How did flightless birds spread across the world?

How Did Flightless Birds Spread Across The World?
How Did Flightless Birds Spread Across The World?

Here's a riddle: how did an order of flightless birds manage to spread to places they would have had to fly to? Rheas live in South America, cassowaries and emus in Australia, kiwis in New Zealand and ostriches in Africa. They're all related and they're all land-dwelling. How does that work?

That's kind of a tough one for evolutionary scientists. The dominant answer is that the birds spread to where they're found today way back before the continents broke up. But DNA tests are starting to make the issue even more puzzling.

A new study from the University of Adelaide looked at the DNA of this big guy, the elephant bird, one of the biggest birds to have ever existed. It lived on Madagascar and died out sometime in the last few hundred years.

The researchers were trying to work out whether the elephant bird was related to the moa, another feathered behemoth from New Zealand that died out around the same time. Instead, it was more closely related to this little guy, New Zealand's iconic kiwi.

And it turns out the moa's closest relative is another chicken-sized bird called the tinamou, which lives - get this - in South America. Another twist: some of them can even fly.

The study's lead author Andrew Cooper told New Scientist, "​In both cases, the moa and the elephant bird, the nearest relative is on the other side of the world."

Analysis of the DNA samples showed the elephant bird and the kiwi last shared a living relative around 50 million years ago - way after the continents had already split up. So Cooper says the answer to how flightless birds spread all over the world is simple: they flew there.

National Geographic's Ed Yong says Cooper's research supports a newer theory about the flightless bird family: that they "evolved from small, flying birds that flapped their way between continents and independently lost the ability to fly on at least six separate occasions."

That would make flightlessness an example of convergent evolution, when two species independently evolve the same traits, like hard shells in different kinds of pill bugs or fingerprints in humans and koalas.

So that's probably how you solve the riddle of the flightless birds. But it turns out there's another interesting subplot here.

Back in 1992, Cooper published a study in Proceedings of the National Academy of Sciences that showed kiwis were likely descended from the Australian emu. And that didn't go over too well in New Zealand.

A writer for the New Zealand Herald said, "Our national bird and treasured namesake ... was, of all things, an Aussie immigrant." and dubbed the study "one of the greatest wrongs ever inflicted upon New Zealanders."

Cooper seems to hope the new study makes up for it, though, saying "​It's great to finally set the record straight. ... I can only apologise it has taken so long!"