Light without flight

Stephens Island wren (Traversia lyalli) drawn by John Gerrard Keulemans
 This week's birds comes once again from New Zealand's extinct passerine birds fauna. I have previously posted about the heaviest bird capable of flight, the kori bustard. Today I introduce you to the Stephens Island wren (Traversia lyalli), weighing around 20 grams and the lighest  bird that is known to have been flightless. It was also one of only three known flightless passerines, all of which were island birds. Were, because sadly they are all extinct.

Flightlessness in birds is common on islands and is an artefact of an evolutionary history devoid of typically mammalian predators. This is evident in New Zealand which has the greatest number of flightless bird species than any other landmass. Subsequently, it is flightlessness that contributes significantly to predation vulnerability and ultimately extinction, another common occurrence on islands in recent times. And it is exactly this which led to the demise of the Stephens Island wren.

Stephens Island, or takapourewa is located at the northernmost tip of the Marlborough Sounds on the South Island of New Zealand. Up until the 1890s it was free from human impact and pests, and had become the last remaining outpost for the Stephens Island wren.  This changed when it was decided that the island would make a perfect place for a lighthouse. The remoteness and untouched nature of the island is beautifully highlighted by Prof. Richard Holdaway who said that it would have been like stepping back in time. Unfortunately for the little wren species, that moment marked beggining of the end. On such a small island, only 1.84 sq km, there is not much place to hide if you are a bird that can't fly, and especially from a cat. The lighthouse keeper, Lyall (from who the wren gets it's species name) had just that, a cat! According to legend Tibbles the cat made short work of the little wren and has been credited with single pawdedly wiping out an entire species. This of course is not true since the bird was previously found on the mainland. There is an historical account of what did happen on the wiki page.

One last thought I will leave you with is this question; if Stephens Island is 3.4km away from the mainland and the Stephens Island wren was flightless, how did it get there?


Paul said...

One could also ask how lemurs reached Madagascar from mainland Africa - a hundred times further than Stephens Island to mainland NZ.

A long way to raft for a mammal, but there they are!

Jarrod said...

Read or heard something recently about a shift in ocean currents that may have been favourable for rafting to Madagascar. But it must have been some time ago when it occurred since lemurs have an exceptionally high order of endemism, right up to the suborder?? level I believe.

Paul said...

Yip - ocean currents seems to be the favourite explanation for Madagascar's lemurs. There was a window between 20-60 Mya where that could have happened. Lemurs were probably earlier rather than later in this window (the authors of a paper last year estimated 50 Mya).

And yet, imagine mammals with their high energy demands rafting for 400km - maybe three weeks - and surviving! Pretty amazing - but helps to explain why so few mammal groups (only 4) are found on Madagascar.

Back to your flightless wren - was there possibly a direct land link to Stephen's Island at some point in its history, considering how close it is to the mainland?

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