Dynamics of calling by tree swallow (Tachycineta bicolor) nestmates

The begging display of nestling passerine birds has become a model for examining the evolution of animal signals. A particular problem for nestlings when transmitting begging signals to parents may be interference from nestmates. The strategies used by nestlings to reduce signal interference have not been studied, yet potentially contribute to the design of these complex displays. In this study, we recorded the begging calls of nestling tree swallows (Tachycineta bicolor) when alone and with a begging nestmate, to determine whether nestlings changed the output, structure or timing of their calls in ways that would reduce acoustic interference. We found that nestlings increased their call rate in the presence of a begging nestmate, but did not alter the length, amplitude or frequency of their calls. They also appeared not to adjust the timing of their calls to avoid those of nestmates. Contrary to expectation, nestling calls became more similar in some aspects when nestmates called together. An increase in call rate in the presence of a begging nestmate should increase the likelihood that a parent detects an individual's calls. However, if all nestlings increase their calling rate in response to competitors, then the overall level of acoustic interference across the brood is potentially increased, an effect exacerbated by the tendency for call similarity to increase when calling together. We discuss how increasing call rate may improve detectability despite this effect and we also examine how an increase in rate and call similarity may serve to produce a strong brood signal.