This is an Accipiter, one of the falcon-like hawks in our area. Falcon-like because they have long tails, long wings and long bodies, similar to a peregrine falcon. They are woodland hawks, their shape designed to help them navigate at speed through treed areas. They nest in the area, occasionally even at the Arboretum/FWG, though not for some years. The cooper's hawk feeds on large prey such as rock pigeons and I've seen them with red squirrels too.