Begging in the absence of parents by nestling tree swallows

Begging by nestling passerine birds has become a model system for studies in animal communication. Although most begging occurs when parents arrive at the nest to feed (here called "primary begging"), it also occurs between feeding visits and immediately after parents leave the nest. Begging in these contexts (here called "secondary begging") may have relatively little influence on the probability of receiving food, but could increase the overall cost of the signal and thus influence nestling begging strategies. The purpose of our study was to determine how often tree swallow (Tachycineta bicolor) nestlings beg in contexts other than to parents with food and to examine what factors influence the frequency of this begging. Secondary begging ranged from 7% of measured begging responses at day 2 to 30% by day 8 and was more frequent when the interval between parental feeding visits was relatively long and when the time to respond to the arrival of parents with food was short. Increases in both age and intervisit interval were associated with decreases in nestling response times, suggesting that secondary begging may be related to the speed with which nestlings respond to stimuli. We discuss possible functions of secondary begging and raise the possibility that it may, in fact, be an error.