I investigated the roles of intraclutch variation in egg volume and hatching asynchrony in the establishment of mass hierarchies in broods of the Brown Pelican (Pelecanus occidentalis). The second egg averaged 2.6% larger than the first and 1.9% larger than the third. In unmanipulated control broods, first eggs hatched an average of 25 h before second eggs, which hatched 40 h before third eggs. On the day the last egg hatched, first chicks averaged 16% heavier than second chicks and 30% heavier than third chicks. Although chick mass at hatching was strongly correlated with egg volume, differences in mass within broods were almost entirely the result of hatching asynchrony. In broods in which I experimentally reduced hatching intervals, initial nestling mass differences were significantly smaller and strongly correlated with differences in egg volume but not with hatching asynchrony. Intrabrood mass differences remained lower in experimental than control broods throughout the first 2 weeks of the nestling period. Furthermore, mass ranks established at hatching were less likely to persist through this period in experimentally synchronized broods than in asynchronous controls. These results indicate that hatching asynchrony promotes the establishment of a more stable size hierarchy. However, the adaptive significance of nestling size hierarchies in Brown Pelicans remains unresolved.
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