SUMMARYParry, V. 1973. The auxiliary social system and its effect on territory and breeding in Kookaburras. Emu 73:81–100.Kookaburras are social and live in territories with either a single permanent mate or a family consisting of a mate and one or more auxiliary members. Auxiliaries form one third of the adult population; they are non-breeding birds, the progeny of previous seasons that remains round the year and aids in defence, nest attendance and provisioning of young. The auxiliary social system operates because of uniform plumage and a strict territorial regime.Size of territory was correlated with the number of birds present at the time when boundary adjustments were made. It is determined by the amount of space the birds can successfully defend. Auxiliaries occupy the same space in the same habitat as breeders, so reducing the breeding potential of the population by about one third.Small clutches hatched asynchronously. Nestlings of pairs and families received the same amount of food and had similar rates of growth but pairs without auxiliaries produced 1.2 fledgelings per pair, those with auxiliaries produced 2.3 fledgelings per pair. Mortality among juveniles of families was less, probably because they received more care than young of pairs.Auxiliaries contributed up to 32 per cent of nest-attendance time and up to 60 per cent of food-items brought, thus allowing the pair of the family freedom from nesting duties not available to single pairs and the opportunity to rear a second brood in good seasons. Because egg-laying to independence from care takes sixteen to eighteen weeks, second broods probably could not be reared by pairs without auxiliaries.The auxiliary system appears to be a long-term adaptation for reducing fertility in accord with longevity incurred from living in the equable Bassian Region. But short-term regulation of the population appears to be achieved through mortality from asynchronous hatching and the ability of families to rear two broods.