He is now in graduate school. Where bats live is dependent on the availability of food. When the food supply declines, usually due to weather, bats have two options. They can hibernate to pass through the low or non-existent food supply period, or migrate to a place with a more abundant food supply. In some areas, bats will also do a combination of both.
Migration involves two parts.
Movement from one location when food is scarce and the return to that same location when the abundance of food returns. There is no clear distinction between migrating bats and hibernating bats. Some bat species, like the silver-haired bat, migrate and hibernate. The same is true for red bats. Red bats migrate from the northern portion of their range and hibernate in the southern portion of their range.