title: The flocking behaviour of wintering turnstones Arenaria interpres and purple sandpipers Calidris maritima
Creators: name: Metcalfe, Neil Benedict
Publisher: name: University of Glasgow
Year: 1984
Subjects: subject: Ecology.

description: The aims of this project were to investigate the extent of social organisation, and the adaptive significance of mixed-species flocking, in two species of waders wintering on rocky shores. The approach adopted was to study firstly the stability of the populations, through monitoring of the movements of marked individuals and the population as a whole. This was followed by more detailed analyses of the associations between individual birds. Several major costs and benefits of flocking were then investigated, with emphasis on how these varied according to the composition and density of the flock, the identity of the individual, the environment and the time of year. The observed flocking dynamics of the two species were then compared to those predicted on the basis of the cost/benefit analyses of flocking, and the mechanisms of these flocking tendencies investigated. The results showed that turnstones Arenaria interpres and purple sandpipers Calidris maritima formed very stable communities, with very limited population turnover during the course of a winter, high survival rates and a high degree of site faithfulness by individual birds between years. Analyses of turnstone movements revealed that birds maintained small home ranges, and so came in contact with only a limited number of other conspecifics. This enabled the formation of a dominance hierarchy, presumably based on individual recognition; dominant turnstones stole food from more subordinate birds whenever possible, leading to high rates of aggression in high density flocks and avoidance of dominants by subordinates. Aggression and interference in feeding constituted a cost of flocking to at least the majority of birds of both species, especially at high densities of conspecifics; major benefits of flocking were concerned with vigilance and the avoidance of predation. Birds gained from increased corporate vigilance at a reduced time cost to the individual when feeding in flocks; the extent of this gain was, however, affected by the species composition and density of the flock, the visibility of the habitat, and the bird's food intake requirement. Thus the optimal flock composition and density was predicted to vary according to the individual's species and dominance status, the risk of kleptoparasitism (which varied between habitats) and the visibility of the habitat. Observed patterns of flocking generally corresponded well with these predictions: birds maintained intermediate flock densities (which were modified by habitat visibility), and reduced their rates of interference with conspecifics by flocking preferentially with those other species with which they shared vigilance. Removal of these other species led to greater aggregation by conspecifics; these aggregations were maintained by birds modifying their foraging search paths so as to remain within flocks, especially when the selective pressures for the maintenance of high conspecific densities were predicted to be greatest. However, there was evidence for variation in flocking behaviour between birds of different dominance status within this general pattern. These findings are discussed in relation to how such details of flocking and social behaviour could affect other aspects of an animal's biology, with particular reference to their implications for the movements and survival of individuals, and hence the population dynamics of the species.