SummaryThe habitat ofLalage sueurii tricolorin north-western Australia is described and, after some notes about locomotion and comfort movements, details on food and feeding habits are given. The food consists mainly of large insects (especially grasshoppers and caterpillars) which are captured on the ground. The male song primarily serves to advertise the territory and is given usually while on the wing. The species breeds during the summer rains. Its breeding season is considerably longer than in other insect eaters, which may be connected with the wide variety of food taken. Within the prolonged breeding season of the species individual breeding and moulting cycles may vary considerably and males in nuptial and eclipse plumage may be found at the same time. In coastal areas Trillers vigorously defend a fairly large territory. In the interior of the continent, there is a trend towards colony breeding, the biological significance of which is discussed. Nesting site and nest are described. Both sexes participate in nest-construction, the male doing 60–70 per cent of the work. Incubation starts the day the second egg is laid. Male and female incubate alternately, with the average period on being 10–15 minutes. All clutches found consisted of 2 eggs. Incubation takes 14 days. The young are fed and covered by both parents until the eighth day after hatching, when the male suddenly ceases to participate in the care of the young. The fledging period is 12 days. Near permanent water the breeding pairs tend to keep their territories all the year round. In drier regions they may be forced to leave their breeding grounds owing to lack of food and water. Some obvious discrepancies between sexual dimorphism and participation in parental care are discussed.