One of the 50 amazing bird species featured in BirdWatching’s 2018 collector’s edition is the Inaccessible Island Rail. About the size of a small sparrow with only rudimentary wings and a reduced tail, it is the smallest flightless bird in the world. Males are slightly larger than females, weighing only about 1.5 ounces.
The bird lives only on uninhabited Inaccessible Island of the Tristan da Cunha archipelago, in the South Atlantic Ocean. The island, formed by a now-extinct volcano 3 to 6 million years ago, is located roughly 3,600 kilometers (2,250 miles) east of Porto Alegre, Brazil, and about 2,800 kilometers (1,750 miles) west of Cape Town, South Africa. Ever since the rail’s discovery in 1923, people have wondered how a small, flightless bird wound up on an island in the middle of the ocean.
Now, researchers from the University of Oregon say they have an answer.
British ornithologist Percy Lowe, who first described the species, surmised that the birds walked to the island on a since-sunken land bridge from Africa or South America, but the later discovery of plate tectonics ruled out that idea, says Martin Stervander, a postdoctoral researcher in the university’s Institute of Ecology and Evolution.
“We found obviously that the birds did not walk by foot,” he says. “They flew or were assisted by floating debris. Whether they flew all the way or were swept off by a storm and then landed on debris, we can’t say. In any case, they managed to make it from the mainland of South America to Inaccessible Island.”
Stervander and his team found genetic evidence that the bird’s closest relative is South America’s Dot-winged Crake. It’s also related to Black Rail, found in a variety of places in the Western Hemisphere, and Galapagos Crake, which also has a reduced flight ability.
Using modern sequencing of mitochondrial and nuclear DNA and phylogenetic methods, Stervander’s team found that the island rail split from the crake about 1.5 million years ago. That’s when the rail colonized Inaccessible Island in a single migration, the team concluded in a paper published by the journal Molecular Phylogenetics and Evolution.
Lowe had placed the rail in the genus Atlantisia, a reference to mythical Atlantis. Stervander’s team recommends that it be reclassified in the genus Laterallus as are the related species.
“We are sorry to be suggesting that we take away this beautiful name, Atlantisia, which is something we can all love,” Stervander says. “But we can now say that the closest relatives of this species are American birds that were given their name before the Inaccessible Island Rail.”
The island’s rail population, Stervander adds, is in a delicate situation. While some 5,500 mature birds now thrive on the island, an accidental introduction of rats or other predators could destroy them. Fossils found on other islands suggest that numerous other flightless rail species had been there but fell to predation with the arrival of humans.
The rail is classified as vulnerable on the International Union for Conservation of Nature’s Red List of species threatened with extinction.
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