I studied the adaptive significance of communal nesting in Greater Anis (Crotophaga major) in the Panama Canal Zone from 2006–2009. The reproductive biology of anis is unique among birds: individuals breed in groups composed of behaviorally monogamous pairs, which build a single nest in which all of the females lay their eggs. Reproductive output is nearly equal among group members, and all contribute to parental care of the shared clutch. Prior to laying her first egg in the communal nest, each female ejects any eggs that have already been laid by fellow group members; after laying, each female ceases ejection. As a result, the first female to begin laying always loses her first egg, whereas the last female to begin laying usually loses none.

Here I show that members of breeding groups are not related to one another, so communal nesting does not appear to be favored by kin selection. Instead, social nesting greatly reduces the risk of nest predation and increases individual reproductive success. Genetic analyses of maternity revealed that females switched positions in the laying order across nesting attempts, so the number of eggs lost to ejection tended to equalize over time. First-laying females also decreased investment in their first-laid eggs—which were likely to be ejected—and increased investment in later-laid eggs. This differential allocation partly explains the tremendous variation in egg size observed in the Greater Ani population. Approximately 75% of nestlings were produced by behaviorally monogamous pairs, whereas ∼20% of nestlings were produced by extra-pair fertilizations within the same social group. Extra-pair fertilizations with individuals outside the social group were rare, accounting for only 4% of nestlings.

Conspecific brood parasitism, in which an extra-group female laid an egg in a communal nest but provided no parental care, was very common in the study population. However, parasitic eggs were typically laid after incubation had already begun at the host nest and were ejected by the hosts. Field experiments demonstrated that hosts are able to recognize and eject freshly-laid, asynchronous eggs because the appearance of the eggshell changes over time. Freshly laid eggs are coated with a white, chalky polymorph of calcium carbonate, which wears off during incubation to reveal the blue shell underneath.