Exposure to traffic noise weakens territory defence in the Southern Emu-wren (Stipiturus malachurus)

Anthropogenic noise has the potential to negatively impact wildlife by disrupting communication and reducing overall fitness. Understanding the effects of traffic noise on signalling behaviour can be important for managing threatened populations. The Southern Emu-wren Stipiturus malachurus is a threatened, sedentary and territorial songbird, with a broad distribution in areas with and without main traffic roads. In this study, we ask if four subspecies of Southern Emu-wren show a similar territory defence response when tested with a traffic noise stimulus, and if either defence behaviour, song characteristics or both change when experimentally tested in relation to distance from the nearest road (proxy for noise exposure). We exposed territory holders to a simulated intruder using playback consisting of two treatments: song only and song with an added traffic noise stimulus. We then measured changes in territory defence behaviour and song characteristics between the two treatments. When exposed to the traffic noise stimulus, Southern Emu-wrens had a weaker behavioural response to a simulated intruder. Irrespective of the playback treatment and distance from the nearest road (noise exposure), Southern Emu-wrens did not change their song characteristics. Response to traffic noise stimulus differed across subspecies as the Mount Lofty Ranges subspecies S. m. intermedius had a weaker behavioural response. The Coorong subspecies S. m. polionotum sang truncated songs compared to all other subspecies. Our results add to a large body of evidence that the territorial defence behaviour of songbirds can be altered by short-term traffic noise exposure. Our study highlights threatened subspecies differences in response patterns to traffic noise that may be important to consider as part of conservation management approaches. Future studies could aim to better understand component traits that show plasticity and individual variability across systems in response to noise from traffic.