Flocking of the White-eye Zosterops lateralis in New Zealand

Abstract

1. The present report forms part of a study of the winter mortality of the White-eye in New Zealand where the breeding success is high. Weekly population counts in a 12 acre area of a semi-natural habitat in Dunedin showed that the increases in number in autumn and spring were the result of partial migration. In these seasons the average flock size also increased (Fig. 1) but nevertheless remained smaller than the average flock size (9.5) recorded in the winter at the feeding station outside the area.2. Frequent interchanges of individuals occurred among the winter flocks but mated birds always remained together. Records of fighting among colour-ringed birds revealed a peck-order in the flock and these, together with evidence obtained by observations of captive birds, prove that there is a definite peck-right hierarchy. After plotting the number of victories against the number of defeats in encounters (Fig. 2) it is apparent that, although some females had more victories than defeats, males were generally more aggressive, and that there were tendencies for dominant birds to fight more often with birds of higher ranks and for subordinate birds to avoid dominant birds.3. Three types of call notes were found which have different functions in the social behaviour of flocking White-eyes: the flight call, the alarm call, and the threat and initiative call notes.4. Pair formation among the first year birds took place during the winter and as these birds gradually left the flock and established territories the size of the flock diminished.5. During the breeding season the principal food of the birds is of the insectspider type. Field observations and feeding experiments showed that during the winter greater interest is taken in plant food. Therefore insect feeding as postulated by Cunningham (1946) as a reason for the origin of flocking is open to doubt and here flocking has been considered from the point of view of migration and predation.6. The importance of behaviour study as an aid to the understanding of speciation was emphasized.