Food limitation in asynchronous bluethroat broods: effects on food distribution, nestling begging, and parental provisioning rules

摘要

Scramble competition models of begging predict that junior nestlings will be more affected by food limitation than seniors. These models assume that food allocation is under offspring control and, hence, predict that this change in food distribution is caused by a differential behavioral response by seniors and juniors. By using the bluethroat (Luscinia svecica svecica) as our model species, we induced food limitation by removing the male parent temporarily. We found that, as predicted, food distribution became more biased in disfavor of juniors when food was limited. However, there was no significant difference in the behavioral responses of seniors and juniors (i.e., positioning in the nest or begging postures) to food limitation that could explain the change in food distribution. Hence, there was no evidence that seniors controlled food distribution. As predicted if parents preferentially fed seniors, nestling rank affected food distribution when controlling for variation in nestling behaviors. Furthermore, as expected if the increased skew in food distribution under food limitation was caused by active food allocation by parents, nestling rank had a greater effect on food distribution under food limitation than under normal conditions. The present study suggests that food distribution in passerine birds is determined not only by nestling behaviors (begging posture and positioning) alone but also by parental preferences for seniors based on nonsignaling cues, such as body size.