How many are we looking at? I ask for one reason only, depending on species it can be a maternal colony. I had a maternal colony of 300 or so little brown bats in my house a few years ago, this time of year the young have been born but not weened. If you seal the entry, you will only get approximately half the adults the remainder will stay with the young. They will die in the attic and create one heck of a smelly mess or try to get out and more than likely find a way in to your living space. I waited for Sept. then sealed the entry.