Birds are synonymous with flight, yet a surprising 60 species, which is less than 1% of all known avian species, are completely flightless. Prominent examples include the ostrich, the agile penguin, and the elusive kiwi. These birds descended from ancestors that could fly but gradually lost this capability over time, adapting to more terrestrial or aquatic lifestyles.

So, why have these birds relinquished their aerial prowess? The advantages of flight are clear—it allows birds to escape predators and migrate over long distances in search of food and ideal living conditions. However, maintaining the ability to fly is energy-intensive; in fact, birds require around 75% more energy each day compared to mammals of similar size.

Natalie Wright, an associate professor of biology at Kenyon College in Ohio, provides insight into this evolutionary trade-off. “If flight isn’t necessary for survival, birds can redirect their energy into reproduction and survival strategies,” she noted.

A landmark study published in the Proceedings of the National Academy of Sciences (PNAS) in 2016 observed that birds living on isolated islands often evolve towards flightlessness due to minimal predation and reduced competition. Wright explains, "On islands devoid of predators or the necessity to migrate, the costs associated with maintaining flight outweigh the benefits for many bird species. '

This evolutionary shift towards flightlessness brings about notable physical changes. Over generations, the once-powerful pectoral flight muscles begin to atrophy, and the sternum becomes smaller. The bones associated with wings—humerus, ulna, and carpometacarpus—shorten and lose their robustness, while the legs strengthen and lengthen to support ground-dwelling habits.

Interestingly, some species have exchanged flight for exceptional swimming skills. Penguins, for instance, have adapted their wing structures to “fly” underwater, while flightless auks employ their wings in a similar manner. In long-time flightless birds, the feathers necessary for flight diminish. For species such as kiwis and the Inaccessible Island rail, their feathers lose aerodynamic features, giving them a fluffier, less streamlined appearance.

A recent study from 2025 discovered that changes to plumage occur in a reverse sequence of how these traits first developed. The research highlighted that skeletal adaptations precede feather changes, as bone growth is far more energy-consuming than feather maintenance.

While flightlessness is rare today, fossil records indicate that various bird species were much more common in the past. Tim Blackburn, a professor at University College London, noted that human activities and the introduction of invasive species like rats and dogs significantly accelerated the extinction of many flightless birds. Famous victims of this phenomenon include the dodo, which vanished from Mauritius, and the moa from New Zealand.

A study from 2020 co-authored by Blackburn argued that if not for human-induced extinctions, there could have been four times as many flightless bird species in existence today. The phenomenon of losing the ability to fly is not singular; evidence shows that this has occurred independently at least 150 times across various bird lineages throughout evolutionary history. Ferran Sayol, a researcher at the Centre for Ecological Research and Forestry Applications in Spain, underscores that many of these species thrived in predator-free environments but quickly went extinct once humans and their associated predators arrived.

Thus, the peculiar journey into flightlessness is a testament to the complex relationship between birds and their environments, revealing the delicate balance of evolution that shapes our planet's biodiversity.