One bat in east house on 6/9/11:

I'm pretty sure that this is the same bat that was in the west house last month, because it was in the exact same position as it was in the west house last month.
Note that this house is mounted to a live tree, which has a long section of trunk with no branches. So while it is generally a bad idea to mount a bat house on a live tree, this shows that in certain circumstances bats will use a house mounted to a live tree.
Here you can see a bit more of the tree:

I was having a terrible time getting the focus right, apparently the bat got fed up with me and crawled into the attic space. This photo was taken 3 minutes after the one above.
