Determinants of Social Rank in Goose Flocks: Acquisition of Social Rank in Young Geese

Abstract

The paper describes a study of social rank acquisition in goslings reared from eggs taken from a full-winged flock of barnacle geese (Branta leucopsis) at the Wildfowl Trust, Slimbridge. Eggs were taken from pairs of known history and the adult's aggressiveness was ranked according to their reaction to humans. This rank was shown to be meaningful intraspecifically both by the outcome of encounters between geese and by the fact that no pairs scored as non-aggressive were able to nest in the preferred colony. A group of goslings reared by their own parents and cross-fostered goslings were also examined. 1. Within a rearing group of goslings (sibling-reared broods), the oldest and heaviest birds ranked highest in the first month and males performed better in encounters than females of the same size in the second month. 2. In encounters between unfamiliar goslings from different sibling-broods in the third month of life, the most important determinants of the new rank were body size, weight and sex. Previous experience also influenced rank; previous success yielded continued success. Goslings lost weight during the test; loss was negatively correlated with rank. The performance of goslings reared without adults bore no relationship to their parent's aggressive score. 3. In the semi-captive flock, parents that scored as "aggressive" reared more and larger goslings than non-aggressive pairs. The rank of these in the third month correlated with their size and sex (independent of size). The cross-fostering experiment suggested that there was a genetic as well as an environmental influence on rank acquisition. 4. In encounters between goslings of similar rank from sibling-broods and parent-reared ones, the latter ranked significantly higher. Parent-reared goslings, though less familiar to the experimental regime, gained weight and goslings from sibling-broods lost weight. 5. Once established, rank order remained stable; the few reversals related either to changes in size or to cooperation between goslings in confrontations. 6. Parental quality clearly affects, through learning and heredity, the physical and social development of goslings, and consequently their chances of survival and reproduction. We suggest that these effects are reinforced by brood size; larger families gain better resources in competitive situations. In wild geese, competitive ability is crucial both to survival in winter and to the acquisition of nesting sites and rearing areas for the young. The paper describes a study of social rank acquisition in goslings reared from eggs taken from a full-winged flock of barnacle geese (Branta leucopsis) at the Wildfowl Trust, Slimbridge. Eggs were taken from pairs of known history and the adult's aggressiveness was ranked according to their reaction to humans. This rank was shown to be meaningful intraspecifically both by the outcome of encounters between geese and by the fact that no pairs scored as non-aggressive were able to nest in the preferred colony. A group of goslings reared by their own parents and cross-fostered goslings were also examined. 1. Within a rearing group of goslings (sibling-reared broods), the oldest and heaviest birds ranked highest in the first month and males performed better in encounters than females of the same size in the second month. 2. In encounters between unfamiliar goslings from different sibling-broods in the third month of life, the most important determinants of the new rank were body size, weight and sex. Previous experience also influenced rank; previous success yielded continued success. Goslings lost weight during the test; loss was negatively correlated with rank. The performance of goslings reared without adults bore no relationship to their parent's aggressive score. 3. In the semi-captive flock, parents that scored as "aggressive" reared more and larger goslings than non-aggressive pairs. The rank of these in the third month correlated with their size and sex (independent of size). The cross-fostering experiment suggested that there was a genetic as well as an environmental influence on rank acquisition. 4. In encounters between goslings of similar rank from sibling-broods and parent-reared ones, the latter ranked significantly higher. Parent-reared goslings, though less familiar to the experimental regime, gained weight and goslings from sibling-broods lost weight. 5. Once established, rank order remained stable; the few reversals related either to changes in size or to cooperation between goslings in confrontations. 6. Parental quality clearly affects, through learning and heredity, the physical and social development of goslings, and consequently their chances of survival and reproduction. We suggest that these effects are reinforced by brood size; larger families gain better resources in competitive situations. In wild geese, competitive ability is crucial both to survival in winter and to the acquisition of nesting sites and rearing areas for the young.