Why do people wear pants? Warmth, decency, self preservation, and respect are all good reasons. But what use would a bird have in donning a pair? That’s the mystery surrounding a group of tropical hummingbirds that, unlike most of their kind, are feathered from head to toe. With an extra layer of fluff around their tarsi (the bird word for legs), these 15 or so species come off as more glamorous than demure, flashing their Renaissance “pants” when they perch or ferociously attack flowers. The evolutionary purpose for their costuming is less clear, though. While the trousered hummingbirds span two genuses (the closely related Eriocnemis and Haplophaedia, which combined are called the pufflegs), the feature seems to only pop up in species living around the Andes mountains. That would point to feathered thighs being a cold-weather adaptation—but as 探花精选 field editor Kenn Kaufman notes, there are species at even higher elevations that go commando. Instead, Kaufman...