SPRING MIGRATION AND DISPERSION OF WOODLAND CARIBOU AT CALVING

Each spring from 1975 to 1985, a small herd of caribou, Rangifer tarandus, migrated in April to the islands in Lake Nipigon, Ontario where they remained for the summer. Three hypotheses for this migration were: (1) to reduce insect harassment, (2) to seek more abundant or nutritious forage, or (3) to reduce predation risk. The food and insect relief hypotheses were neither necessary nor sufficient explanations for the spring migration of the animals. Caribou used the shoreline of the two largest occupied islands even in the absence of insects. There was less green forage on the islands than on the winter range and the smaller islands were overgrazed. The dispersed nature of the population, the use of shore-lines useful for escape, and the avoidance of islands travelled by wolves, Canis lupus, suggest that the migration of this herd was a spacing tactic to reduce predation risk. In other woodland caribou herds in North America, animals are also widely spaced in relatively safe habitats after spring migration, which suggests that migration and dispersion may be a common tactic to reduce predation risk especially for females and neonates. © 1990.