I actually saw this happen, out of the corner of my eye: something dark flashed against the window facing the street, and I heard a thump. Damn kids throwing something, I thought, and then thought again: did we just have a bird fly into the window?
It's a big square window with no interior panes. It was possible. I went out and peered among the hellebores under the window. Yup, on the ground was a small brown bird - lying on its back, with its little feet sticking up in the air, panting rapidly. I wondered if I should try to rescue it, and then thought no, it's likely just stunned. I never saw a bird lie on its back before.
I hadn't, of course, brought my camera, since I wasn't sure what I'd find. I scrambled back in the house, changed to the long lens, and came back out. The bird was still there, now right side up, standing (actually sort of squatting) on the ground. I took a picture:
The bird ignored me. I watched it for a minute and then went back in the house. I went back out to check on the bird maybe 4 or 5 times over the next half hour, and every time it was sitting there staring blankly. Finally, I went out to check and it was looking around, then it turned, hopped a couple of times, and flew off. OK, recovered.
My husband says there's a reason for the term "bird brain."