An Unsual Visitor

I was surprised this morning by a large blur overhead and a great commotion in some low trees. When I went to investigate, I found this lovely creature hunting squirrels that were hiding on an ivy-covered building.



A surprise guest, much less interested in me than I was in him (or her).

Does anyone know precisely what sort of hawk this is?

Update

The conclusion is that the picture above is an immature Red-Tailed Hawk (Buteo jamaicensis). We have, it seems, a few of them as residents on campus. Here are a couple of (somewhat less dramatic) shots I managed to get. This guy is considerably larger than his immature friend (though his friend is probably now about this size).