Penguins do not have the ability to fly, but they have retained their flight muscles and adapted them for swimming. (Photo by JUAN BARRETO via Getty Images)
Birds are commonly associated with flight, but not all birds can take to the air. About 60 species—less than 1 percent of all bird species—cannot fly, including species such as ostriches, penguins, and kiwis. These species evolved from flying ancestors but lost the ability to fly, adapting to life on land or in water.
But what was the reason for the refusal to fly? Why are not all birds able to fly?
Flight is especially important for avoiding predators and traveling long distances in search of food and shelter. However, flying requires a significant amount of energy; birds expend about 75% more energy per day than similarly sized mammals.
“If flight is not essential, birds may survive and reproduce better if they divert their energy resources elsewhere,” Natalie Wright, an assistant professor of biology at Kenyon College in Ohio, wrote in an email to Live Science.
In a 2016 study published in the journal PNAS, Wright and her colleagues noted that birds living on islands, where there are few or no predators and less competition for food and habitat, tend to evolve flightlessness.
“For many bird species that live on islands without predators and without the need to migrate or fly long distances, the costs of flying outweigh the benefits,” Wright said.
The transition to flightlessness causes physical changes in birds. As they evolve, their chest flight muscles shrink. The sternum (breastbone), with its central ridge (keel) where the flight muscles attach, also shrinks, Wright said. The wing bones — the humerus, ulna, and carpometacarpal — become shorter and weaker, while their legs become longer and stronger as an adaptation to a more terrestrial lifestyle, she added.
Some birds have traded the ability to fly for exceptional swimming skills. Penguins, for example, have retained their flight muscles and keels but have adapted them for swimming. “They use their wings for underwater swimming,” Peter Ryan, an emeritus professor of ornithology at the University of Cape Town in South Africa, told Live Science in an email. The flightless great auk (Pinguinus impennis) also uses its wings to move through water.
Birds that are flightless for long periods also lose the long, stiff feathers needed for flight (flight feathers), Ryan said. In some species, such as the kiwi and the inaccessible island rail (Atlantisia rogersi), the body feathers lose their barbules — tiny hook-like structures that normally keep them aerodynamic — giving them a fluffier, fur-like appearance, he added.
A 2025 study published in the journal Evolution found that flightless birds lose feather features in the reverse order of how they originally evolved. The study also found that skeletal changes occur before plumage changes, as it takes significantly more energy to grow and maintain bones than to maintain feathers.
Sourse: www.livescience.com
