I once had a heart-stopping (for me, not the birds!) event, with the glass front - a light tap followed immediately by a tremendous 'thump' - I actually think I 'felt' it as much as heard it! I ran to the glass doors - and there on the deck (with 'stars' actually swirling around its head - or so it seemed ...) was a hawk! Northern Harrier - my guess was, he chased a smaller bird, which hit the glass first, but not hard - and the harrier in hot pursuit could not quite swerve soon enough to miss the impact. I was nonplussed - you could have knocked me over with a feather - first opening the glass door, thinking, "WOW! I'm gonna get the chance to HOLD a HAWK whilst it recovers!!" Then, thankfully, 'calmer voices in my head prevailed', and I realised I dare not reach for the hawk unless I didn't value my fingers, arms, and possibly nose, ears, or eyes! I turned back to the room, trying to decide which to grab - the heavy leather fireplace 'gloves' hanging from the tool rack on the hearth - or the camera. I opted for the gloves - but by the time I had them on, and opened the glass doors, the hawk was already up on its feet, again. I was mesmerised - just stood there, watching him. It was like a cartoon - he staggered a bit, opened his wings, then closed them again, and walked a bit more. Finally, he pushed off the deck, and was back in the air. He took a spot on one of the sweet gum tree branches, and just sat for a while. Bet he had a heck of a headache! On the window, he left the 'dust ghost' from his collision - and just below that 'ghost' there were a few small grey breast feathers, as would be seen after a titmouse or similar size bird hits. The smaller birds do indeed seem to hit with less fatal impact - and I bet the little bird in this event had a whale of a tale to tell, when he got back to his companions!