Vigilance levels in preening Dunlin Calidris alpina.
Individual birds can reduce vigilance for predators by forming flocks (Barnard & Thompson 1985). Flocking allows an increase in overall vigilance thus enabling individuals to devote more time to other functions (Abramson 1979, Thompson & Barnard 1983, Metcalfe 1984). This is only valid, however, when an individual can perceive that one of its companions has detected a predator (Metcalfe 1984).
A study of feeding flocks of Dunlins Calidris alpina showed that individuals regularly ceased feeding to preen. Observations suggested that preening individuals had decreased vision, caused by feathers covering the eyes (Redpath unpubl.), whereas feeding birds did not have obstructed vision. Metcalfe (1984) found that foraging waders increased their vigilance when their visual field was obstructed by rocks. This study examines the effects of the apparent decrease in vision caused by preening on vigilance in Dunlin.