HERITABILITY OF NESTLING GROWTH IN CROSS-FOSTERED EUROPEAN STARLINGS STURNUS-VULGARIS

In altricial birds, growth rates and nestling morphology vary between broods. For natural selection to produce evolutionary change in these variables, there must exist heritable variation. Since nestling traits are not any longer present in parents, traditional offspring-parent r egressions cannot estimate heritabilities of these. In this study, a p artial cross-fostering experiment was performed, where nestlings of th e European Starling (Sturnus vulgaris) were reciprocally exchanged bet ween nests. The experiment demonstrated a significant heritability of nestling tarsus length and body mass, but not of the growth trajectori es followed by individual nestlings. The heritability estimate for tar sus length obtained in the cross-fostering experiment using full-sib a nalysis was lower than those obtained by offspring-parent regressions. This is likely due to a genotype-by-environment effect on tarsus leng th, with nestlings destined to become large but in poor condition havi ng a low probability of appearing as parents. The main reason for the low heritability of growth was probably the large within-brood variati on in growth pattern due to the initial size hierarchy of nestlings. N estlings demonstrated targeted growth, where small-sized nestlings tha t initially grew slower than their siblings, managed to catch up.