The Rough-Legged Hawk breeds in the far north, like the Snowy Owl, but during winter can be found throughout the US, especially in open spaces.
He is a very large hawk with black thumbprints on his wrists - unmistakable in flight. And like most hawks and owls, he is hunting mostly small furry things and vulnerable birds. Like the Snowy Owl, I've seen them in Montauk in the winter.
More about the Rough-Legged here.
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Also, happily perusing a recent birding book - National Geographic's The Complete Birds of North America. It is complete, with a good section on gulls, which I use as a judge of a bird book. It's a companion to NG's Field Guide, and has plenty of information, including habitat, population trends, migratory details, and where populations are expanding and contracting. Good on trends in "accidental" sightings, too. For example, the book notes the increased sightings of Swallow-Tailed Kite in New England - and makes note of its historic populations in the northern USA.