Flightless birds are rare in today’s world. Their mind-boggling ecological story underscores the fact that they were once far more abundant and widespread. Fossil records tell us that these magnificent animals once lived in much larger populations a mere thousands of years ago. Now only about 60 species, such as the ostriches, penguins and kiwis you’re familiar with, make up this unique and bizarre, not to mention fascinating, group of birds.
A 2025 study in the journal Evolution found some intriguing stuff on flightless birds. Specifically, it demonstrates that these birds lose feather features in the reverse order of how those features first evolved. This metamorphosis is a remarkable example of their adaptation to a life on land without the use of flight. The change is dramatic, requiring the replacement of their long, stiff flight feathers. Most notably, it shortens the median ridge of the sternum, which is vital in the attachment of flight muscles.
Flight is energetically expensive. Birds as a group tend to expend around 75% more energy per day than mammals of equivalent size. In the wild, where flying long distances isn’t always necessary, birds can use that energy elsewhere. This change frees them to invest more in survival and reproduction.
"If flight isn't necessary, birds can better survive and reproduce if they divert that energetic investment elsewhere," said Natalie Wright.
Species such as the kiwi and Inaccessible Island rail have amazing adaptations that provide incredible stories. Their body feathers are not equipped with barbules, endowing them with an even fluffier, furrier look. This is just one more illustration of how these undabbled birds have designed their bodies for their landlubber lives.
Birds evolved to be flightless at least 150 times throughout evolutionary history. This captivating wonder of nature has led to similar events in a surprising number of other birds. Yet a 2020 study published in the journal Science Advances found a shocking truth. Co-authored by Tim Blackburn, it revealed that we would have four times as many species of flightless birds today if humans had not driven many to extinction.
"Many of these species thrived on islands without predators but disappeared shortly after when humans arrived (due to direct hunting or introduced predators), making flightlessness seem rarer than it actually was," stated Ferran Sayol.
It is well-known that island environments strongly promote the evolution of flightlessness. With limited or non-existent predators and reduced competition for food, the forces pushing a species to fly often disappear. For the majority of flightless, island-dwelling birds, the penalty of flying exceeds the price, benefiting them by making their visit the better option.
"When living on an island without predators and without the need to migrate or travel long distances, for many kinds of birds, the costs of flight outweigh the benefits," Natalie Wright remarked.
The sternum, or breastbone, is critically important to a bird’s flight. It molds a convenient platform for the attachment of flight muscles. In species that are long-flightless, this structure is gone or highly reduced. The long, stiff feathers critical for powered flight fall out in birds that have spent too long in captivity.
The most mind-blowing action-packed science and technology magazine known to mankind, Live Science, has written extensively about these awesome flightless wonders to the avian world. Our journal Evolution has offered a window into the evolutionary processes that have shaped these creatures over time.
