For last 2 years I have bought some bat guano from 2 guys on ebay. One is Lyle caves who gets his from cave in NW Ark. I have put it on top of house and in a black plastic MW food container underneath on a u bolt- and moved it close to opening. Most will say it makes no difference, but I have seen my numbers increase last 2 yrs. You can see it on the pix I posted last night . I have 87 in there as of last night. Go read article avail abut H.Kizer who did so on his house near Zwolle,La on Toledo Bend. He increase his number dramatically. He is the guy who put a wook duck house on top of bat house and both co-existed fine.
I do believe scent is the key. They won't touch a new house 6" away from this one with 87 and it was put up last May..