Abstract
The breeding biology of the Rock Wren, Xenicus gilviventris, was studied in the Murchison Mountains, Fiordland during the 1983-84 and 1984-85 breeding seasons, observations generally being made of banded birds.
Rock Wrens were found to be monogamous, solitary breeders which have their breeding territories in the subalpine zone of mountainous regions of New Zealand's Southern Alps. Females of this hole-nesting species laid from one to five tiny (mean 2.6g), white eggs at 48-hour intervals, each egg representing a mean13% of the Spring body weight of the female. Eggs were laid in early October through until mid November in thick-walled, well insulated, pouch-like, completely enclosed nests built principally from native grasses and lined with many feathers. The altricial young hatched asynchronously over a 1-3 day period after a 19 to 20 - day incubation phase.
A high degree of parental co-operation was observed in the species, the male sharing equally nest-building, incubation, brooding and the feeding of young. Courtship feeding by the male made a significant contribution to the food supply of the female at all stages of the breeding cycle.
Nestlings developed without down over a 24-day nestling period attaining weights averaging 28% higher than the mean body weight for adults. Some primitive and unusual morphological and pterylographical features were observed during their development.
From about the fourth day after hatching, female nestlings achieved and then maintained higher body weights than male fledglings. When weighed at various times between the end of egg laying and the conclusion of the post-nuptual moult, adult males had a mean body weight of 15.8g while adult females averaged 19.6g. The species was found to show sexual dimorphism not only with respect to differences in body weight, but also in plumage coloration.
Nestlings fledged and became independent within 14-21 days, juveniles establishing territories and finding mates by the end of the summer in which they had fledged.
lt was noted that the species, unlike its confamilial the Rifleman (Acanthisitta chloris), is single-brooded in any breeding season. Also differing from its close relative, the Rock Wren does not exhibit co-operative assistance during breeding, either from sibling or from unrelated helpers.
Adults defended territories ranging in size from 0.6 - 4.2 hectares (the mean being 1.4 hectares). Adults underwent a post-nuptual and juveniles a post-fledging moult within three months after young had hatched.
Each pair of Rock Wrens successfully raised on average 3.2 fledglings per season. 86% of all eggs laid hatched, and of the nestlings which emerged, 79% successfully fledged. Mortality rates for all phases of the bird were found to be relatively low within seasons compared to other New Zealand passerines, but between seasons losses were found to be relatively high, probably reflecting the harsh nature of winter weather conditions prevalent in the subalpine zone of the Southern Alps.
Rock Wrens were found to be sedentary birds maintaining territories which altered little both laterally and altitudinally from one season to another. No migration to lower altitudes was observed during the winter, although strategies for over-wintering were not determined in this study.
Records of banded birds compiled during and since the present study have indicated that Rock Wrens live for at least 5 years.
Features of the breeding behaviour and reproductive phenology revealed in this study have shown that the Rock Wren is remarkably well adapted to survive the harsh and exacting environmental conditions typical of subalpine zones of New Zealand's Southern Alps.