Weeping Willows and Sad Water Lilies

Yesterday I stood on the balcony at half past nine, when it was a little less hot outside then inside my apartment. I listened for bird calls, but there were none. Even the swallows and the seagulls seemed to be asleep already. Instead, there were bats!
They weren't easy to see, but as I was looking at the sky, I noticed those little dark shapes fluttering by. They looked almost like leaves in the wind, but clearly had a mind of their own, flying back and forth in front of me.  
I wonder what species they were. It's impossible to tell. They could have been pipistrelles, given how small they were, but there's no way to be sure. During the day, they probably live in holes in the old trees beyond the lawn, or perhaps in little cracks in the walls of the building.
I hope they caught a lot of mosquitoes.