When i was dating my wife, we were walking up the drive to my cabin, when a Great Horned owl flew over us, from behind. Just a few feet, and as others have mentioned, completely silent. Pretty impressive seeing such a big bird flying without a whisper of a sound. I'd seen it around, usually perched in a large maple tree.
Another different kind of owl, where we are currently living. In the early morning, lying in bed, I heard, what sounded to me, like someone or something making a fairly loud whistle. I opened the window wider and stuck my head out, to see if i could figure out what the heck it was. Then i decided to slip on a pair of pants, still bare foot, and walked outside, toward the whistling, but for the life of me, i couldn't quite figure out where it was coming from. Later that day, i told my nephew about what had happened, he told me it was a young owl, probably not flying yet, calling to it's mother. He said i was lucky the mother hadn't swooped down on me, to protect the young owl. He spend a lot to time out walking around in the dark, in the woods, and told me, the first time he heard that whistle, trying to find out what it was, he saw a young owl, at the base of a tree, and an adult owl, came at him and caught the edge of his sweatshirt hood and clipped the edge of his forehead. A small but bloody cut, like scalp wounds are. Oh, it was Barred owls and we got to watch the youngster grow up, out our family room window. My wife got a cool looking pic of one, that had perched on a post, on our back deck. Here's one of them.
