Ambient noise and the communication of predation risk between parent and nestling tree swallows (Tachycineta bicolor)

Abstract

Ambient noise masks important acoustic signals used in a variety of communication systems. Nestling birds communicate with their parents using loud begging calls that convey their need for food, but these calls can also attract predators to the nest. Parents can counteract this vulnerability by using alarm calls to silence begging nestlings, if predators are nearby. Noise could, however, mask alarm calls and increase predation risk for begging nestlings. In the first chapter of my study, I played back parental alarm calls to broods of 5, 10, and 15 day old tree swallow (Tachycineta bicolor) nestlings to determine if nestlings respond to parental alarm calls. I then played back parental alarm calls to 15 day old nestlings in the presence of white noise to determine if noise affects this response. I found that younger nestlings did not respond to alarm calls, but that 15 day old nestlings responded to alarm calls by falling silent and crouching in the nest. I also found that the magnitude of this response decreased in the presence of noise. Noise interferes with the ability of nestlings to respond appropriately to parental alarm calls, which may, in turn, increase their risk of predation. In the second chapter of my study, I compared features of parental alarm calls to ambient noise levels at nest sites to determine if adult tree swallows adjust their alarm calls to improve signal transmission in noise. I found that tree swallows do not modify their alarm calls in response to ambient noise, suggesting these calls are not altered to improve transmission. Overall, my results suggest that communication of predation risk between parents and nestlings is negatively affected by noise. My results may be the first to suggest that this communication system is negatively affected by noise. The relationship between noise and predation risk is complex, and further work is required to determine if impaired communication could have implications for reproductive success.