Features of begging calls reveal general condition and need of food of barn swallow (Hirundo rustica) nestlings.

Altricial offspring of birds solicit food provisioning by complex begging displays, implying acoustic and visual signals. Different components of begging behavior may function as reliable signals of offspring state and thus reproductive value, on which parents base optimal parental decisions about allocation of critical resources (e.g., food). We experimentally manipulated components of general condition of nestling barn swallows (Hirundo rustica) by (1) altering brood size by cross-fostering an unbalanced number of nestlings between pairs of synchronous broods and thus manipulating the level of within-brood competition for food, (2) injecting some nestlings with a harmless immunogen, simulating an infection, and (3) preventing part of the nestlings from receiving food for a short period while establishing control groups. We recorded rate of begging response by individual nestlings as parents visited the nest and recorded begging calls using a DAT recorder to analyze six sonagraphic features of vocalizations. Our factorial experiment revealed that nestlings deprived of food begged more frequently when parents visited the nest compared to their non–food-deprived nest mates. Food deprivation increased duration of syllables forming begging calls, whereas brood size enlargement resulted in increased latency of response to parental calls. Heavy nestlings in good body condition vocalized at a relatively low peak frequency. To our knowledge, this is the first study in which begging rate and sonagraphic structure of begging calls are shown to reliably reveal a diverse set of components of offspring general state, on which parental decisions may be based.