Of the planet's thirty-nine species of falcon, the largest and most variable is the circumpolar Gyrfalcon (Falco rusticolus). A bird of the arctic, it only occasionally migrates very far south of the Canadian border, although I suspect it does so more than is generally thought, for its long tail and chunky build often cause it to be mistaken for a Goshawk in the air or a Rough-legged Hawk when perched. Judging from the migrants I've seen and trapped in Utah, Idaho and Wyoming, most seem to be young birds, often unwell. In this area wintering Gyrs seem to prey mostly on ducks. The typical plumage is grayish overall, with barred and spotted patterns, but the species varies from individuals that are nearly uniform black to ones that are almost immaculate white. One of my favorites is the "silver phase," where an essentially white bird is heavily marked with charcoal gray.