The Quelea is an extremely gregarious bird living in dense flocks throughout the year. In Senegal the population is found mainly along the river valley in the dry season but it moves north to breed in enormous colonies in the Mauretanian thorn bush country in the rains. In the dry season the diurnal rhythm of the population consists of a morning dispersal for feeding, a mid-day rest period and a nightly congregation into the roost. Abnormal concentrations over man-made feeding grounds are ascribed largely to the gregarious behaviour of the birds (p. 3). Flock movements are described. The Queleas show no flock fragmentation prior to breeding and there is no evidence for an increase in 'individual distance' of the males in flocks prior to the establishment of territories. Around the colonies feeding flocks retain the same gregarious behaviour as observed in the dry season. The life of the colony is described and divided into five phases (p. 9). The species is monogamous and each male builds one nest. He also assists in incubation. The is limited to the nest and its environs. It is defended by supplanting attacks lunges and the depressed posture. These are described in detail and analysed in terms of the tendencies present - viz. to attack, to flee and to remain within the small territory. The complex Tail depressed Threat is interpreted as a transition probably ritualised, that has acquired an especial dual signal function, appearing aggressive to approaching birds but submissive to neighbours. Males in this posture always face approaching birds, thus presenting them with the threatening aspect of the display (p. 21). The sudden removal, by flame throwing, of nests with eggs resulted in the complete breakdown of both territories and pair bonds. The males became very aggressive within an abnormally enlarged 'individual distance' representing an area of territorial defence no longer based on a locality. The females wandered at random between the males. Fragmented and promiscuous courtship occurred at the singed remains of nests. The behaviour thus showed a reversal to an earlier stage. Some nesting material was brought. The earlier the burning during brooding the greater the behaviour reversal observed. The functional significance of the Quelea is discussed. Courtship begins when the nest is half built. The male gives advertisement displays with wings upstretched above the back and the tail vertical. The display functions principally in attracting females which approach firstly in submissive postures. The male's first responses to her are threatening but following his adoption of an Appeased threat posture, the female comes to the nest and hops about in his company. Copulation follows. After pair bond formation a special wing quivering behaviour given mutually appears to reduce further aggressive behaviour between mates. At the nest a change in dominance occurs the female becoming dominant over the male. The courtship postures are analysed briefly in terms of the tendencies present. A number of experiments during incubation showed that egg removal and nest mutilation beyond the cutting of small holes resulted in desertion. Changes in the perceptual situation at the nest site, i.e. the reorientation of the entrances and the changing over of whole nests disturbed the birds, in the first instance, by changing the final routes through the territories prior to entering the nest, thereby increasing the frequency of aggressive encounters between previously peaceful neighbours, and, in the second case, because the males, unlike the females, evidently know their nests as structures apart from their localities. Their behaviour was agitated and indiscriminatingly aggressive including invasions of neighbour territories and examination of the nests there. The feeding of the young by parents is described. At first the young remain near the nest but later wander. Both young and parents recognise each other but the former may solicit food from other adults. A family group territory is shown by parents who prevent the approach of other adults to the young and also drive off other young begging to them. With increased wandering the families break up and the groups of birds mix so that parents feed young indiscriminately and young receive food from adults other than their parents. The young at about eight days (Age 19 days) out of the nest finally move about in small flocks and feed on fallen seeds below the trees. Until their eighth day out of the nest the juveniles show no individual aggression and behave as contact birds. This is contrasted with the typical distance species behaviour of adult birds. The change from the juvenile flocking behaviour to that of the adults evidently occurs as the young leave the shelter of the colony. The vocabulary of the species is summarised in an appendix.