NEED AND NESTMATES AFFECT BEGGING IN TREE SWALLOWS

We conducted an experiment on nestling tree swallows (Tachycineta bico lor) to examine predictions from signalling models for the evolution o f conspicuous begging behaviour. Specifically, we examined the relatio nship between (1) nestling begging intensity and hunger, (2) begging i ntensity and parental provisioning and (3) begging intensity and nestm ate condition. Forty broods of 9-day-old nestlings were removed from t heir nests for 1 h and assigned to one of the following three treatmen ts: (1) all nestlings in the brood deprived of food (n = 13), (2) all nestlings in the brood fed (n = 11) or (3) half the nestlings in the b rood deprived and half fed (n = 16). Videotapes before and after the t reatments showed that begging intensity increased in broods in which a ll of the nestlings had been deprived and decreased in broods in which all of the nestlings had been fed. Deprived nestlings in the half-and -half treatment did not change their begging intensity in response to treatment, while fed nestlings in this treatment group showed a decrea se in begging intensity. Parent tree swallows increased their feeding rate to deprived broods and decreased their rate to fed broods. Within broods, parents decreased their feeding rate to fed nestlings, but sh owed no significant change in feeding to deprived nestlings. Our resul ts suggest that begging intensity is influenced by hunger and that par ents appear to respond to variation in begging intensity. The begging of nestmates also appears to influence begging independently of need. These results are consistent with predictions derived from signalling models of begging.