Introduced red deer Cervus elaphus competing for tussock were a major factor in the post-1940s decline10, while a series of unusually harsh winters appears important in the recent fluctuations1,3. Predation by introduced stoat Mustela erminea could be significant2,3,4 but remains speculative5. Other potential competitors or predators are introduced brush-tailed possum Trichosurus vulpecula and the threatened Weka Gallirallus australis5. The level of inbreeding in females appears to be related to the low hatching and fledging success exhibited by small island populations12. Radio-tags have been shown to increase daily energy expenditure, this may influence mortality, particularly in hard winters13. On some Tiritiri Matangi at least there is probably some predation by Swamp Harrier Circus approximans15. The small island populations may be close to carrying capacity14: on Tiritiri Matangi the 2000/2001 breeding season was largely unsuccessful, primarily due to the increase in territorial disputes amoung proximal family groups15. Habitat quality on some of the islands is probably in decline as reafforestation reduces the area of foraging habitat15.
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