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 levels in larger groups that can cause higher feeding rates if the time thus saved is used for foraging. We adopted a within-subject procedure whereby in each of six replicates we observed one adult male searching for food in groups varying in size from one to five. As predicted, vigilance levels decreased with group size up to four birds but increased in groups of five. Birds also collected seeds more quickly in larger groups, reflecting (i) a decrease in vigilance levels in larger groups, (ii) a decrease in handling time, and (iii) an increase in search speed. Nearest neighbour distances were smaller in larger groups, a potential confounding factor in the analysis of vigilance time. Although the results are partly consistent with the many-eyes hypothesis, we suggest that the arrow of causation from vigilance to feeding time could be reversed: the increase in feeding rates in larger groups, with a concomitant decrease in vigilance levels, would be caused by increased competition.