Hatching asynchrony results in age and size hierarchies within broods, and the subsequent asymmetric competition among siblings has important consequences for nestlings' fitness. In this study, we compare the growth of Barn Swallow (Hirundo rustica) nestlings in relation to their order of hatching. The aim was to test the prediction that early-hatched nestlings develop differently from late-hatched nestlings, which should be under greater pressure to trade investment in growth in favor of traits important to simultaneous fledging. Early-hatched nestlings were always larger than late-hatched nestlings, but when the age difference was taken into account, the two classes of nestling gained mass and head-bill length in similar ways, including having similar asymptotes, as predicted by nonlinear curve models. For wing length, however, late-hatched nestlings reached the inflection point of growth sooner than early-hatched nestlings, and although scaled rates of wing growth were similar, early-hatched nestlings had significantly longer wings, both before fledging, when the oldest nestling was 14 days old, and as suggested by the asymptotic, age-independent values derived from the nonlinear curve models. This finding suggests that nestlings hatched later preferentially develop body mass and the skeleton at the expense of wing feathers. As swallows rely on their wings for foraging and avoiding predators, this pattern of resource allocation is likely to have negative consequences for the late-hatched nestlings.
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Vol. 111 • No. 4