Reproductive success in relation to egg size was studied in European Starlings (Sturnus vulgaris) by swapping whole clutches between nests at the stare of the incubation period. Egg size did not reflect parental quality as no measure of reproductive success was correlated with the foster mothers' mean egg size. There was a significant positive relationship between the mean size of the cross—fostered egg and the subsequent mean size of hatchlings. The mean size of cross—fostered eggs did not affect hatching success or nestling growth rates, and initial nestling size differences between broods with large and small eggs persisted for <1 wk. No effect of mean egg size on mean nestling survival could be detected. Furthermore, a partial cross—fostering experiment, where nestlings were swapped between nests the day after hatching, failed to demonstrate any lasting effect of egg size on nestling size. It is suggested that mean egg size may only influence reproductive success during particularly inferior environmental conditions.
In altricial birds, growth rates and nestling morphology vary between broods. For natural selection to produce evolutionary change in these variables, ther must exist heritable variation. Since nestling traits are not any longer presents in parents, traditional offspring-parent regressions cannot estimate heritabilities of these. In this study, a partial cross-fostering experiment was performed, where nestlings of the European Starling (Sturnus vulgaris) were reciprocally exchanged between nests. The experiment demonstrated a significant heritability of nestling tarsus length and body mass, but not of the growth trajectories followed by individual nestlings. the heritability estimate for tarsus length obtained in the cross-fostering experiment using full-sib analysis was lower than those obtained by offspring-parent regressions. This is likely due to a genotype-by-environment effect on tarsus length, with nestlings destined to become large but in poor condition having a low probability of appearing as parents. The main reason for the low heritability of growth was probably the large within-brood variation in growth pattern due to the initial size hierarchy of nestlings. Nestlings demonstrated targeted growth, where small-sized nestlings that initially grew slower than their siblings, managed to catch up.
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