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Breeding biology of radio-tagged Kakapo Strigops habroptilus on Stewart Island was investigated from 1977 to 1988. Some males moved several kilometres from their ranges occupied in the non-booming season (May-November) to track-and-bowl systems. Occupancy of track-and-bowl systems varied markedly from one booming season (December-March) to another, from no visits in some booming seasons to breeding years when most males occupied their systems nightly. Descriptions and rates of calling for three types of calls by males are given. Breeding probably occurred in 1978, and nests were found in 1981 and 1985, but radio-tagged females did not nest in 1982-84 and 1986-88. Females visit males at track-and-bowl systems for mating in mid-summer (January-February), and then returned to their home ranges to nest. Nests were under thick vegetation, in short holes in banks or rotten trees at ground level. Clutches consisted of 2–4 eggs, and incubation lasted about 25 days. One female left her eggs and young nestling unattended at least once a night for 1–4 hours. Thus, Kakapo eggs and young chicks were very vulnerable to predation by introduced predators. There was no evidence that any male assisted a female during incubation or nestling-rearing phases. The chicks were brooded by day until about 30 days old, after which the female roosted nearby. The chicks left the nest at about 10 weeks old, but remained nearby for a further month. Phenological observations of Kakapo food plants suggest that to stimulate and sustain breeding, Kakapo on Stewart Island are dependent upon abundant fruit from a few irregularly mast-fruiting tree species.