Hohl, L.; Yelimlies, A.; Akcay, C.; Kleindorfer, S.

Acoustic communication between animals is increasingly disrupted by noise in human-altered environments. This can make signals less effective and create selection pressure to increase their detectability. A good example of this is bird song. Song 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 that differ greatly in traffic noise exposure. We conducted simulated territorial intrusions on Galapagos yellow warblers (Setophaga petechia aureola) living on Santa Cruz (high traffic) and Floreana (low traffic) islands. 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). Although we found that physical response levels did not differ between islands or treatment types, birds on each island adjusted their songs differently in response to noise. In particular, birds increased the minimum frequency of their songs in response to the noise treatment, and the increase was more pronounced on Floreana Island (low traffic) than on Santa Cruz (high traffic). In contrast, song duration increased in response to noise treatment only on the high-traffic island. Our results suggest behavioural flexibility in birdsong in response to traffic noise, which appears to depend at least in part on prior experience with traffic noise.