The Tasmanian native hen, Tribonyx mortierii. II. The individual, the group and the population

Abstract

A partly isolated natural population of Tasmanian native hens was studied in southern Tasmania for 3 yr. The population contained about equal numbers of pairs and trios, the latter consisting of two males, often brothers, breeding with the same female. Trios appeared to depend on the observed shortage of females in the population, apparent even among juveniles, and did not appear to be the cause of this unequal sex ratio. Males and females shared all duties more or less equally within the group. The group was well integrated and there was little friction between its members. All groups defended permanent territories, to which they were restricted. The territories of older birds were larger than those of birds in their first year. Display was usually sufficient to make trespassers withdraw from the territory. Nevertheless actual fighting was quite common, although serious wounds were rare. Young birds left their parents spontaneously around the age of 1 yr. Most of them wandered ("roved") out of the study area almost immediately but some tried to form groups and establish territories there. Only a few succeeded in the face of the opposition from surrounding groups. Such conflicts were the most vigorous of those observed. Territorial behaviour greatly reduced the density of the population during the breeding season. The frequency of conflict per group doubled when the population doubled. There was some indication that this increased conflict reduced breeding success because of interference with parental care. Birds breeding in their first year were more sexually active but less sexually proficient than older birds. First-year birds laid slightly smaller clutches, and laid two clutches much less often, than older birds. Largely as a result of these differences, older groups reared more young in a season.