Workload inequality increases with group size in a cooperatively breeding bird

Groups of animals often cooperate on shared tasks, but individual contributions are not necessarily equal: individuals can specialize on different tasks (division of labour) or vary in total work performed. Either of these patterns can have fitness consequences. However, few studies have examined division of labour and workload inequality in vertebrates. We analysed individual contributions to parental care in the greater ani, Crotophaga major, a bird that nests communally in groups of two or three pairs and up to two nonbreeding helpers. We examined nearly 600 h of video footage of 10 groups to quantify individual contributions to nest building, incubation, brooding and nestling feeding. There was no clear evidence of division of labour; rather, contributions across some behaviours were positively correlated, suggesting individual differences in overall work performed. Indeed, helpers and one of the breeding pairs in three-pair groups contributed significantly less than the other two pairs. Inequality in overall workload, as measured by the Gini coefficient, was significantly higher in three-pair groups than in two-pair groups, as predicted by collective action theory. However, there were no significant differences in inequality of specific behaviours between two- and three-pair groups, possibly due to low sample size. Finally, anecdotal observations of aggression directed at the pair that contributed the least to parental care in three-pair groups suggested that the lowest-contributing pair's access to the nest might be limited by such aggression, which could be driven by conflicts of interest over group size and membership.