Flightless bird theory in a tailspin
There are three species, none larger than your average farmyard hen. With their unusual body feathers, kiwis resemble giant rats. Flightless, they have no tails and their little wings are tucked away. The eyes are tiny; these nocturnal birds rely on smell to find their way around, probing the ground for insects with long de-curved bills. Pairs nest in burrows, laying one or two eggs, each a quarter of the mother’s weight. The males of the two smaller species do the incubating. Great spotted kiwis share the task, which can take up to 90 days. A week after hatching, the chicks can feed themselves. Now, a paper published in the journal Science, has revealed more kiwi peculiarities.
The ‘ratites’ have an ancient lineage. With the exception of the kiwis, all are huge. Their ancestors competed with the dinosaurs for food. When, 65 million years ago, a global catastrophe killed off the famous reptiles, the warm-blooded ratites inherited the southern earth.
Flying birds need more food than flightless ones. When the dinosaurs died out, there were as yet no predatory mammals so the early ratites could afford to give up flying. Many birds have a tendency to do this; in much more recent times, species such as the dodo, finding themselves on predator-free islands, did the same. For the ratites, there was an additional bonus; they no longer needed to be weight-watchers. In time, they became so large that even fox-sized predators, when they evolved, couldn’t harm them. The ostrich, thanks to its great size, excellent eyesight, stamina, and the ability to deliver a lethal kick, managed to hold its own even after big cats and hyenas appeared in Africa.
The ostrich is the largest bird alive today. Two other ratite families, the elephant birds of Madagascar and the moas of New Zealand, were even bigger. The presence of moas may explain why kiwis are small; to avoid competition, they hunted little invertebrates rather than compete with their huge vegetarian neighbours.
The moas and the elephant birds, alas, were hunted to extinction by man. All we have is their egg-shells and bones. Kieren Mitchell, and colleagues at the University of Adelaide, extracted DNA from two elephant bird skeletons. This would confirm, they thought, that the giant’s nearest relative was the ostrich. The researchers were in for a shock, however; DNA analysis showed the closest kin were New Zealand kiwis.
How could they possibly have a common ancestor? It had always been assumed kiwis descended from flightless emus and cassowaries, isolated on New Zealand when it separated from Australia about 75 million years ago. The discovery has upset the theory of the origins of the ratites in general.
Molecular clock studies show the two groups did not diverge until 15 million years ago. Their descendants had to cross the ocean to get to New Zealand and Madagascar and the only way they could do that was by flying. The kiwi and the elephant bird, therefore, had flying ancestors which went their separate ways and each, in due course, became flightless.
The ratites, these results seem to show, retained the power of flight much later than was previously thought. If so, how are kiwis related to the other flightless giants, the South American rheas, the ostriches of Africa, and Australia’s emus and cassowaries? Is it possible all are descended from one source species, a primordial bird which flew down-under from the great northern landmass? Was there a ratite equivalent of Adam and Eve?
* Mitchell, K. et al. 2014. Ancient DNA reveals elephant birds and kiwis are sister taxa and clarifies ratite bird evolution. Science 344: 898-900.
CONNECT WITH US TODAY
Be the first to know the latest news and updates
