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

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 vocalisations in response to playback of conspecific song (control) and conspecific song coupled with traffic noise (noise treatment). We found that physical response levels mostly depended on treatment and distance to the nearest road: on both islands, birds living adjacent to the nearest road increased their aggression levels with experimental noise compared to control, while birds living farther away decreased their aggression levels. Birds on all islands irrespective of their distance to a road increased minimum frequency of their songs during the noise treatment. However, change 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.