Senior and junior nestlings in asynchronous bluethroat broods differ in their effectiveness of begging

In accordance with theoretical models of begging as an honest signal, empirical studies have found that begging by nestlings reflects need and that parent birds respond to variation in begging activity. However, parent birds hatch their broods asynchronously and the resulting within-brood size-asymmetries might induce differences in the begging tactics of senior and junior nestlings. In the present study, we tested for long-term effects of initial variation in size-asymmetry on food allocation, visual begging and physical competition among siblings by experimentally generating asynchronous and synchronous bluethroat (Luscinia s. svecica) broods. Seniors were fed more often than juniors. However, there were no differences in the prefeeding begging behaviour of seniors and juniors. Thus, seniors were more effective at begging (i.e. had higher returns for a given level of begging) than juniors. There was no evidence that the higher effectiveness of seniors was due to juniors being excluded from the most favourable positions in the nest. Seniors in asynchronous broods might have been more effective at begging because they controlled food distribution through other behaviours than those analysed by us, or because parents preferentially fed seniors. Finally, we found that juniors begged for longer than seniors in asynchronous broods. This suggests that juniors compensated for their less effective begging by increasing their post-feeding begging effort. Our results provide evidence that marked initial size-asymmetries lead to a divergence in the begging behaviour of seniors and juniors.

Per T. Smiseth and Trond Amundsen, Evolutionary Ecology Research, 2002, 4: 1177-1189