The Breeding Biology and Habitat Relationships of the Yellowhead
This study aimed to find an explanation for the decline of yellowheads and formulate recommendations for management and further research on the species. There were three main lines of investigation: basic population ecology and behaviour; the effect of introduced predators on breeding; and the habitat relationships of the species. A detailed study of a yellowhead population in the Eglinton Valley in Fiordland National Park was undertaken. Birds were caught and banded and their behaviour, breeding and survival monitored for 4 years. The relationship between yellowhead distribution and vegetation, topography, and fertility were investigated in part of Mt Aspiring National Park during one summer. Yellowheads suffered high rates of predation from stoats during "plagues" that occurred after heavy beech seeding. Three aspects of yellowhead biology made them vulnerable to mammalian predation: (1) they nested in holes and predators killed not only eggs and nestlings, but also incubating adults; (2) only the females incubated, thus losses to predators had a greater effect on the population than if equal numbers of males and females were killed; and (3) yellowheads nested later than most other forest passerines and were still nesting when stoat numbers reached their summer peak. Though the yellowhead's hole nesting habit made them vulnerable to mammals it restricted nest parasitism and predation by long-tailed cuckoos and hole nesting is likely to have evolved in response to cuckoos. Yellowheads were found to be tall forest specialists; they occurred more frequently in tall forests than short ones, and preferentially used the largest trees. Their choice of nest sites had no effect on their preference for any forest types. The forests they favoured grew mainly on fertile valley floors at low altitudes. Yellowhead populations in "good habitats" raised two broods a year and these populations are probably sufficiently productive to withstand stoat plagues occurring once every 5 years, the average frequency of this event. Populations in "poor habitats" raise only one brood and their productivity is probably insufficient to match losses to stoats. Such populations are probably slowly declining, and are very vulnerable to extinction. A habitat suitability index was devised and forests in the north of the South Island from which yellowheads have disappeared, were compared with those in the south where yellowheads persist. Northern forests were as good for yellowheads as southern ones. Thus, the combination of habitat preference and predation cannot account for the recent disappearance of yellowheads from the northern half of the South Island. The decline in yellowheads was attributed to both predation by introduced mammals and competition with introduced vespulid wasps. Predation may have eliminated yellowheads from podocarp-dominated forests where predator numbers are constantly high, but they survive in some beech forests where predator numbers rise only once every five years. However, even within beech forests only the most productive populations are sufficiently productive to survive predation and these populations are probably susceptible to competition with wasps which eat large numbers of invertebrates. Yellowheads are likely to be more vulnerable to wasp competition than other forest insectivores because: (1) predation has reduced their productivity more than other birds because they nest in holes; (2) they are specialised in low altitude, tall forest that the wasps also favour; (3) their breeding is later than most other forest birds and their period of juvenile dependence much longer. Yellowheads are still feeding fledgling yellowheads at the time when wasps numbers reach their peak in the autumn, whereas the offspring of other forest birds are independent by this stage.