Tiny rock wrens or pīwauwau (Xenicus gilviventris) are found above the treeline, at between 900 and 2,500 metres in altitude. They live under large boulders at the foot of screes and rock falls.
In summer rock wrens hop and buzz about on short rounded wings, feeding on insects or collecting materials to build their nests. Once winter snows arrive, they survive in pockets of space in their rock or scrub shelter. Rock wrens fly close to the ground, their flights rarely exceeding 30 metres. They move astonishingly quickly, diving in and out of crevices. On the ground they bob energetically up and down, flicking their wings.
A rock wren is 10 centimetres long, from its bill to the end of its short tail. The topside colour of males varies regionally from green to brown. Females are brown above, tinged with yellow. Both sexes have yellow flanks, a pale underside and long cream eyebrow lines. They have slender black bills and, like others in this family, long toes and short wings. Females weigh 20 grams and males 16 grams.
Rock wrens belong to a special group – the endemic New Zealand wrens, which form the ancient family Acanthisittidae. Part of the large group known as passerines or perching birds, they form a suborder distinct from all the others (which are oscines or songbirds).
The rock wren and the rifleman or tītiti pounamu (Acanthisitta chloris) are the only surviving members of this suborder. They are not closely related to true wrens.
Rock wrens are confined to separated pockets in the mountains of the South Island – in north-west Nelson, the Victoria Range, and on eastern and western slopes of the Southern Alps down to Fiordland. The population is small and in decline, as they are vulnerable to predators, including stoats and mice. A 2008–10 attempt to establish a population on predator-free Secretary Island in Fiordland appears to have been successful.
Feathering its nest
In the 1920s, ornithologist Herbert Guthrie-Smith counted 791 feathers lining one rock wren nest on the Mackinnon Pass, Fiordland. Most belonged to weka, but there were also many kiwi and kākāpō feathers, and a few from kea and pigeons. Guthrie-Smith noted that the nest was fresh and dry, despite recent deluges.
Rock wrens build elaborate nests in rocky crevices, either at ground level or up in bluffs, or sometimes tucked under subalpine scrub. They excavate soil, and build a bulky structure of dried grasses, moss, skeleton leaves and tussock, which they line with feathers. The nest is fully enclosed except for a tiny side entrance tunnel. The male feeds the female as she prepares to lay three or more cream-coloured eggs, usually in October or November. Both parents incubate the eggs, and take turns feeding the chicks.
Rock wrens eat a variety of invertebrates, including beetles, spiders, flies, caterpillars and caddisflies. They also eat berries and grass seeds.