We asked our friend and local hawk/wildlife photographer Goggla for insights into who this might be...
Looks like an adult, but I'm not sure who it is. Definitely not Christo (not his coloring), but it could be one of the Washington Square hawks who have become mysterious. They did not nest in WSP last year, but no one knows if they nested somewhere else or not. Union Square was always part of their territory and they'd often perch on the Con Ed building lantern on 14th Street. That pair is different now than the ones Roger Paw blogged about, so their habits are not really known.
Check out Goggla's photo site for more on our resident red-tailed hawks, Christo and Amelia, and others.