The formation and function of mixed-species wader flocks in fields
The formation and function of mixed-species wader flocks in fields in Aberdeenshire were studied during 1976-79. Four species - oystercatchers, lapwings, redshank and golden plover - were common in fields in winter, and fed frequently in mixed flocks. Flocks of waders congregated in fields with high prey densities and experiments with model lapwings and oystercatchers showed that the four species were attracted to fields where other waders were feeding. These findings suggest that mixed flocking in waders facilitates the location of favourable feeding areas. The ingestion rates of oystercatchers, lapwings, redshank and golden plover were measured to test the hypothesis that, on the feeding grounds, members of wader flocks interact with each other in some beneficial way to increase their feeding efficiency. With one exception, the ingestion rates of waders were unaffected by changes in flock size, density and composition. Lapwings in conspecific groups of 2 - 10 fed at faster rates than those in groups of other sizes, but the reasons for this were not identified. These results show that waders did not increase their feeding efficiency by foraging in flocks. The dispersion patterns of the four species and the types of interference that waders incurred by feeding in flocks were examined. Differences in dispersion patterns between the species were not correlated with methods of hunting. Aggressive interactions between conspecifics occurred frequently in dense flocks, but interspecific encounters were rare. The plovers were parasitised heavily by black-headed and common gulls during the day, but not at night, when feeding also took place. Ways in which plovers could minimise the risk of sustaining gull attacks were investigated. The time that waders devoted to being alert did not change with flock size and composition, but individuals probably gained some protection from predators by feeding in mixed flocks.