THE EFFECT OF GROUP-SIZE ON VIGILANCE AND FEEDING RATE IN SPICE FINCHES (LONCHURA-PUNCTULATA)

We investigated group-size effects on individual vigilance levels and feeding rates in captive groups of spice finches (Lonchura punctulata) . The ''many-eyes'' hypothesis predicts a reduction in vigilance level s in larger groups that can cause higher feeding rates if the time thu s saved is used for foraging. We adopted a within-subject procedure wh ereby in each of six replicates we observed one adult male searching f or food in groups varying in size from one to five. As predicted, vigi lance levels decreased with group size up to four birds but increased in groups of five. Birds also collected seeds more quickly in larger g roups, reflecting (i) a decrease in vigilance levels in larger groups, (ii) a decrease in handling time, and (iii) an increase in search spe ed. Nearest neighbour distances were smaller in larger groups, a poten tial confounding factor in the analysis of vigilance time. Although th e results are partly consistent with the many-eyes hypothesis, we sugg est that the arrow of causation from vigilance to feeding time could b e reversed: the increase in feeding rates in larger groups, with a con comitant decrease in vigilance levels, would be caused by increased co mpetition.