The energetic cost of begging in nestling passerines

A critical assumption of many of the current discussions of parent-offspring conflict and sibling competition is that the begging behavior of nestling birds is costly, either in terms of energetics or increased risk of predation. I measured the energetic expenditures associated with the begging of nestling birds using closed-chamber respirometry and found this cost to be surprisingly low. Active metabolic rate (AMR) while begging was 1.05 times the resting metabolic rate (RMR) in European Starlings (Sturnus vulgaris) and 1.27 times the resting metabolic rate in Tree Swallows (Tachycineta bicolor). The cost of 1 s of begging was 0.001 J/g in European Starlings and 0.008 J/g in Tree Swallows; this cost increased with age for all nestlings. Results of measurements on five other species are consistent with these values. The ratio of AMR:RMR did not change with ambient temperature for either Tree Swallows or European Starlings, but data for all seven species pooled did show a significant decrease in energetic costs with temperature. The amount of time spent begging had a negligible effect on cost for both Tree Swallows and European Starlings. Likewise, the intensity of the begging display had no effect on the AMR:RMR ratio in either species alone, but was positively correlated for the seven species pooled. Compared to the energy requirements for other avian behaviors, the cost of begging is low. Most discussions of the evolution of begging behavior in nestling birds have assumed that begging is costly. Based on my results, the assumption that begging is energetically costly needs to be reexamined and, until then, conclusions of models dependent on this assumption should be considered tentative.