I looked as closely as I dared. And it turned out to be a group of bats. But they didn't have normal wings. As I studied them, I realized that they had six arms. And instead of wings formed from the skin between long fingers, they had flaps of skin that stretched between the wrists of their arms like the skin of a flying squirrel.
In my dream, I fished out my phone, and tried to take photos of the six-legged bats. It was hard, because I didn't dare clean the window, it was too disgusting, and I couldn't feel sure that it would be safe to do so. So I stretched my arms out with the camera, as close as I could, and as steady as I could and took a few shots. The skin around their arms was a little translucent because there was street (or moon) light behind them. It was fascinating.
After I woke up, I had to try and sketch them, to remember the dream.