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Insurance value of "extra" eggs is thought to influence clutch size in avian species in which offspring loss invariably results in fewer young being raised than the number of eggs laid per clutch. Mock and Parker were the first to suggest formally that marginal offspring may also serve an insurance function in species with less frequent offspring loss. By their mathematical definition, marginal offspring represent insurance reproductive value (RVi) when they survive following the death of a sibling. We suggest that RVi includes two categories of offspring: (1) replacements for failed senior offspring (substitute offspring, RVs), and (2) extra survivors of random mortality (lottery offspring, RVl). These categories differ in whether survival of the marginal offspring is conditional on the death of an older sibling: substitute offspring survive only if a senior sibling dies, while lottery offspring survival is independent of sibling mortality. The relative importance of these two categories of marginal offspring varies with the frequency of brood reduction.
Oikos © 1996 Nordic Society Oikos