The Stephens Island Wren, Xenicus lyalli or often Traversia lyalli*, was a small flightless bird. Its bill was only 14 millimeters long, its tail 17 millimeters, and the males were smaller than females. Its colours were a variation of olive green and brown. It could also sing.
Originally, the Wren prospered on the main islands of New Zealand. When the Maori brought the Polynese Rat to the area with them, the bird was wiped out on mainland New Zealand. Only a small population survived on what was later known as Stephens Island.
Stephens Island is a patch of land of 2.6 square kilometers off the northern tip of the South Island of New Zealand. It was untouched by any creature foreign to the New Zealand ecosystem all the way to the 1890s, when humans arrived and built a lighthouse.
You may have noticed that I have used the past tense when describing the bird. I'm getting to that.
In 1894 David Lyall was the keeper of the lighthouse on Stephens Island, and the island's only human occupant. Keeping a lighthouse is a lonely duty, so he brought to the island a cat, Tibbles, to keep company.
One day Lyall noticed that Tibbles had developed quite an aptitude for hunting birds. In fact, the cat brought home over a dozen specimens of the same tiny bird. It did not feed on them, though, so the dead birds were in good condition.
Lyall managed to take a few of Tibbles's little trophies and send them to Wellington. A specimen ended up with a member of the British Ornithological Club, Lord Rothschild, who identified it as a new species and named it Traversia lyalli after the human discoverer. A wren also found its way to the famous New Zealandian ornithologist, Sir Walter Buller, who also named the bird. It was later concluded that Buller, in fact, had named the bird first.**
The flightless Wrens were helpless against this new predator in their little habitat. Curiously, Lyall, one of the few men who have seen a living Stephens Island Wren, described their gait as that of mice. The birds were also nocturnal, and hopped and scurried between boulders, looking for insects. Lyall never saw any of the birds fly, and the shape and size of their wings confirmed that they were flightless.
It apparently did not occur to anyone that the source of Wrens might be exhaustible. After a while, Tibbles stopped bringing in wrens to the lighthouse. The entire species had been eradicated by the cat.
This exctinction is remarkable in many ways, even if it is tragic. In the 19th century, such speed meant that the ornithologists of the world would find out about the birds existence and disappearance at the same time. It is also the only recorded instance of a single creature, human or otherwise, wiping out a species whose population had remained stable for centuries.
Fortunately, the cat's careful treatment of the wrens means that there are several good specimens of it in museums all over the world.
A paper by R. Galbreath and D. Brown of the Ornithological Society of New Zealand argues that the extinction of the wren was not quite as dramatic as the usual tale goes. The paper states that specimens of the Stephens Island Wren were discovered all the way until 1899, and that Stephens Island was inhabited by several cats after 1894, sparing Tibbles from bearing the guilt all by itself.
All the more reason to agree with Sir Walter Buller's sentiment in 1905, however:
And we certainly think that it would be as well if the Marine Department, in sending lighthouse keepers to isolated islands where interesting specimens of native birds are known or believed to exist, were to see that they are not allowed to take any cats with them, even if mouse-traps have to be furnished at the cost of the state.
* Xenicus is the genus of New Zealand wrens, who actually have no direct relation to real wrens at all, and are named such only because their appearance resembles real wrens. Traversia is a separate genus that was established for the Stephens Island wren alone. Both names for the wren are in current use. Nobody seems to have gotten around to deciding which one is correct, but then again, a little extinct bird is not high in priority.
** None of my sources are direct on this, but I would wager that Buller placed the bird in the Xenicus genus while Lord Rothschild placed it in the Traversia one.
A page with a drawing of the bird: