Egg ejection risk and hatching asynchrony predict egg mass in a communally breeding cuckoo, the Greater Ani (Crotophaga major)

The Greater Ani (Crotophaga major) is a neotropical cuckoo in which several females lay eggs in a single nest. Group members synchronize egg laying and compete for reproduction by ejecting early-laid eggs from the communal nest. Eggs are large (∼17% of female body mass) and vary greatly in size. I assessed the effects of egg size, hatching asynchrony, and position in the laying order on the survival and growth of nestlings to test the hypothesis that females invest more resources in eggs that are likely to give rise to surviving offspring. The egg's position in the female's laying sequence was a significant predictor of egg mass, with first-laid eggs and last-laid eggs consistently smaller than those in the middle of the clutch. Females that initiated laying in the communal nest almost always lost their first-laid eggs, and these females also exhibited the most extreme variation in egg mass. Nestlings from last-laid eggs were more likely to hatch asynchronously and starve before fledging, and the mass of last-laid eggs decreased as the degree of hatching asynchrony increased. Ejection and starvation probabilities were not affected by egg mass; therefore, low survivorship of first-laid and last-laid eggs was due solely to their position in the laying order, not to their smaller size. These data suggest that individual females allocate more resources to eggs that are likely to survive to fledging and that reproductive competition among communally breeding females explains much of the variation in egg size in this species.