The average bat would hibernate every winter, similar to the snakes and frogs. They would sleep for at least a month, but the period could last as long as half a year. The Vampire Bats, on the other hand, were a unique species that did not need to hibernate at all.
Aside from their ability to go about their business year round without hibernating, their lifestyles were still similar to that of the average bat. Under normal circumstances, they would rarely come out during the day, unless they caught the scent of a Beastman or felt they were in danger.
When it was deep into the evening, many wild beasts would flock to drink by the Black Water Lake. That was the time when the Vampire Bats were most active. They would appear in groups, swarming toward the wild beasts like flies to drink their blood.