Many island rails are flightless because on a small island without threatening predators they do not need to fly or move long distances. Flight makes intense demands, with the keel and flight muscles taking up to a quarter of a bird's weight. Reducing the flight muscles lowers the metabolic demands, and reduces the flightless rail's energy needs. For this reason, flightlessness makes it easier to survive and colonize an island where resources may be limited. Flightlessness can evolve extremely rapidly in island rails; it took as little as 125,000 years for the Laysan Rail to lose the power of flight and evolve the reduced, stubby wings only useful to keep balance when running quickly.