Factors affecting formation of flocks of unusual size and composition in Black-necked Storks ('Ephippiorhynchus asiaticus') in Australia and India

The Black-necked Stork ('Ephippiorhynchus asiaticus') usually occurs singly, in pairs or in family groups, and only very rarely in flocks. We describe records of flocks of this species in the Australian and Indian subspecies and discuss the possible reasons for their occurrence. Records of flocks were fewer from India, but flock-sizes were significantly larger. Factors related to flock formation, such as season, habitat quality and water availability, differed between Australia and India. Flocks occurred predominantly in autumn in Australia, whereas there was no relationship between season and flocking in India. Flock-sizes did not differ seasonally for either subspecies or between subspecies. Flocks were composed either of adults only or combinations of young birds and adults. In Australia, occurrence of different types of flocks was not affected by season or habitat. A combination of season and local habitat conditions appear to be affecting formation of flocks in this species.