Why Some Birds Cannot Fly?

When it comes to birds, people may immediately think of flying, but in fact, some birds do not fly, such as penguins, which primarily inhabit the Southern Hemisphere. But since these birds are classified as birds, why don’t they fly?

According to a report on the Live Science website, there are about 60 species of birds (less than 1% of all bird species) that do not fly, including ostriches, penguins, and kiwis. These birds have lost the ability to fly that their ancestors had and instead have adapted to life on land or in water.

Flying ability is particularly useful for avoiding predators, traveling long distances to find food, and seeking favorable living conditions. However, flying requires a lot of energy – birds consume about 75% more energy per day than mammals of similar size.

Natalie Wright, an associate professor of biology at Kenyon College in Ohio, told the website via email that if birds do not need to fly and can invest their energy elsewhere, they can better survive and reproduce.

Wright and her colleagues published a paper in the Proceedings of the National Academy of Sciences (PNAS) in 2016, pointing out that birds living on islands often have few or no predators and less competition for food and habitat, which is why they often evolve to be flightless.

Wright said, “For many species of birds, when they live on predator-free islands and do not need to migrate or travel long distances, the cost of flying outweighs the benefits.”

During the evolution of becoming flightless, birds’ bodies also undergo changes. For example, their chest flight muscles shrink, their breastbones and keel (where the flight muscles attach) become smaller, and their legs become longer and stronger to adapt to a terrestrial lifestyle.

Although some flightless birds have lost the ability to fly, they have developed swimming abilities. For example, penguins retain their flight muscles and keel but use them for swimming. Peter Ryan, an honorary professor of ornithology at the University of Cape Town in South Africa, described it as “flying underwater with their wings.”

Ryan mentioned that for birds that have been flightless for a long time, the long and stiff feathers needed for flying also disappear. Birds like kiwis have lost barbules (tiny hook-like structures that usually help maintain aerodynamic efficiency) on their feathers, making them look fluffier, akin to fur.

Tim Blackburn, a professor of invasion biology at University College London, said that while flightless birds are not commonly seen today, fossils indicate that they were more widespread and diverse thousands of years ago. However, the arrival of humans along with animals like rats and dogs has posed a threat to these birds as predators.

Ferran Sayol, a researcher from the Centre for Ecological Research and Forestry Applications in Barcelona, Spain, mentioned, “Many of these species thrived on predator-free islands, but disappeared shortly after human arrival (due to direct hunting or introduction of predators), making flightless birds appear rarer than they actually were.”