The study is based on 18 months intensive field-work during which 212 Australasian harriers were trapped, retrapped, measured, sexed, aged, individually marked and observed. Fortnightly observations of the individually marked population were made over a further seven months. The Australasian harrier and European marsh harrier are considered to be conspecific. Evidence is presented showing that there is no valid reason for considering Circus aeruginosus of the Pacific Islands to be a different subspecies from C. aeruginosus of Australia and New Zealand. During the breeding season ten territories in the 12 km 2
study area averaged 31 ha, nest sites averaged 910 m apart, pairs' overlapping home ranges averaged 9 km. 2
and favourite hunting areas 3 km, 2. A high population density of one bird per 50 ha was calculated. A low fledging success rate of 1.8 young per successful pair and 1.1 young per nest site, and two cases of polygyny were recorded during two breeding seasons. Territorial and courtship behaviour, nest parameters and the parental division of labour is described. Seasonal movements and the dispersion of all age and sex classes from the study area at the end of the breeding season are described. Most (66.7%) individually marked adults returned after the autumn dispersal phase and established winter home ranges averaging 9 km. 2
. The home range of an adult female in open farmland was calculated to be 14 km 2
using radio-telemetry techniques. A non-breeding season population density of one bird per 80 ha was calculated. Communal roosting, which occurred throughout the year, is discussed. Four hundred and seventy food items were identified in the diet from pellets, prey remains, stomach contents and field observations. In descending order of numerical importance in the diet were mammals (46.4%), introduced passerines (29.0%), insects (7.6%), game birds (6.7%), birds' eggs (4.8%) and aquatic prey (4.6%). Australasian harriers ate significantly greater numbers of live prey than carrion annually. Adults took significantly greater numbers of agile food items than juveniles. Females ate significantly more large (>200 g) and fewer agile food items than did males. Seven search techniques and five attack techniques, including some buteonine techniques, are identified and described in the Australasian harriers' wide range of hunting techniques. Ninety five attacks on prey are recorded and 15.8% of these were successful. Adults were significantly more successful hunters than juveniles. Co-operative hunting, hunting in the daily cycle, feeding behaviour at carrion, interspecific competition for carrion, interspecific disruption of hunting and prey escape tactics are described. From a computer analysis of hunting behaviour data it is concluded that adult males are more manoeuverable and less conspicuous than adult females and juveniles because they flew significantly lower and faster. Adult males also hunted, to a significantly greater degree, those habitats where there were greater numbers of agile prey. The hunting inexperience of juveniles was quantified. The Australasian harrier is moderately sexually dimorphic. Current hypotheses proposed to explain the degree of sexual dimorphism in raptors and why the females of most raptor species are larger than males are critically reviewed.