In many organisms, siblings compete actively for limited parental resources, often with fatal results for some brood members. Sometimes this rivalry is accentuated by overt aggression and the establishment of dominance hierarchies. The evolution of self-promoting behavior that sacrifices close kin is believed to be constrained by two main factors: (i) the genetic relatedness between survivor and victim and (ii) the energetic costs associated with vigorous competitive begging and/or fighting. In this study, these energetic costs will be measured in three-chick broods of cattle egrets (Bubulcus ibis), an abundant colonial bird that practices siblicide, using "doubly labelled water" (a mixture of stable isotopes). The behavior of chicks injected with small amounts of this compound will be monitored closely during all waking hours over a two-day period, after which analysis of a tiny blood sample will reveal the total energy spent. This method will be applied to chicks of various activity levels, so that the unit costs of each focal activity (begging and fighting) can be determined. This exercise will allow testing and modification of current intra-family conflict theory, as it directly addresses the premise that begging and fighting are expensive. It will also provide information about the efficiency of parental care, by showing how much of parentally supplied food energy is used for growth, as opposed to self-protection. On a more practical level, this knowledge, and the approach itself, is likely to be applicable to various rare and endangered species (e.g., whooping cranes, various eagles, brown pelicans) whose nestlings compete fatally.
