Intraflock variation in the speed of escape-flight response on attack by an avian predator

The benefits of flocking to prey species, whether through collectivevigilance,dilution of risk, or predator confusion, depend on flock membersrespondingin a coordinated way to attack. We videotaped sparrowhawksattackingredshank flocks to determine if there were differences in thetimingof escape flights between flock members and the factorsthat might affect anydifferences. Sparrowhawks are surpriseshort-chase predators, so variation inthe time taken to takeflight on attack is likely to be a good index ofpredation risk.Most birds in a flock flew within 0.25 s of the first birdflying,and all birds were flying within 0.7 s. Redshanks that werevigilant,that were closest to the approaching raptor, and thatwere close to theirneighbors took flight earliest within aflock. Birds in larger flocks tooklonger, on average, to takeflight, measured from the time that the first birdin the flockflew. Most birds took flight immediately after near neighborstookoff, but later flying birds were more likely to fly immediatelyaftermore distant neighbors took flight. This result, alongwith the result thatincreased nearest neighbor distance increasedflight delay, suggests that mostredshanks flew in responseto conspecifics flying. The results stronglysuggest that thereis significant individual variation in predation riskwithinflocks so that individuals within a flock will vary in benefitsthatthey gain from flocking.