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Unexpected twist in evolution of flightless birds

An adult brown kiwi (Apteryx australis) beside the egg of a huge elephant bird (Aepyornis maximus). A new genetic study led by Kieren Mitchell and Alan Cooper at the University of Adelaide indicates that these two bizarre and dramatically different birds are close cousins.
[Photo by Kyle Davis and Paul Scofield, Canterbury Museum]

Ratite birds, some of the largest flightless birds, live all over the world, and now a new study published in the 23 May issue of the journal Science suggests they spread so far over not because big landmasses split up, forcing their separation, but because their ancestors flew far and wide. It was only after separating, this study says, that most members of this group lost the ability to fly.

Ratite birds include winged creatures of all shapes and sizes, ranging from the large African Ostrich to the now extinct Madagascan elephant bird to the chicken-sized New Zealand kiwi.

Scientists have long thought that they evolved into these various species after the supercontinent Godwana split apart.

Godwana included Africa and Madagascar, the first two landmasses to split off. Thus, models of how ratite species developed into such diverse bird types suggest the African Ostrich and the Madagascan elephant bird are the oldest branches on the ratite family tree that they started it all.

Most analyses do point to the ostrich as a very old ratite, but the position of the elephant bird on the ratite family tree has been unclear.

Now, to help figure out just where the elephant bird sits, Kieren J. Mitchell of Australia's University of Adelaide and colleagues sequenced and analyzed DNA of two species of elephant birds, leading them to a surprising result: this large, plant-eating bird is the closest living relative to the small, omnivorous kiwi. And ostriches thought to be a common ancestor to all the ratites, including elephant birds and kiwis are only distantly related to these species.

Thus, say the authors, the great many ratite species we see today didn't starting with a common flightless ancestor, a big ostrich that once driven apart from other ratites by continental separation diversified into the many ratite species we see today.

Instead, it appears ratite species diversity was driven by the divergence of species from ancestors capable of flight, ancestors like the elephant bird that flew long distances to new corners of the world, after which they evolved the inability to fly.

Elsewhere, the inability to fly is almost only seen on islands lacking big mammalian predators. So, the authors suggest, the evolution of ratite birds to the large flightless birds we see today likely began in the period after the mass extinction of dinosaurs, the major predator of the time.