Predation, scramble competition, and the vigilance group size effect in dark-eyed juncos (Junco hyemalis)

In socially feeding birds and mammals, as group size increases, individuals devote less time to scanning their environment and more time to feeding. T his vigilance "group size effect" has long been attributed to the anti-pred atory benefits of group living, but many investigators have suggested that this effect may be driven by scramble competition far limited food. We addr essed this issue of causation by focusing on the way in which the scan dura tions of free-living dark-eyed juncos (Junco hyemalis) decrease with group size. We were particularly interested in vigilance scanning:concomitant wit h the handling of food items, since a decrease in food handling times (i.e, scan durations) with increasing group size could theoretically be driven b y scramble competition for limited food resources. However, we showed that food-handling scan durations decrease with group size in an environment wit h an effectively unlimited food supply. Furthermore, this food-handling eff ect was qualitatively similar to that observed in the duration of standard vigilance scans (scanning exclusive of food ingestion), and both responded to changes in the risk of predation (proximity of a refuge) as one might ex pect based upon anti-predator considerations. The group size effects in bot h food-handling and standard scan durations may reflect a lesser need for p ersonal information about risk as group size increases. Scramble competitio n may influence vigilance in some circumstances, but demonstrating an effec t of competition beyond that of predation may prove challenging.