The Great Black Bear Migration

Do black bears migrate? Well, it depends on the meaning of the word migrate. If by migration you mean regular season movements from breeding areas to feeding grounds, then probably not. However, if you take a wider view and define migration as repeated, predictable movements outside the animal’s normal home range in response to changes in resource abundance (or quality), then black bears migrate - or at least black bears in Minnesota do.

Black bears (
Ursus americanus) are known to move up to several hundred kilometers in late summer in search of high quality food sources (Figure 1). These movements occur at when black bears are feeding voraciously (hyperphagia) in order to fatten up prior to hibernation. But, is this really a migration? Are these movements really predictable?

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Figure 1. Minimum convex polygons of black bear home ranges and measurements of forays outside the bear’s home range (from Noyce and Garshelis, 2011)

For over a decade (1981-1990) researchers in northern Minnesota have been radio tracking black bears.
Noyce and Garshelis (2011) examined the data for 206 black bears of both sexes from the time they were first collared (many were fitted with collars as yearlings in their mother’s den). The bear’s home ranges were determined and movements outside the home range were classified as either seasonal forays, if the bear returned to its home range, or dispersal if the animal never returned. In addition, the researchers assessed food availability and quality across north-central Minnesota to see if the seasonal movements by black bears were correlated with food resources.

In northern Minnesota, black bears, of both sexes and all age classes, made long distance forays in late summer (Figure 2). The timing and destinations varied from year to year, but the typical pattern was for bears to depart their home ranges in August, travel southward to feed on abundant acorn and berry crops, and return to their summer ranges by October. The median distance traveled was 10 kilometers for females and 26 kilometers for males. The maximum foray was 168 kilometers.

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Figure 2. Food abundance indices for acorns (a) and chokcheerys (b) in north-central Minnesota and the percentage of late-season migrations by bears in 1981–1990. (from Noyce and Garshelis, 2011)

Although individual bears did not always travel to the same resource patch each year, these seasonal migrations were common when high quality resources were only available outside the bear’s summer range. Black bears are omnivorous, but prior to hibernation they must rapidly put on sufficient fat reserves to last the winter months. At this time of year, bears feast on a smaller subset of energy rich foods. Like other mobile mammals with restricted diets, black bears are forced to leave their summer ranges when key food resources dwindle. They make long distance seasonal movements to high quality food patches, eat voraciously to store fat, and return to their more familiar ranges in time for hibernation.



References

Noyce, K., & Garshelis, D. (2010). Seasonal migrations of black bears (Ursus americanus): causes and consequences Behavioral Ecology and Sociobiology, 65 (4), 823-835 DOI: 10.1007/s00265-010-1086-x