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Did Ratites Fly to New Zealand?

flying_ratite.gif The consensus is that kiwi branched off the ratite evolutionary tree long after New Zealand separated from Australia. That means they had to cross 2000 km of ocean, and there’s no record of a flightless bird ever dispersing across an ocean barrier, no matter how small. That means they flew. A Tertiary dispersal and radiation of ratites, with several flightlessness transitions, is the least improbable explanation of their current distribution, and it has the advantage of being consistent both with the fossil record and with a (sensibly) calibrated molecular tree.

The standard account is that the ratite birds are descended from a flightless ancestor that roamed all over the southern supercontinent Gondwana. As Gondwana started to fragment, populations of this flightless ancestor were isolated on drifting continents, and eventually evolved into the different ratites we know today. When did this happen? Well, the first split in the ratite family tree is between moa and everything else. This is assumed to coincide with the separation between New Zealand and Australia, about 80 million years ago, which then calibrates the tree.


The problem with this is that ostrich, elephant bird, and kiwi splits all happened later, long after the places they live today had been isolated by oceans. This means the flightless ancestors would have had to cross the sea, and flightless birds are notoriously terrible dispersers. In fact, I’ve yet to find one example of a flightless bird species crossing an oceanic barrier, no matter how small.

The Cretaceous model has almost all living groups of ratite birds originating before the extinction of the dinosaurs. There are no ratite fossils until after this extinction, though. And in addition one has to make up stories about the ancestors of ostriches rafting on India and dispersing across Asia to Africa, for which there’s also no evidence.


An alternative, which has been kicking round for a while but I don’t think had been properly set out until I put it in my dissertation, is that the ratites evolved in the Tertiary from a flying ancestor which flew to the different fragments of Gondwana. This has the advantage of matching the fossil record, and the fact that almost all flightless birds descend from flying ancestors that became isolated on islands.


The only implausibility with this model is that flightlessness has to have occurred multiple times—in every ratite lineage. Recent DNA work in fact strongly suggests that the flighted tinamous belong right in the middle of the ratite tree, necessitating multiple flightlessness. And some groups, like rails, do seem to go flightless whenever possible, so the scenario is not too far-fetched—certainly not as far-fetched as a flightless kiwi swimming or rafting to New Zealand.

(Presented at the Society for Avian Paleontology and Evolution meeting, Sydney, September 2008.)