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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.