The Tasmanian native hen, Tribonyx mortierii. III. Ecology

Abstract

Tasmanian native hens live in open grassy areas near water, often on the floors of river valleys. They are most abundant at altitudes below 700 m although they occur up to 1100 m. They are secondary grazers and in agricultural areas rely largely on the sward created by sheep and rabbits. Originally their habitat was probably produced by marsupial grazers (and still is in occasional remote virgin habitats), and by the bushfires of the Aborigines. Since European settlement farming has appreciably extended their range. Their main food is fine young green herbage, especially grasses, but they also eat seeds, particularly in summer. In a study area near Hobart they mainly ate the predominant introduced temperate pasture plants. The young plants they eat are rich in starch, which is probably digested with the aid of the well-developed intestinal caeca. Only small quantities of insects are eaten. The food supply runs short in summer. It is suggested that territorial behaviour ensures for a family group a food supply which generally tides it over the summer. Territoriality considerably reduced the density of the population of the study area during the breeding season, although there were signs that sometimes a severe summer food shortage might reduce it further. The onset, duration, and success of breeding were all influenced to some extent by the food supply, which in turn depended on rainfall. Of the eggs laid, 55 % produced young which survived to the age of 4 months or more. Although infertility, climatic factors, and straying caused some of the failures of the remaining 45%, predation was probably the most important cause. Adult mortality in the study area was very low. Territoriality enforced the dispersal of most first-year birds to areas up to 9 miles outside it; and in these areas mortality, mostly due to man, was often much more severe, eliminating most of these surplus birds. The evolution of flightlessness in the Tasmanian native hen can be explained as resulting from selection for the redistribution of locomotory musculature to the legs, in the absence of conditions which require flight in continental gallinules. Gallinules, although poor fliers, need flight to escape climatic vicissitudes when they inhabit a continent such as Australia. The regular and equable climate of the island has removed this need in the case of the Tasmanian native hen.