Revista Brasileira de Ornitologia

Abstract

Sibling competition may reduce reproductive success as well as growth and future survival of the offspring involved. In birds, this may influence the evolution of reproductive strategies concerning how adults may control or limit such competition. For example, it is hypothesized that size hierarchy in the young may reduce sibling competition and result in less costly reproduction for the parents and increased survival of the young. This hierarchy may be generated by reduction the interval between egg laying and incubation beginning with the first egg and hatching will then follow the same order as egg laying (hatching asynchrony). Conversely, when incubation begins near the last day of egg laying, hatching may be more or less synchronous and all young are the same size (hatching synchrony). We tested these hypotheses in nests of the Rufous Hornero (Furnarius rufus) by experimentally manipulating recently hatched young to generate two treatment types: synchronous and asynchronous nest treatments. Young were moved from one nest to another as necessary to generate the two treatments, in which a weight differential > 20% was considered asynchronous and < 20% was considered synchronous. Young from asynchronous nests grew larger than young from synchronous nests. In asynchronous nests, when mortality occurred, the smallest individuals always died, while in synchronous nests, any size was equally likely to perish. Yet, surprisingly, mortality was lower in synchronous nests, contrary to prediction. Thus, some of the predictions of sibling competition were supported in this study, while others were not. Long term studies may better answer these questions, especially in systems such as this one, where wide annual variation in reproductive success may occur.

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