Birds on Islands Are Losing the Ability to Fly

Flight muscles come with a cost. Even at rest, larger ones require more energy to maintain. So if birds can get away with smaller ones, evolution pushes them in that direction. Large flight muscles are especially useful when birds take off. That’s the most energetically demanding part of flying, and the bit that’s most important for escaping from ground predators. If such predators are absent, birds can take off at a more leisurely pace, and they can afford to have smaller (and cheaper) flight muscles.