Nature

This strange, flightless bird evolved into existence twice across millennia

Unlike human history, when nature repeats itself, there’s usually a good reason. For the white-throated rail (Dryolimnas cuvieri), all it took was a peaceful reef Eden, free from predators and competition.

On the remote Aldabra atoll in the Indian Ocean, scientists have discovered that the same species of bird has lost its ability to fly on two separate occasions.

Hundreds of thousands of years ago, members of this intrepid rail species flew from their native home in Madagascar to a ring-shaped atoll in the Seychelle islands. Safely feeding and reproducing in this veritable Eden, they had no need to fly and so, like the dodo, the species became landlocked.

What the birds didn’t see coming was the tide. Roughly 136,000 years ago, the Aldabra atoll slipped below the waves, and the flightless rail was no more.

When the atoll once again resurfaced, about 36,000 years later, it wasn’t long before the same thing happened all over again. In short, that very same species in Madagascar flew to the very same atoll and gave rise to another species of flightless rail, the one that still exists there to this day (Dryolimnas cuvieri aldabranus).

It’s the first time that scientists have ever found evidence of this happening in rails, or any other bird for that matter.

“Only on Aldabra, which has the oldest palaeontological record of any oceanic island in the Indian Ocean region, is fossil evidence available that demonstrates the effect of changing sea levels on extinction and recolonisation events,” the authors write.

“Conditions were such on Aldabra, the most important being the absence of terrestrial predators and competing mammals, that a Dryolimnas rail was able to evolve flightlessness independently on each occasion.”

Screen Shot 2019 05 15 at 5.12.59 pm(Hume & Martill, Zoological Journal of the Linnean Society, 2019)

It sounds incredible, but the researchers say the evidence is irrefutable. Comparing the bones of a fossilised rail from before the ocean inundation event and a rail from after, the researchers found that the wing bones were well on their way to becoming useless and the ankle bones showed signs of taking on more weight – both distinct qualities of flightlessness.

It’s as if the ancient, now extinct bird has come back to life once more; far from being some wild coincidence, the authors think it has to do with the rail’s persistent migrations to isolated islands around Madagascar.

“Fossil evidence presented here is unique for rails, and epitomises the ability of these birds to successfully colonise isolated islands and evolve flightlessness on multiple occasions,”says author Julian Hume, an avian paleontologist at the Natural History Museum in the UK.

This study has been published in the Zoological Journal of the Linnean Society.

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