Falcon

I think it is at least
One of a pair who terrorize the pigeons in the tall tree in front of my office window
They make a lot of noice, which doesn't bother me and shit upon the window sill (a lot)
Today I saw one of them attack a pigeon in flight, he hit it, but couldn't get a hold (they look heavier than the falcon)
Not easy to get a decent picture, they are very fast