Intrinsic and extrinsic factors modulating vigilance and foraging in two gregarious foragers

A continuous balance between costs and benefits dictates individual vigilance and foraging dynamics. In group-living animals, understanding the resulting trade-off is often complicated by multiple confounding effects. Vigilance and foraging levels may be the result of intrinsic (e.g., body size, trophic ecology, migratory phenology) and extrinsic (e.g., flock size, edge effect, group dynamism) factors, potentially differing between species, individuals, and contexts. We explored this idea by investigating intrinsic and extrinsic factors influencing vigilance and foraging behavior of two sympatric gregarious bird species that differ markedly in body size and foraging strategies (Greylag Goose Anser anser and Common Crane Grus grus), during their non-breeding period. Interspecific differences were detected in activity allocation and in response to group-related variables. For both species, time spent in vigilance decreased with increasing flock size and with increasing distance from the edge of the group. While cranes allocated the resulting time to foraging, the same did not occur in geese. Changes in individual position in the group (i.e., peripheral vs. central or vice versa) elicited a prompt behavioral change (i.e., vigilance vs. foraging or other activity). Temporal changes in activity budgets were reported for geese but not for cranes, with a decrease of vigilance and an increase of foraging as winter progressed. Results allowed to disentangle the role of multifactorial determinants of vigilance and foraging, in turn increasing our understanding of underlying forces driving the evolution of behavioral traits and of group-living. Individuals need to trade-off vigilance and foraging, but species differ in how they manage this. We show that vigilance in individuals of two gregarious bird species differing in body size and diet decreased with increasing group size and distance from the edge of the group. However, the two species varied in how they allocated the freed-up time. These results increase our understanding of forces driving the evolution of behavior in group-living animals.