Home field advantage, not group size, predicts outcomes of intergroup conflicts in a social bird

Research on cooperatively breeding birds usually focuses on social dynamics within the breeding group, but conflict between groups can also affect individual fitness and the evolution of sociality. Here we investigate the causes and consequences of competition between groups of communally breeding greater anis, Crotophaga major, over a 10-year field study. Social groups were spatially clustered into loose aggregations that showed a moderate degree of reproductive synchrony. However, competition between neighbouring groups for nesting sites was intense, occasionally leading to wholesale destruction of a group's nesting attempt and abandonment of the site. We documented 18 cases in which a group's entire clutch of eggs was ejected from the nest during the laying or incubation period, often accompanied by behavioural observations of conflict with a neighbouring group. Clutch destruction typically occurred when two groups attempted to nest in close proximity on high-quality sites: nearest-neighbour distance and nest site type were the strongest predictors of clutch destruction. Surprisingly, group size did not predict whether or not a group's clutch would be destroyed, and small groups sometimes ousted larger groups. By contrast, ‘home field advantage’ did have a significant effect: groups that had previously nested on the site were more likely to destroy the clutches of newly established groups, and this effect increased with the number of years that the group had nested there. Together, these results support previous evidence that competition between groups for high-quality nesting sites is an important driver of communal breeding, and they highlight the importance of location and past history in determining the outcome of intergroup contests in social species.