Please Explain: What Kind of Bird Is This?

We spotted this bird in a tree just south of the lake in Central Park when it suddenly took off and flew right above our heads. It seemed huge-- like four times the size of your average pigeon-- and it seems to have some distinct markings on the stomach. Any ideas, especially from our ornithologist friends?

Update: Joe says it's a Cooper's Hawk-, which looks pretty similar-- can anyone confirm?