Foraging in groups offers animals many benefits, such as increased efficiency of finding resources and increased vigilance against predators. However, group foraging also increases competition for resources and interference competition can lead to some group members being excluded altogether from resource exploitation. The costs of interference competition are expected to be unevenly distributed across group members, with subordinate individuals suffering more negative consequences than dominant individuals. Models suggest that individuals with unpredictable access to resource should develop strategies to manage uncertainty that may arise as a result of interference competition.
We assessed whether foraging rate was negatively impacted when we experimentally increased interference competition by reducing food availability in a field study with 132 individually marked black-capped chickadees (Poecile atricapillus). Further, we looked at whether individuals that are faced with higher uncertainty— notably females and juveniles, which in chickadees are subordinate to males and adults, respectively— manage unpredictable access to food by adjusting their foraging rate when interference competition is increased.
Results/Conclusions
Overall, experimentally reducing food availability decreased foraging rate in a population of free-living black-capped chickadees, which suggests that our manipulation was effective at manipulating interference competition. Further, we also found significant among-individual variation in how chickadees responded to increased competition. When we looked at factors that may affect individuals’ responses to increased competition, i.e., sex and age, we found that younger birds reduced their foraging rate significantly more compared to older birds when faced with interference competition. Surprisingly, sex did not significantly affect foraging rate in response to changing competition. This suggests that younger birds may not be able to manage uncertainty as well as older birds, because if subordinate individuals were affected by increased competition in the same way, we would have expected that females would also have lower foraging rate compared to males under increased interference competition. Our study demonstrates that manipulating food availability can reduce foraging rates through increased interference competition, and that while birds respond consistently different to changing competition intensity, age may play an important role in offsetting the costs of subordination in a group foraging organism.
