Aggression and multi-modal signalling in the European robin (Erithacus rubecula)

Urban habitats are polluted with acoustic noise, which may disrupt communication between urban-living wildlife. Urban birds rely heavily on acoustic signals in many contexts, such as mate attraction and territorial defense. Aggressive interactions may escalate more rapidly when communication between rivals is disrupted. One way to respond to this challenge is to shift signaling effort to a less noisy modality. A common inhabitant of wooded areas and urban parks, European robins (Erithacus rubecula) use acoustic and visual signals during territorial defense. The present study investigates how European robins holding territories in urban and rural areas respond to simulated intrusions with or without experimental noise playback. We predicted that under experimental noise, European robins would show an increase in aggressive behaviors and shift their signaling effort from the acoustic modality to the visual. Since urban robins may have adapted to increased acoustic noise conditions, we expected them to show a steeper increase in aggression and a more prominent shift to the visual modality than that exhibited by rural robins. While urban robins were generally more aggressive, noise playback increased aggression only in rural robins. In accordance with the multi-modal shift hypothesis, noise playback resulted in lowered song rates in only urban robins. However, we did not detect a significant effect of noise playback on visual signals. The results suggest that plastic responses to transient increases in noise may be shaped by longer-term processes in urban European robins.