The role of feeding regularity and nestling digestive efficiency in parent-offspring communication: An experimental test

Current theory suggests that by responding to offspring food solicitation, or begging, parents improve the efficiency by which they convert parental investment into offspring fitness. However, the proximate mechanisms of this conversion are not entirely clear. One potential function of responding to begging is to maintain feeding regularity. Feeding at regular time intervals may improve offspring fitness through increasing digestive efficiency, securing food receptivity, and reducing excessive begging displays. To examine the adaptive value of parental responsiveness to begging, we simulated either responsive or non-responsive mutant parents while hand-raising nestling house sparrows (Passer domesticus). In a previous study we tested parental responsiveness per se, without changing feeding regularity. Here, we tested the impact of the very likely possibility that non-responsive parents also cause greater variability in the intervals between visits and feedings than do responsive parents, by experimentally scheduling either extremely variable or regular time intervals between visits and/or feedings in a two-way design. Our results show that nestling growth, digestive efficiency and begging intensity were not affected by the level of feeding or visit regularity. However, within the regular feeding treatments (but not within the variable feeding treatments), digestive efficiency was positively correlated with nestling begging levels, and negatively correlated with how persistent the experimenter had to be to induce chicks to accept food. These results suggest that nestlings are quite resilient to variable feeding schedules and that parents have therefore little to gain from regular feeding as long as they provide an adequate daily amount of food. Nestlings that are regularly fed even when they are satiated, however, may exhibit some reduction in digestive efficiency. These data imply that digestive efficiency decreases only when the digestive system is very close to being full. Thus, when mediated by a reduction in begging displays near satiation, increased digestive efficiency imparts a previously unappreciated physiological benefit to signalling offspring and their responsive parents.