Emus, kiwis, ostriches, and penguins are just some of Earth's roughly 60 species of flightless birds. But why do they still have wings if they have lost their original function?

Their wings are vestigial structures that had a necessary function at some point in their ancestors' life but have lost it in modern species. According to ZME Science, scientists assume that the evolution of flightlessness occurred independently in different bird lineages, and genetic data suggest that this happened at least five times in history.

Not All Birds Can Fly: Take A Look At How These Feathered Critters Use Their Wings on Land
(Photo: Unsplash/Catherine Merlin)
Male Ostrich display: "shaking a tail feather" to attract female attention.

Why Do Some Birds Not Fly?

Evolution is truly a mystery because why else would some birds abandon a trait, like flight, that gives them an advantage in attaining resources?

Abigail Kaparis, one of the authors of the 2018 study, explained that the most common theory that explains the reason behind the flightless birds is that the Cretaceous-Paleogene Extinction 66 million years ago opened up many new niches in the absence of three-quarters of Earth's species.

Some birds filled the roles of non-flying reptiles, decreasing their reliance on the flight. Instead, they increasingly relied on their feet for transportation and body size to ward off predators and have better access to food sources.

Moreover, several lineages of one of the largest families of birds today, called ratites, shifted away from traditional niches of birds to perform ecological functions seen today in large mammals. Ratites include the ostrich, which is the largest ratite in modern times.

The paper further explains that it seemed inefficient to supply a part of the body that is not being used, especially since resources cannot be wasted on wings that do not serve their original purpose. Eventually, birds with smaller wings have higher resource use and fitness efficiency, resulting in overall wing size decreasing and, over time, becoming non-existent.

Previous studies also showed that birds that lost the ability to fly continue to use their wings and develop pectoral muscles, such as the penguins that use their wings for swimming which is also why they retain their keel and strong chest muscles. According to a 2019 study, different groups of ratites lost their ability to fly independently, with which the driving force is mutations in regulatory DNA.

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Vestigial Structures are Not Functionless

Per the Australian Academy of Science, vestigial structures in flightless birds are not entirely functionless. They have a different function than flying birds and their ancestors.

Some of these functions could be more complex, like the ostrich wings used to help the birds balance when running and courtship displays. But not just birds; even humans have certain features that are no longer used for what they were once intended.

For instance, the appendix in humans is a long thought classic example of a "useless organ." But a 2013 study founds that it does have a role to play in the immune system to regulate pathogens by assisting the digestive system movement and waste excretion.

So, why do these non-adaptive or non-functional features have not disappeared? It is because there is little reason for them to do so. But if these structures are disadvantageous and put the organism's life at risk, it would relatively rapidly vanish from a population.

Evolution is still happening, and what is present today could be experiencing a stage in the disappearance of an old function or honing a new one.

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