Condition-Dependent Begging Elicits Increased Parental Investment in a Wild Bird Population

Abstract

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 experimentally tested whether begging serves as a negatively condition-dependent signal of need or a positively condition-dependent signal of quality. Across multiple years, we supplemented nestling house wrens with food shortly after hatching and simultaneously manipulated corticosterone levels to simulate the hunger-induced increase in glucocorticoids thought to mediate begging. This allowed us to also test whether begging is simply a proximate signal of hunger. Days after supplementation ended, food-supplemented nestlings were in better condition than nonsupplemented nestlings and begged for food at an increased rate; their parents, in turn, increased provisioning to a greater extent than parents of nonsupplemented 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.

Keywords: corticosterone; glucocorticoid; house wren; life history; parent-offspring conflict; parental care.

Figure 1

Schematic depiction of the timing of our treatments (feeding of food and corticosterone) and observations of nestling begging and adult provisioning (growth data from Bowers et al. 2015c).

Figure 2

Interactive effects of food and corticosterone supplementation were apparent within the first 5 min of treatment application on day 4/5, but disappeared in min 6–10 (A). Food supplementation affected nestling begging during the entirety of our hour-long observation on this day (B), with unfed nestlings begging at a higher rate than nestlings fed either food and water or water only. Plotted are least-squares means ± SE. Filled black symbols depict experimental (Exp) nests for either the corticosterone (A) or food (B) supplement, open symbols depict control (Con) nests in which nestlings were fed the vehicle only, and filled grey symbols depict unmanipulated, natural (Nat) nests that were not supplemented.

Figure 3

Effect of brood size on begging vocalizations on day 4/5 in the time since application of treatments (i.e., 0–10 min is the first 10 min since feeding nestlings and the beginning of the observation).

Figure 4

Effects of food supplementation. (A) Nestling growth from the onset of the manipulation (day 2 posthatching) to the end of supplemental feeding (day 5 posthatching). On both days, nestlings were weighed before the treatments were applied. (B) Total food provisioning by both parents (deliveries per hr) in relation to begging vocalizations 4/5 d posthatching (vocalizations per nestling per hr), immediately following the feeding treatment. (C) Total food provisioning in relation to begging vocalizations (as in B) 7 d posthatching, two days after the supplemental feeding had ended. (D) Nestling prefledging mass. All panels depict least-squares means ± SE. Black symbols depict experimental (Exp) nests (nestlings fed food dissolved in water), open symbols depict control (Con) nests (nestlings were fed water only), and grey symbols depict unmanipulated, natural (Nat) nests that were not supplemented.

Figure 5

The probability that a given nesting attempt would produce at least one recruit to the breeding population in relation to the average prefledging mass of nestlings within broods. The solid curve represents the predicted value and dashed curves the 95% CI.

Figure A1

Accuracy of Raven software. Accuracy was assessed using ten randomly selected clips (10 sec each) from each begging file (total of 100 sec per file). The sum total of detections is compared with the actual number of begging signals produced by nestlings. The dashed diagonal indicates perfect correspondence. Each point represents a single file (N = 195).