Bats in the Belfry

A few weeks ago, we started hearing a tapping on our bedroom wall at night. I assumed it was a tree branch scraping the side of the house or a leaf stuck in the shingles and blowing on the wind. So I cleaned up any debris I saw and thought it would go away. Unfortunately it hasn't - and it so bad last night that Melissa moved into the guest room!

If there's nothing outside, it must be inside.  Fearing that could be bugs we called an exterminator today.  Melissa led him through the house and attic, and he didn't find anything.  That's good, right?  Maybe not.

He's 99% confident that its not bugs, mice or rats (thank Christ, I would have moved immediately if it were rats).  But he does think its either squirrels, birds or bats.  There's another woodpecker hole (http://www.ithinkweoverpacked.com/2009/07/weekend-honey-do-list.html) that I didn't see and therefore didn't patch.  That's where they'd be getting in.  And he didn't see any sign of a nest, so they haven't set up camp yet.  Yet! They're just coming and going at night.

Now, how do I get them out?  I could:
a) plug the hole. But if they're still inside they'd be stuck and either die and rot, try to dig another hole or try to get into the first floor.
b) sit in the attic all night and kill them when they show up.  That's neither humane nor reasonable.
c) get a one-way door that lets them out but not in.

Option C is really my only option. I'm going to stop at Home Depot tonight and pick one up. Will I get to install it tonight?  Probably not. Unless its a bat it will probably be indoors at night. I don't want to be up on a ladder when it shoots out the hole because of my hammering.  So odds are we'll have this thing in the house at least until the weekend.

Man, this sucks!