Muting individual nestlings reduces parental foraging for the brood

Nestling birds use vocal and visual behaviours when soliciting food from parents. Such behaviours serve at least two discrete functions: (1) to induce parents to bring more food; and (2) to influence how food is allocated among brood members. Playback experiments have shown that vocalizations serve function 1. Do they also function to influence intrabrood allocation, as contemporary begging theory suggests, or is that governed chiefly by the nonvocal components of begging (neck stretching, gaping, jockeying for position within the nest)? We tested this hypothesis using a novel nonsurgical muting technique to decouple the vocal and visual components of begging in nestling red-winged blackbirds, Agelaius phoeniceus. Single chicks that were muted temporarily (1 h) continued to be fed at roughly the same rate as either the same individual prior to muting or sham-muted nestlings in the same brood. Parents reduced provisioning rates by increasing nest attentiveness in response to changes in the begging behaviour of the brood following treatment. These changes included less time spent begging (visual and vocal) accompanied by a reduction in the collective vocalizations of the brood. Our results suggest that vocalizations function primarily to regulate parental foraging rates, and visual begging displays function primarily to access food (competition). (C) 2002 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.