This is the time of year when the weather starts warming up and bats begin to come out of hybernation and looking for food. But bats have a very high metabolic rate, if they don't find food they rapidly tire and end up landing whereever they might be. I suspect that's what's happening with the two bats you found.
Familiars are not physical animals, they are magical servitors or constructs created by the magic user. You could make one in the form of a bat, but real bats are not familiars.