Less than one percent of all bird species—around 60 in total—are unable to fly. This is surprising, considering that birds are primarily associated with flight. Ostriches, penguins, kiwis, and other feathered creatures descended from ancestors that could fly but lost this ability as they adapted to life on land or in water. But why did they give up flying? The ability to soar through the skies is particularly advantageous for evading predators or traveling long distances in search of food and favorable living conditions. However, flying requires a significant amount of energy. Birds expend 75 percent more energy during flight than similarly sized mammals. “If flight isn’t essential, birds can survive and reproduce more effectively by redirecting their energy resources elsewhere,” explained Natalie Wright, a researcher at Kenyon College in Ohio, to Live Science. A few years ago, her team published a study in the journal PNAS on this topic. The findings indicated that island birds, which encounter few predators (or none at all) and experience less competition for food and habitat, tend to lose their ability to fly. This leads to certain physical changes. Over time, the flight muscles in birds shrink. The sternum (breastbone) and its central ridge—the keel to which the flight muscles attach—also become smaller, Wright noted. The bones in their wings shorten and lose strength, while their legs, adapted for a terrestrial lifestyle, grow longer and stronger.
How else do these evolutionary changes manifest? Some birds have traded their flying abilities for exceptional swimming skills. Penguins, for instance, have retained their flight muscles and keel but have repurposed them for swimming. “They use their wings to fly underwater,” explained Peter Ryan, an honorary professor of ornithology at the University of Cape Town in South Africa. In birds that haven’t flown for extended periods, the stiff feathers necessary for flight disappear, Professor Ryan pointed out. In some species, like the kiwi, the feathers on their bodies lose tiny hook-like structures that typically help maintain aerodynamic efficiency. This gives them a fluffier appearance, the expert added. According to a study published this year in the journal Evolution, flightless birds lose feather characteristics in the reverse order of how they originally evolved. The authors concluded that skeletal changes occur in tandem with feather changes, as maintaining and growing bones requires significantly more energy than supporting feathers.
While flightless birds are now relatively rare, fossils indicate that thousands of years ago, they were much more widespread and diverse. Tim Blackburn, a professor at University College London, shared this insight. However, the arrival of humans and animals like rats and dogs on their territories led to the extinction of these birds. “Having lost the ability to fly, they didn’t have the time to redevelop this useful trait,” said Professor Blackburn. This resulted in the rapid extinction of iconic species such as the dodo in Mauritius, the moa in New Zealand, and many others. As Professor Blackburn noted, there could have been four times as many flightless bird species on Earth today if not for the extinctions caused by human activity. “Many of these species thrived on islands without predators, but they disappeared after humans arrived,” confirmed Ferran Sayol, a research fellow at the Center for Ecological Research and Forestry Applications in Barcelona, Spain.