Why have some birds lost the ability to fly?

Flight is an expensive strategy. Flight muscles make up about 20 per cent of the total weight of a flying bird and reducing this muscle mass allows birds to survive on a lower-energy diet.

Flying lets birds escape from predators but remote islands often don’t have land predators anyway. For ostriches and emus, evolving a large body was a better defence against predators than flying.

Water-loving penguins, meanwhile, probably evolved from diving birds whose bodies were specialised for swimming at the expense of flying.