Help! What is this bird?

While on route to the Niagara Falls we stopped at a picnic spot in the Niagara Gorge. I quickly spotted several birds of prey, obviously common to the area, but I've no idea what they are! My bird books only cover Europe! Can any one help please! The were buzzard sized, but more streamlined, especially the narrow tail and appeared quite dark in colour whether viewed from above or below. The bill appeared slightly hooked pointing downwards in flight. The image is a collage of three photos.

Done