Productivity and mortality of mohua (Mohoua ochrocephala)

Mohua (Mohoua ochrocephala) breeding and mortality were studied in the Eglinton Valley, Fiordland National Park, New Zealand. Mohua on the valley floor (380 m a.s.l.) bred from early October until March, though birds at higher altitude in the Eglinton and elsewhere started later. In low altitude areas, most pairs raised two broods a year, but elsewhere they seemed to raise only one. Eggs were laid daily, and incubation began with the laying of the last egg and lasted about 20 days. The nestling period was about 23 days. Clutches contained 1-5 eggs, most often three. During three breeding seasons there was no significant nest predation by introduced mammals, but in 1990/91, when stoat (Mustela erminea) numbers were high, 67% of nests and 50% of nesting females were destroyed by stoats. When stoat numbers were low, the productivity and mortality of double-brooded mohua were well within the ranges recorded for other forest-dwelling bird species, and the productivity of single-brooded mohua was lower than that of most other forest-dwelling passerines. During years when stoat numbers were high, the productivity and mortality of even the double-brooded mohua were lower than for most other forest-dwelling passerines. There is some evidence that there were density dependent mechanisms influencing the productivity and survival of chicks.