Poll: do you know birds? 60 mph+ bird flying "erfly stroke" style, what bird?

Fastest bird I've ever seen, doing a erfly stroke with each wingflap. With each wingflap it went down several feet and then back up. This is not the kind of bird that flaps its wings a few times real fast and then glides for a while. My first thought was that someone shot something from a cannon it flew by so fast, then I saw it was a bird. Size midway between a sparrow and a crow possibly, dark color, maybe black. It took it about 4 or 5 wingflaps to travel 200-300 feet and be obscured behind some trees. Flew straight as a crow, except for the up and down movements.