At around dusk (a little before actually) go outside and look for them to fly OUT of the attic. You may need to enlist the help of some neighbors to "stake out" the attic. When you find where they are flying out, you have now found where they are getting in. Then go into the attic or on a ladder from the outside - plug up the opening. If you decide to go into the attic (Thats the way I do it)be prepared for a foul stench. Take a towel to breath through.