Abstract

Animals that feed socially may benefit from the presence of others without necessarily improving their feeding rates. Feeding rates may actually decrease in the presence of others due to different forms of competition and interference (kleptoparasitism, aggression). Recent work with starlings ( Sturnus vulgaris ) shows that social foragers may also experience self-imposed costs to their feeding rates in order to remain in the presence of group members, despite a lack of physical interaction or overt aggression. I studied whether nutmeg mannikins ( Lonchura punctulata ) feeding from depletable patches alone or with a heterospecific or conspecific flock in a separate cage adjacent to the food patch experience such self-imposed feeding costs. The birds experienced self-imposed changes to their foraging behaviour which led to decreased feeding rates. Subjects fed more slowly in treatments with conspecifics despite travelling more quickly between patches. The decrease occurred because they spent more time idle near the flock instead of feeding from the patch or immediately travelling to the next one. This seems to be a cost related to the maintenance of group cohesion. The birds also reduced their scanning time and rate near conspecifics without experiencing a concomitant increase in feeding rate consistent with the group-size effect. This result may point to a form of interference usually attributed to reduced quality of or access to the food resource caused by group members.