Birds on an island in the Indian Ocean evolved flightlessness twice

Fossil evidence has revealed that the Aldabra rail, the last surviving native flightless bird in the Indian Ocean lost the ability to fly not only once, but twice over a 20,000 year period. This is one of the fastest recorded timelines of a bird losing its ability to fly.

The new research, led by Natural History Museum paleontologist Dr Julian Hume, explores the surprisingly complex history of the rail, a descendent of the flying white-throated rail. Published in the Zoological Journal of the Linnean Society, it reveals fossil evidence of the flightless rail on Aldabra dating back 136,000 years.

The Indian Ocean was once home to a whole menagerie of flightless birds, the most famous being the dodo of Mauritius. As humans spread out across the ocean and colonised these islands, each of these bird species spiralled into extinction except the Aldabra rail (Dryolimnas cuvieri aldabranus). The Aldabra sediments show that the island had been completely submerged by the ocean on multiple occasions, eradicating all life on its shores.

But in the years following these 136,000-year-old rail fossils, the white-throated rail became flightless again around 118,000 years ago.

'Aldabra went under the sea and everything was gone,' explains Julian. 'There was an almost complete turn over in the fauna. Everything including an endemic crocodile and duck, as well as the tortoise and the rail went extinct. Yet as the Aldabra rail still lives on today, something must have happened for it to have returned’

Immediately after the last inundation event around 118,000 years ago, Julian found a leg bone in the fossil, which showed the species was more robust and heavier than the flying rail, and was losing its ability to fly once again.

'There is no other case that I can find of this happening,' explains Julian, 'where you have a record of the same species of bird becoming flightless twice. It wasn't as if it were two different species colonising and becoming flightless. This was the very same ancestral bird.'

The research suggests that once the sea levels dropped again and Aldabra reappeared, the white-throated rail once again recolonised the islands and became flightless, giving rise to the modern birds we see today.

This means it does not take much for evolution to favour flightlessness on an island in which there were no terrestrial predators and plenty of food on the ground, so there was no need to develop those wing muscles at all.

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