Solicitation displays reliably reflect oxidative damage in barn swallow nestlings

Reproduction entails parental decisions over allocation of limiting resources. 'Begging' solicitation displays by the offspring have been hypothesised to evolve as reliable indicators of quality, allowing parents to adopt optimal allocation strategies. To function as such, however, begging signals must covary with offspring traits that affect viability. Here we tested whether oxidative damage of nestling barn swallows predicts begging bout duration, call amplitude and intensity of postural displays both under normal food provisioning and under food shortage. Oxidative damage as determined by normal metabolism, pathological conditions and functioning of the immune system can serve as a comprehensive indicator of offspring condition because it can profoundly affect viability, senescence and reproduction. Begging bout duration was negatively predicted by oxidative damage but only after food deprivation. Postural begging negatively covaried with oxidative damage, at least when nestlings were confronted with related nest mates. Plasma antioxidant capacity did not predict begging behaviour. Thus, we show for the first time that behavioural begging covaries with a comprehensive index of chick condition, lending support to reliability of such begging traits also as signals of quality rather than need or hunger only. Future studies are candidate to decouple the effect of hunger state and individual condition on behavioural begging.