges; the proportion of Scots pine Pinus sylvestris stands, Norway spruce Picea abies stands, mire, deciduous, and clear-cut forest habitats did not differ, nor was there a difference in snow depth. Similarly, within the group of migrants, we detected no differences in habitat composition or snow depth between the summer ranges they just left, and the winter ranges they moved to. Snow quality (as indexed by the depths to which moose sank) did not differ between the two seasonal ranges of migrants, but calves sank less deeply in ranges of migrants than in ranges of residents, suggesting that snow quality may play a role in the selection of ranges by moose or their decision to migrate/remain resident. Thus, habitat composition and snow depth were important at lower levels of habitat selection, but differences among home ranges were not dramatic. The observation that snow quality differed significantly between ranges of residents and migrants suggests that future studies of migration might profitably investigate snow quality.