Green and his fellow researchers have set up temporary quarters inside one of the area’s many private hunting lodges, and its walls are lined with the taxidermied heads of native and exotic game animals. But Green, a professor of ecology and evolutionary biology at the University of Michigan and a National Geographic Explorer, has eyes only for the three dozen monarchs he captured earlier in the day. He applies a dot of epoxy between the wings of the butterfly in his hand, then affixes a custom-designed sensor—a stack of computer chips powered by a miniature solar panel that together weigh less than three grains of rice. The soft flutter of wings is the only sound in the room.