Flocking Preferences in Relation to Vigilance Benefits and Aggression Costs in Mixed-Species Shorebird Flocks

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Abstract
Wintering turnstones Arenaria interpres have previously been shown to gain antipredator vigilance benefits when foraging from flocking with certain other wader species (purple sandpiper Calidris maritima, oystercatchers Haematopus ostralegus and redshanks Tringa totanus) as well as conspecifics. They also experience less competition and aggression when flocking with these other species than when associating with conspecifics. Cost-benefit models therefore predict that they should prefer mixed-species to single-species flocks. This was examined both by measuring the species compositions of flocks in which foraging turnstones were found and by recording the response of turnstones to natural and manipulated reductions in the density of these other species. Turnstones showed a decreasing tendency to flock together, and an increasing tendency to associate with other species, as the area available for foraging increased with the dropping tide. A decrease in the abundance of oystercatchers and redshanks in the spring (due to their earlier migration to breeding areas) led to greater clumping by turnstones, so that flock densities around individual turnstones were maintained. In contrast, purple sandpipers, which do not gain vigilance benefits from oystercatchers and redshanks, showed no tendency to preferentially associate with them and did not change their own dispersion when these left. Turnstones also formed denser flocks in response to short-term experimental reductions in the abundance of other species. The results indicate that turnstones adjust their own dispersion in relation to the abundance of certain other species.