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

The benefits of flocking to prey species, whether through collective vigilance, dilution of risk, or predator confusion, depend on flock members responding in a coordinated way to attack. We videotaped sparrowhawks attacking redshank flocks to determine if there were differences in the timing of escape flights between flock members and the factors that might affect any differences. Sparrowhawks are surprise short-chase predators, so variation in the time taken to take flight on attack is likely to be a good index of predation risk. Most birds in a flock flew within 0.25 s of the first bird flying, and all birds were flying within 0.7 s. Redshanks that were vigilant, that were closest to the approaching raptor, and that were close to their neighbors took flight earliest within a flock. Birds in larger flocks took longer, on average, to take flight, measured from the time that the first bird in the flock flew. Most birds took flight immediately after near neighbors took off, but later flying birds were more likely to fly immediately after more distant neighbors took flight. This result, along with the result that increased nearest neighbor distance increased flight delay, suggests that most redshanks flew in response to conspecifics flying. The results strongly suggest that there is significant individual variation in predation risk within flocks so that individuals within a flock will vary in benefits that they gain from flocking.