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The hawks must fly into the glass at some high speeds, I'm just happy they haven't blown out the window yet. We had a pheasant crash through our double-pane kitchen window. We found him next to the woodstove, 12 feet from where the window was.

I hope it doesn't happen again, but I'm afraid it surely will.

I had a grouse break out a kitchen window in a cabin we once owned. What a mess that was because it was winter and we were not there and it wasn't discovered until weeks later.

Grouse look like a football when flying, compact and they get up to a good speed very quickly.
I think that many times birds see their reflection in the window and fly towards it. I have also heard that during a certain time of the year, there is a berry, not sure which one it is, but when the birds eat it they become disoriented, almost like getting drunk. This might explain some of the encounters with windows. I think they call that FUI.