Parent-absent begging in barn swallow broods : causes of individual variation and effects on sibling interactions and food allocation

Altricial offspring solicit parental care using complex begging displays. Although such solicitations are mainly performed towards parents, nestlings of several birds often beg when parents are not at the nest. This parent-absent begging (PAB) has been interpreted as a mistake in correctly detecting parental arrival, but it may also reliably signal individual need to nest mates, potentially affecting intra-brood competition for food. Here, we focused on the proximate factors that may determine frequency and intensity of PAB, including brood size and sex ratio, as well as individual age, sex and body mass, in natural barn swallow (Hirundo rustica) broods, under conditions of normal and experimentally reduced food intake. We also investigated the relationship between PAB and begging intensity at parental arrival and food intake in the two different experimental conditions. The frequency of PAB was larger after food deprivation than before, indicating that PAB reliably reflects hunger level. In addition, nestlings increased their own begging intensity upon parental arrival after performing PAB. Concomitantly, siblings decreased begging effort, irrespective of sex or body mass, but this occurred only when nestlings were normally fed, suggesting that PAB diminishes scrambling for food by nest mates. Finally, such a reciprocal begging modulation resulted in a larger chance of obtaining food at the subsequent feeding event for nestlings performing PAB under normal food provisioning. Within-brood signalling in the absence of parents can therefore play an important role in determining competitive strategies among siblings and affecting parental decisions on food allocation in altricial broods.