The Phainopepla is a singular bird of the Southwest. There is, for instance, the songbird’s unusual looks: With its shining black plumage, fierce crimson eyes, and punk-rock crest, the male has been likened to a goth cardinal (it’s actually a member of the silky flycatcher family). And then there is that intimidating name, which comes from the ancient Greek for “shining robes." Adding to its allure is the mystery surrounding the Phainopepla’s sex life: It’s long been suspected that the birds breed twice per year, in two completely different habitats—something only two other avian species are known to do. But proof has been elusive. Two years ago Dan Baldassarre, an ornithologist at SUNY Oswego, set out to solve the mystery. “I’m always interested in bizarre behaviors that are sort of well-known by birders but that haven’t been poked and prodded much in an empirical way,” he says. The Phainopepla’s breeding behavior has baffled birders since as early...