How Large Cooperative Groups Avoid Local Competition

Large cooperative groups are a common sight in nature. Their existence is puzzling, however, because local competition should keep groups relatively small. A simple but untested way large groups can avoid local competition is by increasing their resource base. We conducted a systematic review and phylogenetic meta-analysis to look for evidence of this effect in wild populations of cooperatively breeding birds. Across 634 groups from 29 species, group size and resource availability (territory area) were strongly positively correlated (Pearson’s r = 0.52). Furthermore, when a specific group changed size, its territory changed size correspondingly (N = 34 groups). Our results support the prediction that large groups can avoid local competition by increasing their resource base and explain a sizeable fraction of group size variation in nature (R2 = 0.27).