AbstractAs a consequence of the anti-predator functions of flocking it is predicted that as flock size increases individual birds will (i) spend less time vigilant for predators and more time feeding, and (2) show fewer escape responses. These predictions are corroborated here in experiments with captive flocks of the red-billed weaverbird Quelea quelea varying in size from I to 32 individuals. In Experiment i the presence of companions reduced feeding latency and increased the rate of pecking. Feeding behaviour by the companion(s) was not necessary to produce these effects and did not produce additional facilitation of the rate of pecking, but feeding companions were more potent facilitators of the onset of pecking than non-feeding companions. The number of companions influenced different measures of feeding in different ways but facilitation reached a ceiling with three companions. These and other findings in the social facilitation literature are interpreted in the light of various propositions concerning the functions of flocking behaviour. In Experiment 2 undisturbed birds with companions made less frequent head-turns (possibly vigilant responses), wing-flicks (flight intention movements), hops and flights than birds on their own but the number of companions had no effect on the frequency of these responses. All responses correlated significantly and positively across individuals. Solitary individuals were more often perched, and less often on the ground, than those in flocks, a finding compatible with greater safety in flocks in the normal feeding habitat of the species. These (undisturbed) birds, in flocks of 1, 2 and 4, had lower rates of head-turning than those in another experiment (LAZARUS, 1979), which had been subjected to 10 alarm stimuli; in larger flocks there was no difference in the rate of head-turning. An alternative interpretation of the experimental results is evaluated and a possible reason for the weak effect of flock size considered.