Biometrics, sexing criteria, age-structure and moult of Sooty Oystercatchers in south-eastern and north-western Australia

Abstract

Since 1980, 856 Sooty Oystercatchers (Haematopus fuliginosus) have been caught and banded in Australia by the Victorian Wader Study Group (n = 801, from southern Australia) and the Australasian Wader Studies Group (n = 55 birds, all from north-western Australia). The biometric data collected were analysed for differences between sexes and ages and for geographical differences. Females were larger than males in all biometric dimensions, in all age-groups and in both geographical regions. For both subspecies-H.f.fuliginosus in south-eastern Australia and H.f. opthalmicus in north-western Australia -length of bill (exposed culmen) was the most discriminating parameter for determination of sex. Weights of adult females increased markedly towards the end of the moulting period. In south-eastern Australia, adult males considerably outnumber adult females in catches. Furthermore, adult males also outnumbered adult females in late autumn-early winter samples; at other times, and in other age-groups, the sex-ratio was more evenly balanced. Differential survival or spatial distribution of females and males are proposed as the mechanisms causing male-biased tertiary sex-ratios in catches. South-eastern Australian birds were larger and heavier than their northern counterparts. Furthermore, comparisons between Sooty Oystercatchers and the sympatric congener, the Australian Pied Oystercatcher (H. longirostris) showed the former to be significantly larger and heavier in both parts of the country.