One can reasonably assume that the ability to fly evolved in birds because it provides more opportunities to secure food and also to escape from terrestrial predators. Physiologically, however, flying is an expensive business. A 17 gram swallow flying at 35 kilometers per hour expends about 50 calories of energy per kilometer flown whereas a 130 gram dove forfeits 390 calories for the same distance traveled. Flying doesn’t come cheap and when the advantages of flight no longer pay for the energy cost, wings may shorten or disappear over a period of time.
As a rule, flightlessness is associated with geographical isolation and relative absence of predators. Such protective environment is found especially on oceanic islands. For the island birds flying could in fact be hazardous as it increases their chances to being blown away during strong winds. There are 10 kinds of flightless birds like ostrich, cassowary, penguin and emu which have lived in such habitats and have lost their ability to fly.
Flightless bird (Wikipedia)
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