The breeding ecology of the South Island Fernbird in Otago wetlands

Abstract

The fernbird is a small inconspicuous passerine with relatively poor flying capabilities. It is found occupying areas with a generally dense cover of low vegetation from sea level to over 1000 m elevation. With the drainage of wetlands and the clearance of vegetation for farm development schemes or the planting of exotic pine forests, suitable habitat is diminishing at a rapid rate. Previous investigations on fernbirds have pointed out the relative stability and security of populations on remote islands and the vulnerability of small mainland populations in the face of habitat destruction.

The aims of the present study were to investigate the breeding ecology and habitat preferences of a population of fernbirds in one of New Zealand's few remaining extensive wetland areas and to determine whether management guidelines need to be implemented to ensure long-term survival.

To fulfill these aims I monitored the activities of individually colour-banded fernbirds over three complete breeding seasons. Sightings of recognizable individuals contributed to the mapping of territories. Nest sites were located and monitored. Information was collected on the length of the breeding season, the number of clutches produced during the season, the size of clutches, the duration of the incubation and nestling periods and an assessment of nesting success. The growth, development and feeding ecology of nestlings was also monitored in order to assess any differences with respect to brood-size, the prior nesting experience of parents or habitat quality.

Fernbird males are larger than females. Fernbirds are long-lived and sedentary, this combination apparently favouring the formation of long-term pair-bonds, possibly for life. Pairs occupy large territories that provide all of the resources for rearing young. Territory fidelity continues over several years, with a tendency for minor positional shifts to occur over the winter.

The preferred habitat is characterized by an assemblage of plant species that contribute to an extremely uneven canopy profile. Areas with significant amounts of surface water enhance this uneven profile. The preference of long-term residents for this habitat is reflected in the greater production of fledged young from this area.

Nest sites are selected for the degree of cover, but various factors influence the choice of location. Both sexes contribute to nest construction, incubation of eggs, brooding and feeding young. Nest initiation is highly asynchronous and the season can last more than six months.

The predation of nests by harriers, primarily during the nestling stage, is the most significant mortality factor. Introduced mammals account for a small proportion of losses, primarily in the drier areas of the wetland.

A comparison of the growth and development of nestlings from broods of various sizes indicated that there were some differences. Larger broods compromised a lower body weight at fledging for mature function in skeletal characters.

Despite the low levels of nesting success (30-40%) the long breeding season, the ability to produce more than one brood and the ability to increase clutch-size with no apparently greater risk, indicates that the population is capable of long-term survival. As long as there are no significant reductions to the size of the Lakes Waipori and Waihola wetlands or to the flow of water through the wetland, management policies are not required.