Galápagos yellow warblers differ in behavioural plasticity in response to traffic noise depending on proximity to...

Acoustic communication between animals is increasingly disrupted by noise in human-altered environments making signals less effective. Birdsong is a signal used in agonistic interactions between territorial rivals, and birds may modify their song and singing behaviour in response to noise. However, if these modifications are still ineffective, this can lead to increased conflict between rivals. Here, we asked whether experimental traffic noise induces immediate changes in acoustic characteristics of song and aggressive behaviour in populations and territories that differ greatly in traffic noise exposure. We conducted simulated territorial intrusions on Galápagos yellow warblers, Setophaga petechia aureola, living on Santa Cruz (high traffic) and Floreana (low traffic) islands. Territories were either adjacent to the nearest road or at least 100 m away from it. We assessed the focal birds' physical response levels and recorded their vocalizations in response to the playback of conspecific song (control) and conspecific song coupled with traffic noise (noise treatment). We found that, on both islands, birds living in territories adjacent to roads increased their aggression levels with experimental noise compared to control, while birds living farther away from roads decreased their aggression levels. Birds on both islands increased minimum frequency of their songs during the noise treatment, irrespective of their distance to a road. However, changes in peak frequency and duration of their songs depended on the habitat they live in. Our results suggest behavioural flexibility in territorial responses and birdsong in response to traffic noise, which appears to depend at least in part on prior experience with traffic noise.