“Condition-dependent begging elicits increased parental investment in a wild bird population”
E. Keith Bowers, Jonathan B. Jenkins, Alexander J. Mueller, Kelly D. Miller, Charles F. Thompson, and Scott K. Sakaluk (May 2019)
Are baby birds begging or boasting? Positively condition-dependent begging ultimately enhances offspring survival
Altricial young beg for food from their parents across a diverse array of taxa. Expression of this behavior consists of a combination of auditory and visual stimuli, but exactly what offspring are communicating to their parents remains controversial. The most popular hypothesis to explain the functional message encoded by offspring begging postulates that begging signals offspring ‘need’ to parents, and that parents should respond to the degree to which feeding a particular offspring will benefit that individual offspring’s fitness, and, thus, parental fitness. This hypothesis makes several assumptions that are often not satisfied in natural families, raising the possibility that begging offers alternative kinds of information to parents. Here, investigators at the University of Memphis and Illinois State University test the hypothesis that the body condition of nestlings positively affects their begging and, consequently, the parental provisioning of food, offspring growth, and long-term recruitment of offspring into the breeding population. To enhance nestling condition, they experimentally supplemented nestling diets for four days posthatching by pipetting food into their mouths, and also manipulated glucocorticoid levels to simulate the transient increase in corticosterone induced by hunger, believed to mediate begging. In the short term, begging increased with experimental increases in glucocorticoid levels, but this effect depended on nestling satiety. Thus, glucocorticoids promoted begging as an immediate manifestation of offspring hunger. However, days after the food supplementation ended (when there was no effect of glucocorticoid supplementation), previously food-supplemented nestlings were in better condition than non-experimental nestlings and begged for food at an increased rate; their parents, in turn, increased provisioning to a greater extent than parents of non-experimental young, as begging positively predicted food provisioning. Food-supplemented nestlings, therefore, attained above-average pre-fledging body mass, which predicted their recruitment as breeding adults in the local population. Thus, begging signals appear to have communicated offspring condition or quality to parents, eliciting increased parental allocation to enhance offspring survival.
The coevolution of parental supply and offspring demand has long been thought to involve offspring need driving begging and parental care, leaving other hypotheses underexplored. In a population of wild birds, we tested experimentally whether begging serves as a negatively condition-dependent signal of need or a positively condition-dependent signal of quality. Across multiple years, we food-supplemented nestling house wrens shortly after hatching, and simultaneously manipulated corticosterone levels to simulate the hunger-induced increase in glucocorticoids thought to mediate begging. This allowed us also to test whether begging is simply a proximate signal of hunger. Days after supplementation ended, food-supplemented nestlings were in better condition than non-supplemented nestlings and begged for food at an increased rate; their parents, in turn, increased provisioning to a greater extent than parents of non-supplemented young, as begging positively predicted provisioning. Food-supplemented nestlings, therefore, attained above-average condition, which predicted their recruitment as breeding adults in the local population. Glucocorticoids increased begging in the short-term, but this transient effect depended on satiety. Thus, glucocorticoids promoted begging as a proximate response to hunger, whereas the longer-term changes in nestling condition, begging, and food provisioning suggest that begging ultimately signals offspring quality to elicit increased investment, thereby enhancing offspring survival.