Inter‐group conflict in a cooperatively breeding bird: New insights into “home field advantage”

Conflicts between groups of animals have individual‐level fitness consequences that can influence the evolution of social behaviour. In the cooperatively breeding greater ani ( Crotophaga major ), groups occasionally destroy the eggs of their neighbours, causing the attacked group to abandon their nest. Prior research suggested that such conflicts occur when two groups build nests in close proximity, and that established groups tend to evict newcomers. However, inter‐group conflict had never been directly observed. Here, we report the first photographic evidence of egg destruction by greater anis. Twelve artificial nests containing clay eggs were placed in the field with the intent of attracting nest predators. Camera footage revealed that although only two nests were discovered by heterospecific predators (white‐faced capuchins, Cebus capucinus ), five nests were visited by resident anis, who in three cases damaged the clay eggs by grasping them with their bills and ejecting them from the nest cup. Nests were discovered between 2 and 96 hr after being placed in the field (mean = 34 hr) and were visited up to 18 times by up to 4 individuals simultaneously. The distance between artificial nests and the nearest known ani territory ranged from 0.13 to 0.28 km (mean = 0.2 km). Resident groups did not subsequently breed at the nest sites that they attacked, consistent with the hypothesis that the main benefit of inter‐group conflict is to reduce competition for local resources rather than to usurp nest sites. This accidental experiment reveals that greater anis closely monitor nesting activity near their territories, which may contribute to the strong “home field advantage” that resident groups hold over intruders.