Why Some Birds Don’t Fly: The Evolution of Flightlessness

The evolution of flightlessness in certain bird species has long intrigued scientists, prompting questions about why some birds, like penguins, ostriches, and kiwis, have forsaken the skies. While the concept of birds being synonymous with flight is ingrained in popular belief, the reality is that a mere fraction, approximately 60 out of thousands of bird species, are flightless. These include well-known examples such as ostriches, penguins, and kiwis, which descended from ancestors capable of flight but gradually lost this ability, adapting instead to terrestrial or aquatic lifestyles.

The decision to relinquish flight is a complex interplay of evolutionary pressures. Flight, though advantageous for escaping predators and seeking resources over vast distances, demands a significant amount of energy expenditure. In situations where flight is not essential for survival, birds may redirect this energy towards other vital activities such as reproduction and foraging. According to Natalie Wright, an associate professor of biology at Kenyon College, birds residing on predator-free islands, where the need for long-distance travel is minimal, often evolve towards flightlessness due to the high energetic costs associated with flight.

This transition to flightlessness entails a series of physiological transformations in birds. Over time, their pectoral flight muscles diminish, while the sternum and wing bones undergo structural alterations to accommodate their new lifestyle. Penguins, for instance, repurpose their flight muscles for swimming, utilizing their wings as efficient propellers underwater. Additionally, flightless birds exhibit changes in their plumage, with some species losing the specialized flight feathers essential for airborne locomotion, resulting in a fluffier appearance.

Although flightless birds are relatively scarce in the modern avian world, the fossil record paints a different picture of the past. Tim Blackburn, a professor of invasion biology at University College London, highlights that human-induced factors, such as the introduction of predators like rats and dogs, played a pivotal role in the rapid decline of numerous flightless bird species. Iconic examples like the dodo and moa faced extinction due to their inability to evade these newfound threats, emphasizing the vulnerability of flightless birds in the face of human activities.

Recent research, including a 2020 study led by Ferran Sayol from the Centre for Ecological Research and Forestry Applications in Barcelona, sheds light on the prevalence and extinction of flightless birds throughout history. The study reveals that human-driven extinctions have significantly reduced the diversity of flightless bird species that would have existed today under different circumstances. This underscores the profound impact of human actions on avian biodiversity and emphasizes the delicate balance between flight and flightlessness in the evolutionary narrative of birds.