How many birds does it take to put a flock to flight?

Flights from foraging flocks of sanderlings, Calidris alba, were observed at Redcar, northeast England, in order to investigate how individual decisions to fly related to the behaviour of other flock members. Flights of sanderlings tended to be either of all birds in the flock on the ground, or of single birds or groups representing only a small proportion of the foraging flock. The latter accounted for the majority of movement events but after weighting by flock size the former accounted for the majority of bird movements. As flock size increased, the number of birds that flew without the whole flock taking flight increased. Thus, as flock size increased, it took more individual departures before the whole flock took flight. When flocks were disturbed by people or dogs, they were more cohesive, with more departures being of whole flocks. (C) 1997 The Association for the Study of Animal Behaviour.