The effects of mixed-species flocking on the vigilance of shorebirds: Who do they trust?

Abstract

The benefit of reduction in individual anti-predatory vigilance with increasing group size has previously been demonstrated only within single species. Here the effect of mixed species flocking on vigilance is investigated in two species of wading bird wintering on rocky shores. Both turnstones, Arenaria interpres, and purple sandpipers, Calidris maritima, ‘share’ vigilance with conspecifics, but also with some other waders; the extent of sharing appears to depend on the relative size of, and habitat overlap with, the other species. Vigilance is not shared with much larger species, nor with those rarely found in the same areas. There is no sharing of vigilance between neighbouring birds unable to see each other. Variation in vigilance level results from an alteration in both rate of scanning and duration of single scans; these two variables are adjusted independently over the range of densities experienced. This is due to constraints on a high scanning rate (which may reduce feeding efficiency) and a short scan duration (which cannot be lower than the minimum required to take in the necessary information).