We watched them for a little while to figure out what they would possibly be doing hanging out on second-storey windows. We realized they are eating bugs that come up to the glass, attracted by the light shining through it. These little tiny green frogs grab mosquitos and moths that come up near the glass. It is really fun to watch them hard at work.
This is one of the little frogs that came out the other day as I was watering a flower bed on the ground down below the windows.