Ambient noise and parental communication of predation risk in tree swallows, Tachycineta bicolor

Ambient noise can mask important acoustic signals used in a variety of communication systems, especially if signals are not adjusted to improve transmission in noise. Nestling birds communicate with their parents using loud begging calls that convey their need for food, but that also attract predators to the nest. Parents can reduce this vulnerability by using alarm calls to silence begging nestlings. Noise could, however, mask alarm calls and increase predation risk, unless parents can adjust their calls to circumvent the noise. Here, we determined whether the response of nestling tree swallows to parental alarm calls is altered in the presence of noise, and whether parents adjust alarm calls in noise to improve call transmission. We found that, in noisy conditions, nestlings continued calling and failed to crouch in the nest in response to parental alarm calls, in contrast to their behaviour in quiet conditions. None of the call features that we measured varied in relation to ambient noise levels at the nest. Our results suggest that noise could interfere with the ability of nestlings to respond appropriately to parental alarm calls and, in turn, could increase predation risk, although further work is needed to test this possibility.