A growth cost of begging in captive canary chicks.

Nestling birds solicit food from adults by using begging displays that appear paradoxically costly and wasteful. Theoretical work suggests that the evolution of such exuberant offspring behavior reflects parent-offspring conflict over the supply of parental investment. Originally, extravagant begging was seen as a means of psychological trickery by which offspring could wheedle additional resources from resistant parents. Subsequently, costly begging came to be viewed as the hallmark of resolved parent-offspring conflict, serving either to prevent escalated scramble competition or to enforce honest signaling. However, the theoretical assumption of costly solicitation has been called into question by the low level of energy expenditure measured empirically during begging. This finding has prompted new theoretical work that shows that begging can be cost-free and yet still resolve parent-offspring conflict. Here, I report that begging is more costly than recent work suggests. My experimental evidence from captive canaries demonstrates a marginal cost of begging through impaired growth. Furthermore, I argue that previous studies of energy expenditure during solicitation do not measure the cost of begging, as defined theoretically. More generally, my results may account for the evolution of nestling growth rates, as well as the observation that begging is typically most flamboyant in older offspring.