Why Would a Bird Stop Flying? Giving Up Something Expensive

Being able to fly imposes requirements across the whole body. The flight muscles are the largest in a bird and constitute a substantial share of its weight, and maintaining muscle is metabolically expensive whether it is used or not. The skeleton must be light, which limits how robust it can be. Body size is constrained, since the power needed rises faster than the power available as an animal grows. Wings must be kept in condition through moulting and preening. And the whole arrangement demands a high metabolic rate. If a bird's circumstances remove the need for flight, all of that expenditure becomes available for something else, which is why the loss happens so readily.

Flightlessness evolves under a consistent set of conditions:

• •Islands without ground predators, which is by far the commonest setting
• •Abundant food available on the ground or in shallow water
• •Large body size, which makes flight expensive and walking efficient
• •Aquatic lifestyles where wings are repurposed for swimming, as in penguins
• •It has evolved independently many dozens of times across unrelated groups
• •Rails have lost flight more often than any other family, repeatedly on different islands

The large flightless birds of the southern continents were long assumed to share an ancestor that was already flightless, with their distribution across Africa, South America, Australia and New Zealand explained by continental drift carrying them apart. Genetic evidence has substantially revised that. The group's closest relative turns out to be a small flying South American bird, and the pattern of relationships indicates that flightlessness and large size evolved several times independently within the group rather than once, with the ancestors dispersing by flying and losing flight afterwards in each place. Extinct giants including the moa of New Zealand and the elephant birds of Madagascar fit that revised picture, and the moa's closest relative is a flying bird in South America.

Several groups repurposed wings for underwater propulsion, and the physics explains why they cannot do both well. Water is around eight hundred times denser than air, so a wing efficient for flying is far too large and flexible for swimming, and a wing efficient for swimming is too small and stiff for flight. Penguins went entirely to the second, with dense bones, stiff flattened wings and no flight at all, and they are among the most accomplished swimmers of any bird. Auks in the northern hemisphere occupy the same role while retaining flight, which they manage badly in both media, and the extinct great auk had gone the same way as penguins. Diving birds that still fly have visibly compromised wings and have to work extremely hard to get airborne.

The conditions that permit flightlessness are exactly the conditions humans destroy, which is why flightless birds are so heavily represented among extinctions. Islands without predators acquire them the moment people arrive with rats, cats, pigs and dogs, and a bird that nests on the ground and cannot escape has no defence at all. The dodo is the standard example and is one of very many, with estimates suggesting that a majority of the flightless rails that existed across Pacific islands were eliminated following human settlement, which amounts to hundreds of species known mainly from bones. Several surviving flightless species persist only on predator-free islands maintained deliberately, and their conservation consists largely of keeping mammals out.

Flight requires large expensive muscles, a light skeleton and a high metabolic rate, so birds abandon it whenever the benefit falls below that cost. Islands without ground predators are the usual setting and rails have lost flight repeatedly. Genetic evidence shows the large southern flightless birds lost flight independently several times rather than inheriting it from one ancestor.