Why can’t some birds fly? The science behind flightless birds

We often spot birds soaring through the skies, gliding with effortless ease, flapping across oceans, or flocking through trees. And one of the most iconic aspects that help the birds through this is flying, but not all of them take to the skies. In fact, some birds, like penguins, ostriches, and kiwis, don’t fly at all. But why does that happen if birds evolved to fly?Flightless birds make up less than 1% of all bird species today, yet they tell an interesting evolutionary story. The skeletal structure, feathers, and wings of the flightless birds have adapted in unusual ways to survive on land or in water. When survival no longer required flight, evolution found new paths for these grounded birds.Around 60 bird species today, including penguins, ostriches, and kiwis, have done exactly that. These birds evolved from flying ancestors but gradually lost the ability to take flight. Instead, they developed physical traits better suited to running, swimming, or moving on dense forest floors.One major reason is that they lack the keel, which is a ridge on a bird’s breastbone where powerful flight muscles attach. Flightless birds like the ostrich, cassowary, kiwi, and rhea belong to a group called ratites, which do not have a keel. Without this important structure, flying becomes nearly impossible.Some species, like the kakapo, a flightless parrot, may have vestigial keels, small, leftover versions of this bone from their flying ancestors. Penguins, however, are unique. They still have a well-developed keel, but their heavy bodies and small wings make true flight impossible. Instead, they have become master swimmers.Flying requires a massive amount of effort, where birds burn about 75% more energy per day than similarly sized mammals.Wright’s 2016 study in PNAS found that birds living on islands with few predators and no need to migrate often evolve toward flightlessness. “When living on an island without predators and without the need to migrate or travel long distances, for many kinds of birds, the costs of flight outweigh the benefits,” she said.Over time, as these birds adapted to life without flight, their muscles and skeletons also changed. “Their pectoral flight muscles shrink,” Wright said. “The sternum... becomes smaller,” and their wing bones shorten while leg bones become sturdier.Peter Ryan, a professor emeritus of ornithology at the University of Cape Town in South Africa, says that long, stiff flight feathers can disappear entirely, and some birds even lose the hook-like structures called barbules that keep feathers aerodynamic, making them appear fluffier, like the kiwi.A 2020 study led by Ferran Sayol found that flight was lost independently at least 150 times in bird history. “Many of these species thrived on islands without predators but disappeared shortly after when humans arrived,” he said.