The role of hatching asynchrony and sibling rivalry in pelican brood reduction

Evolutionary theory predicts that organisms should maximize their reproductive success, yet in some species parents appear to induce mortality in identifiable classes of offspring, resulting in brood reduction. Pelicans are a widely cited example of parentally induced intense competition among offspring that leads to obligate brood reduction, based largely on detailed studies of one species, the American white pelican (Pelecanus erythrorhynchos). I used the results of a field study of Australian pelicans (Pelecanus conspicillatus) and a literature review to test whether parentally induced conflict between offspring leads to brood reduction among pelicans generally. The field study showed that parentally induced hatching asynchrony and aggression between siblings were key determinants of brood reduction in Australian pelicans, which occurred in 49% of nests. The literature review showed that hatching asynchrony occurred in all pelican species. However, the incidence of brood reduction varied greatly, from 100% in some populations of brown pelican (Pelecanus occidentalis) to 0% in Dalmatian pelicans (Pelecanus crispus), demonstrating under-appreciated variation in the incidence of brood reduction among pelicans. This variation in the incidence of brood reduction corresponded to the incidence of sibling aggression, suggesting that, in some circumstances, pelican nestlings might be able to overcome parentally induced inequities attributable to hatching asynchrony.