Flight

Most birds fly, with some exceptions. The largest birds, the Ostrich and the Emu, are earthbound, as were the now-extinct Dodos and the Phorusrhacids, which were the dominant predators of South America in the Cenozoic period. The non-flying penguins have wings adapted for use under water and use the same wing movements for swimming that most other birds use for flight. Most small flightless birds are native to small islands, and lead a lifestyle where flight would confer little advantage.

Among living animals that fly, the Wandering Albatross has the greatest wingspan, up to 3.5 meters (11.5 ft); the Great Bustard has the greatest weight, topping at 21 kilograms (46 pounds). [link]