Saturday, October 8, 2011

Stephens Island Wren (Xenicus lyalli)

Rising to heights of around 300 m, Stephens Island looms off   the northernmost tip of Marlborough Sound on South Island of New Zealand. The island is tiny (2.6 km2), but it is a refuge for many animals that have disappeared from the mainland since the arrival of Polynesians.

On this prominent lump of rock, there once lived a small bird known as the Stephens Island wren. This bird was unrelated to the familiar wrens of the Northern Hemisphere and actually belonged to a small group of perching birds endemic to New Zealand. The remains of this small bird have been found at various sites throughout the main islands of New Zealand, and it seems that Stephens Island was the last refuge for this bird following the arrival of humans and the animals they brought with them. One animal in particular, the Polynesian rat, wreaked havoc among the populations of New Zealand’s small endemic birds. Stephens Island served as a refuge for the wren for hundreds of thousands of years, and even after the Polynesians and their animals wiped out these small birds on the mainland, the population on Stephens Island was safe—until the arrival of Europeans.

The British commandeered New Zealand as an extension of their growing empire, and in their learned opinion, what Stephens Island needed more than anything was a lighthouse to warn ships away from the rocks. In June 1879, a track to the proposed site for the lighthouse was cleared, and five years later, the lighthouse went into operation. In itself, the  lighthouse was no threat to the wren, but in those days, lighthouses were operated by people, and people have pets—often, cats.

At some point in 1894, a pregnant cat was brought to the island, and it seems that no sooner had she arrived than she gave her new owners the slip and escaped. This unassuming cat probably didn’t realize how special she was. No predatory land mammal had ever set foot on Stephens Island, and the animals on this forested outcrop were woefully ill prepared as they had never encountered any mammal, let alone one with the predatory proclivities of the domestic cat. In June 1894, one of the off  spring of the escaped cat was apparently taken in by one of the assistant lighthouse keepers, David Lyall. Lyall had an interest in natural history, and he was intrigued by the small carcasses his young pet brought back from its forays around this previously untouched island. The carcasses were those of a tiny bird, but of a sort that Lyall had never seen. With a hunch these birds were something special, he had one sent to Walter Buller, an eminent New Zealand lawyer and ornithologist, who immediately recognized the sorry-looking carcass as an undescribed species. The bird was definitely a type of New Zealand wren, related to another small New Zealand bird, the rifleman. Unlike the rifleman, the Stephens Island bird was flightless. The larger group to which these birds belong, the perching birds (passerines), has only a couple of flightless representatives.

The only information we have on the way the Stephens Island wren lived comes from the limited observations made by Lyall. According to the only person who saw this species alive, it “ran like a mouse” and “did not fly at all.” This is about the sum of the information we have on the living bird, but the structure of the bird’s skeleton and plumage allows us to investigate if Lyall was correct. The skeleton of this tiny bird bears all the hallmarks of a species that had given up the power of flight, and the plumage does not appear to be up to the job of flapping flight. We can’t rule out the possibility that this tiny bird ran and leapt or glided to catch aerial insects, but it would not have been capable of flapping its wings to any great effect. The great tragedy is that this tiny bird died out before we could learn anything more about it.

In 1894, Lyall brought a total of 16 to 18 specimens of Stephens Island wren to the attention of the scientific establishment. It is not clear if his cat caught all of these, but late in 1894, news of this bird had circulated in the ornithological community, and some collectors were willing to pay big money for a specimen—Lionel Walter Rothschild, the famous British collector, purchased nine specimens alone. With such a high price on the heads of these diminutive birds, can we be sure that Lyall didn’t go and catch some himself to supplement his income? We’ll never know, but the cats and the greed were too much for the Stephens Island wren, and before 1894 was out, the species was extinct—discovery and extinction all in the space of one year. This is pretty impressive, even by human standards of devastation. 

Source: Wikipedia


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