A Population Study of the Weka Gallirallus Australis on Kapiti Island

Abstract

This study investigates the territoriality, time budget, diet, feeding, breeding and moult of a population of wekas (Gallirallus australis) on Kapiti Island in order to model population dynamics there; and investigates the interrelationship between anatomical changes associated with flightlessness, and the ecology of the weka. The origin of the Kapiti weka population, and the effect that the population is having on other biota of the island, are also investigated. The principal period of field work was between September 1979 and March 1982. However, as results of many aspects of the biology of the weka were inconclusive during this period, and some sections use information obtained up until February 1985. The weka population comprises sub-adult and adult individuals without territories, and year round territorial pairs. The means and extent of territorial changes are described, and the implications of territoriality on the stability, dynamics and density of the weka population are investigated. Stability of pair bonds and territorial boundaries are considered important influences on population size and composition. Weka behaviour is described, and the time budget of the sexes and resident components of the population investigated. Few differences are apparent in the time budgets of the sexes, either monthly or throughout the day. Differences are found between territorial and non-territorial individuals and between the breeding and non-breeding seasons. The annual diet of Kapiti wekas is determined and is compared with other populations. The weka's ability to change feeding effort as food availability evidently changed is also investigated. The weka is found to be a generalist and opportunist feeder, to have preferences and control feeding effort. Foraging is compared with predicted food requirements, to assess the effect that feeding efficiency has on weight changes, and ultimately on the incidence and timing of breeding and population dynamics. Food availability, foraging time and breeding are found to play important roles in population condition regulation. Pair condition ultimately determines the level of breeding. The breeding performance and the 'fledging' success of the study population over five seasons is described. The population produced more young than were required to fill spaces in the breeding population. Density dependent and density independent factors are investigated to ascertain their role in population control. The factors controlling the inconsistent breeding and the social structure and interaction of the population, are considered to be having an important bearing on population dynamics. Weka moult is investigated to find whether it effects other aspects of the life cycle, and how it differs from other flightless and Volant rails. The energetic requirements of moult appeared to be of little consequence to the weka. The number and sequence of plumages were similar to other rails. Moult overlapped breeding in periods of higher food availability. The weka is considered to be having little impact on the long-term survival of any other biota in similar habitats on Kapiti Island. The external morphology of Kapiti wekas is found to resemble that of the western weka (Gallirallus a. australis), however the origin of the population requires further investigation, because of predicted external similarities between mixtures of sub-species and the possible complications of environmental effects. Olson's (1973) claim that neoteny confers morphological flightlessness in the Rallidae is challenged. An alternative argument is presented, namely that a concurrent series of external and internal changes occur with flight loss in the Rallidae; and that allometric changes and associated epigenetic effects result in new morphologies, not retentions of states of the juvenile of the ancestor. The differing expression of some character states in any of a variety of related rails is interpreted as phylogenetic recombinations of ancestral potential (Croizat 1968), with the similarity of these changes being due to the directional evolutionary tendencies of the ancestor (Heads 1983). Panbiogeographic analysis (Croizat 1958) of the group of rails evidently related to the weka (Olson 1973a), challenges Olson's (1973) view of the antiquity of the rails on 'continental islands', and the fast rate of evolution of flightlessness. It is suggested that the ancestor of the weka was flightless, and that the weka and the banded rail share no immediate ancestry. The survival of the weka despite habitat modification and the effects of introduced fauna is suggested to be due to the species ecological plasticity in feeding and breeding. Such tolerance is only able to be maintained where food is collectable at or above energetic requirements throughout the year, or when food availability and feeding rates allow for periods of fat storage and fat attrition. It is suggested that this relationship is the key to weka population survival and it is argued that food availability reduction in the past by introduced mammalian browsers and competitors has led to the current patchy distribution of populations. This situation is predicted to get worse on the main islands in the next century and it is suggested that offshore island weka populations should be managed to safeguard the species.