Competition and waste in the communally breeding smooth-billed ani: effects of group size on egg-laying behaviour

We investigated the effects of group size on egg-laying behaviour in the communally breeding, joint-nesting smooth-billed ani, Crotophaga ani. We tested the predictions of the competitive female egg-investment hypothesis, which states that females in plurally breeding, joint-nesting systems respond to increased group size and egg-laying competition by trying to skew the contents of the final incubated clutch of eggs in their own favour by tossing and/or burying competitors' eggs and by producing more of their own eggs. Results supported the predictions of the hypothesis as both the number of eggs produced per capita and the number of eggs lost per capita increased with increasing group size. Egg tossing and burial behaviours were almost entirely restricted to multifemale groups and 56% of the 829 eggs laid in communal nests were lost to either egg tossing or burial. As a consequence of this egg-laying competition, the number of eggs incubated per capita decreased with increasing group size. Large groups laid more eggs and took more time to synchronize laying than smaller groups. Finally, we found that chicks hatched late within a communal clutch were more likely than earlier hatching chicks to die during the first 5 days of life. We conclude that communal life in anis generates competition and egg production waste that reduces short-term per capita reproductive benefits. Long-term data are needed to clarify individual benefits associated with communal breeding in this species.