Begging and digestive responses to differences in long-term and short-term need in nestling pied flycatchers

Offspring begging provides parents with an honest signal of short-term nutritional need (i.e. hunger). However, offspring that have experienced contrasting levels of long-term food intake may beg differently for a given level of short-term need, perhaps as a result of developmental differences in behaviour and/or physiology. We tested for the effects of both short- and long-term need on begging and digestion by manipulating food intake rates of the junior and senior nestlings from broods of pied flycatchers, Ficedula hypoleuca. Artificial food intake at natural levels created stable and normal levels of begging in both senior and junior nestlings, over 10 h of hand-feeding individuals in isolation. In both classes of nestling, higher than natural levels of food intake led to similar progressive linear declines in begging effort, while lower than natural levels of food intake led to similar progressive increases in begging. However, there were no corresponding changes in digestive efficiency (assimilable mass coefficient). Consistent with previous studies, junior nestlings begged at consistently higher rates throughout compared with senior nestlings. There was also evidence that digestion by junior nestlings was slightly more efficient, but individual variation in begging effort (and change in begging effort) did not correspond to differences in digestive efficiency. These results show that begging does not reflect digestive efficiency across a range of natural food intake rates, and suggest that the honesty of offspring solicitation signals, such as the differences in begging between junior and senior nestlings, are not maintained by contrasting digestive development or function.