There’s a rocky island in the South Atlantic Ocean so remote that it is known as Inaccessible Island. No humans and few animals dwell there, but among the creatures that call the island home is the Inaccessible Island rail, the world’s smallest flightless bird still in existence. Since the creature was first described in the 1920s, scientists have wondered how it managed to reach its far-flung habitat. Now, as Sarah Laskow reports for Atlas Obscura, a new study may shed light on the enduring natural mystery.
Inaccessible Island rails are tiny little things, weighing less than a chicken egg, with brown feathers, black feet and bright red eyes. The birds exist only on Inaccessible Island, which spans just 4.6 square miles and is located more than 1,550 miles from any mainland.
Percy Lowe, a British physician, was the first to describe the Inaccessible Island rail in 1923. He found them so different from any other extant rail, a large family of birds known formally as Rallidae, that he classified them under their own genus, Atlantisia, in reference to the fictional island, and named the species rogersi after the chaplain who was the first to collect the specimens. Lowe posited that the unique rails made their way from Africa or South America to Inaccessible Island by hopping across a now-submerged land bridge.
The new study, published recently in Molecular Phylogenetics and Evolution, relied on more advanced analytical tools than Lowe had at his disposal. An international team of researchers, led by lead author Martin Stervander of Lund University, who is now a postdoctoral researcher at the University of Oregon, sequenced the DNA of a male Inaccessible Island rail, captured in 2011, and then compared the data to genetic sequences of other rails. They found that the bird’s closest living relative is the dot-winged crake, which dwells in southern regions of South America and belongs to the Laterallus genus. Both species are also closely related to the black rail of South and North America. And, crucially, the common ancestor of all of these birds could fly.
In light of this new genetic evidence, the researchers conclude that the Inaccessible Island rail originated in South America, and should be classified as a member of the Laterallus genus. They also posit that the bird made its way to its remote habitat about 1.5 million years ago—not on foot (or claw, as it were), but by flying at least part of the way.
“Whether they flew all the way or were swept off by a storm and then landed on debris, we can’t say,” Stervander says. “In any case, they managed to make it from the mainland of South America to Inaccessible Island.”
For the sea-faring rails, Inaccessible Island was a utopia, filled with plenty of food and devoid of predators. The researchers believe that over time, the birds lost their ability to fly because they simply had no use for it. “[T]hey had all their food from walking around and there was nothing to escape from,” Stervander tells Ryan F. Mandelbaum of Gizmodo. “[T]here’s not much need for flying.”
This evolutionary story is not, in fact, limited to Inaccessible Island rails. According to the study authors, 53 extant or recently extinct rail species exist only on specific islands, and at least 32 of these species have lost or dramatically reduced their ability to fly.
Some 5,500 rails live on Inaccessible Island today, and in spite of their geographic isolation, they are a vulnerable species. Should foreign predators, like rats, be introduced to their habitat, the flightless birds would likely be decimated. Fortunately, as Laskow notes, the risk is low: Few humans try to make their way to the island, which true to its name, remains as inaccessible as ever.