When ornithologist Francisco Sornoza visited Ecuador’s Cerro de Arcos in April 2017, he was hoping to find interesting birds but had no idea he would discover a new species. He had made the 15-hour drive from Quito to this windswept Andes mountain region, named for its rocky arches, to help El Oro provincial government assess the area’s potential for attracting ecotourism. After hiking for three hours, Sornoza—Pancho to his friends—and companions stopped to rest on a high, flat rocky expanse. As they chatted, a small white streak boldly darted through their group, and then settled on a nearby tree. It was a hummingbird, Sornoza realized, and at first glance it looked like a hillstar—a group of species that are adapted to cold, high-elevation environments and live in areas to the north and south. But, he says, something was off: “Why,” he asked himself, “is a hillstar here?” He was far from their documented range, and the bird’s head was an artist-palette mix...