Our strong impression at the time was of a large bird, the size of a smaller hawk. It seemed that we'd seen a peregrine falcon. Upon reflection and looking in the books, however, it would seem that this is not the case. The light nape of the neck is more characteristic of a kestrel. A peregrine would more likely have a dark neck giving it a helmeted look.

Crane Swamp, Northborough, MA, 8/21/11