SUMMARYWyndham, E. 1980. Diurnal cycle, behaviour and social organization of the BudgerigarMelopsittacus undulatus.Emu 80: 25–33.The BudgerigarMelopsittacus undulatuswas studied in the field in eastern Australia. Flocks occurred throughout the year, during most diurnal activities and at most stages of the life cycle. Daily activity began about sunrise and ceased during dusk. It consisted of feeding in the morning and afternoon and resting and preening in the foliage of trees during the middle of the day. Birds drank intermittently and at no specific time of the day. Different ages and sexes mixed in flocks and there was no obvious hierarchical construction nor high-and low-status groups within flocks or populations. At times, flocks occurred that were composed predominantly of young birds. Eight discrete calls were identified. Slight differences between juveniles and adults, little sexual dimorphism and few discrete calls suggests that the social organization of the Budgerigar is simple compared to that of the Eastern RosellaPlatycercus eximius.Behaviour of birds in the field is compared to that described from studies of domestic birds in cages.