In Indiana this temperature increase is critical. If baby bats are three weeks late in reaching the flying stage, they do not have time to eat enough bugs to gain enough fat to make it through the winter. Coming up with evolutionary scenarios to explain this might appear to be pretty easy. Bats that congregate survive, and bats that do not die. The fact is that all bats seem to have this behavior.
Red bats cluster in broad leafed trees.
Seminole bats roost in clumps of Spanish moss. Wood bats cluster in hollow trees. In many of the locations, bats would not have to have this behavior to survive because the temperatures of the environment are high enough to avoid it. It appears that this is a genetically designed characteristic that allows bats to live in all kinds of temperatures and environments, and still have the ability to survive and produce offspring.