Anthropogenic noise impacts territorial defense in a non-urban songbird

The challenge anthropogenic noise poses to songbird acoustic communication is well-documented from the signaler's perspective, but less is known about its effects on signal receivers. This study used playback experiments to simulate intrusions on hermit thrush (Catharus guttatus) territories along an urbanness gradient, examining the effects of both ambient and experimentally added noise on territorial defense behavior. Ambient noise did not impair signal detection or song amplitude adjustments, suggesting that background levels in these habitats remained below the species' critical hearing ratio. However, males in more urban territories exhibited stronger aggressive responses-consistent with the urban anger hypothesis-despite hermit thrush being a primarily non-urban species. In contrast, experimentally added noise delayed approach responses, reduced aggressive displays, and triggered increased song amplitude, indicating a masking effect on signal perception or interpretation. These findings highlight key differences between chronic ambient noise and intermittent noise. The study reveals that while hermit thrush maintain effective signal transmission under moderate ambient noise, transient disturbances disrupt territorial defense and may reduce fitness. By combining observational and experimental approaches, these results underscore the need to differentiate noise types when assessing anthropogenic impacts on wildlife and call for broader investigations across urban-rural gradients to guide conservation efforts.