FIG. 23-4 Present distri 

 from distributions a* th 



winter, on the tips of birch, aspen, cedar, balsam fir, 

 and various other shrubs and small deciduous trees 

 (Shelford and Olson 1935). Small mammals are 

 abundant. In northern Michigan, the populations of 

 two species each of mice, chipmunks, and shrews 

 varied from 6.2 individuals per hectare (2.5/acre) in 

 jack pine to 12.5 (5.0/acre) in black spruce, 16.0 

 (6.4/acre) in hemlock, 19.5 (7.8/acre) in a white- 

 cedar swamp, and 28.2 (11.3/acre) in white birch 

 (Manville 1949). 



Perennial animals that remain active over winter 

 have a high tolerance of low temperatures and use 

 food not readily obscured by snow (Snow 1952). 

 The large mammals become browsers in the winter. 

 Wapiti chew bark patches off aspen trees when other 

 forage is difficult to find. Scars thus formed are ideal 

 sites for the development of fungus disease (Packard 

 1942). Birds feed on seeds extracted from the cones 

 of the coniferous trees, on buds, and on bark insects. 

 When the seed crop fails, large numbers of pine 

 siskins, pine and evening grosbeaks, red crossbills. 



:a. Post-Pleistocene di 

 (from Peterson 1955). 



and white-winged crossbills emigrate southward into 

 the United States. Small ground and subterranean 

 animals are well insulated under the snow where 

 temperatures even in the far North may drop only a 

 few degrees below freezing (Pruitt 1957). Some 

 birds, such as the grouse, roost at night in cavities 

 formed in snowbanks. 



Less than half of the nesting bird population of the 

 western forest biociation migrates for the winter, 

 and then only to lower altitudes on the mountains. In 

 contrast, the birds of the boreal forest are acclima- 

 tized to warm climate, and over 80 per cent migrate 

 hundreds of kilometers to the south. A few mam- 

 mals also migrate, such as the hoary bat in the East 

 and the wapiti and mule deer down the mountain 

 slopes in the West. 



Insects virtually dominate the forest, at times. 

 Vast numbers of mosquitoes and flies force moose to 

 spend much of the summer submerged in water, and 

 are generally annoying to other animals and man. 

 The larch sawfly has spread across Canada and the 



Coniferous forest, woodland, and chaparral biomes 309 



