In spring pregnant cows lead the migration to the calving grounds, where each will give birth to a single calf. Newborns can walk within an hour and in a few days they can outrun a person. In mid-summer the harassment of mosquitoes and flies can drive the animals to windy coastal areas or old snow patches for relief. In late August or early September the herd begins to drift toward its winter range. Mating occurs enroute during late September and October.
Caribou are well adapted to winter conditions.
To cope with scarcities and hardships they reduce food intake and lower their metabolic rate. Caribou shift winter ranges from year to year, which minimizes overgrazing. Since heavy snow or ice conditions can make it difficult to dig down to food, they often winter in forested areas where snow cover may be less and lichens growing on trees can be eaten. The great migrations of wild herds in groups of thousands are an inspiring sight. But much of their movement is simply the gradual drifting of a widely dispersed herd.