Family: Rallidae; Genus: Gallirallus; Species: australis
Weka, also known as woodhens, are flightless birds endemic to New Zealand. Four subspecies of Weka are found in New Zealand; the photo above is of a Stuart Island Weka (Gallirallus australis scotti). Weka are about the size and weight of a large chicken, however, Stuart Island Weka are the smallest of the four subspecies.
At present, it is thought that the Weka diverged from ancestors that flew to New Zealand less than 10 mya (Gibbs, 2006), and developed flightlessness once on the islands. Because the ancestors of the Weka migrated to New Zealand so recently, the flightless Weka is thought to have evolved by a process known as vicariance where a subpopulation of a species is cut off from the general population by a barrier, in this case the ocean, and evolves separately.
New Zealand is an island environment that originally lacked terrestrial mammalian predators, and so it is not surprising that many of the birds evolved flightlessness once established here. Flighying is metabolically and physiologically taxing, and is thus selected against in situations where long-distance trips are unnecessary. Ancestors of the Weka used flight to colonize the remote island habitat of New Zealand as did many other birds, however, long range dispersal ability can be deleterious to a species once it has been established in such an environment where strong winds could alter the flight course of individuals in the species. In response to such evolutionary pressures, the Weak lost the ability to fly and settled down for a more terrestrial existence.
Ghosts of Gondwana: The History of Life in New Zealand.—George Gibbs. 2006, reprinted 2007. Craig Potton Publishing, Nelson, New Zealand.