Why some animals lost the ability to fly

Flightlessness is not a primitive condition that all birds share. It is a secondary change: a lineage whose ancestors could fly gradually lost powered flight, while its wings, muscles and skeleton changed in different ways. The pattern has appeared repeatedly among birds, and also in insects and other animals. Each case has its own history.

What changes when flight is lost?

Powered flight requires a coordinated body plan. The wings must generate lift and thrust, the chest anchors large flight muscles, and the skeleton must balance strength with low mass. When selection no longer rewards flight, these structures can change over generations. A bird may develop smaller flight muscles, shorter wings or a heavier body, while its legs become more important for walking or running. The exact changes differ among lineages; flightlessness is not a single anatomical switch.

Energy is one part of the explanation. Maintaining and using flight muscles is costly, but a flightless animal may still need strong muscles for swimming, display or rapid movement on land. Researchers therefore compare anatomy with ecology rather than assume that every grounded bird simply saved energy. The evidence also shows that flight can be lost more than once within a broader group, as happened among rails.

Why islands matter

Islands can change the balance of risks. Where large mammalian predators are absent, a bird may gain less from escaping into the air. Food on the ground or in wetlands can favour walking, wading or diving. Over time, some island lineages became flightless. This is a recurring association, not a rule: islands differ, and many island birds remain capable fliers.

The Aldabra rail illustrates an unusual twist. A flightless rail population lived on the atoll, disappeared when the island was submerged during a high sea level, and a flying rail later recolonised it. Fossils indicate that flightlessness evolved again in the recolonising lineage. This is an example of repeated evolution, sometimes called iterative evolution, rather than the same individual population returning.

Flightlessness can also be advantageous in water. Penguins use their wings as flippers, sacrificing aerial flight for efficient underwater propulsion. Their bodies are not simply reduced versions of flying birds: wing bones, feathers and muscles have become suited to a different medium. Moas and ostriches took other routes, relying on large bodies and powerful legs.

How scientists test the pattern

Researchers combine fossil anatomy, living species and evolutionary trees. A phylogeny helps estimate how many times flight was lost, while fossils can show when those changes occurred. Extinct birds matter: many flightless species disappeared after human arrival, so a living-only survey hides much of the diversity that once existed. Studies that include recently extinct birds have found flightlessness spread across far more bird families than the present-day record alone suggests.

Flightlessness is a trade-off, not an evolutionary failure. A lineage may exchange dispersal by air for diving, running, ground feeding or survival in a particular habitat. The outcome depends on predators, food, body size and geography. For this reason, the story of flightless animals is best told as many independent experiments in evolution rather than one universal explanation.

For a wider comparison of how powered flight arose in different groups, see the evolution of flight in insects, pterosaurs, birds and bats. Modern and extinct birds also show how lineages can change rapidly when island ecosystems are transformed.