To the small group of young scientists tracking rare birds through the steep, muddy rainforests of Maui, it was a thrilling opportunity: a visit from the eminent ornithologist who literally wrote the book on the group of species they were studying. They begged their boss, Hanna Mounce, to arrange a meeting. The ornithologist agreed to join them at their field site, a remnant stretch of native forest on the misty, windward shoulder of Maui’s towering Haleakala volcano. But all he seemed to talk about was how dark the future looked for Hawaiian birds, how lucky the team was to see them before they inevitably died out. “He said, ‘I don’t work in Hawaii anymore, because there’s no real hope,’ ” remembers Mounce. “And I was, like, ‘Get away from my field techs!’ ” Mounce can’t stand that kind of talk, though she hears it all the time because in the conservation world Hawaii is most renowned for extinction. The arrival of Polynesians and then Europeans...