Noise impairs the perception of song performance in blue tits and increases territorial response

Details in birdsong parameters convey information about fitness, quality and motivational state of the signaller. Perception of these song details may affect decision making of receivers in territorial defence and mate choice. Whether the message in the song is perceived or not may have major consequences for the birds’ reproductive success. Consequently, birds may suffer fitness consequences from masking by other sounds in the environment. We conducted two different playback experiments to test whether song consistency, a sexually selected performance trait expressed in the temporal and spectral parameters of song, is perceived under different noise conditions. In the first experiment, we found that blue tits, Cyanistes caeruleus, are less able to assess the performance levels of song, but still detect the song stimuli under experimentally high broadband noise levels. Blue tits also responded with more songs overall, independent of song stimulus variation to playback of song stimuli under noisy conditions. When song stimulus variants were exposed simultaneously with a narrow band of noise in the second experiment, blue tits showed reduced capacity to discriminate only when the band of noise overlapped in frequency with the song. Results from this experiment support the notion that it is indeed the masking, rather than nonauditory effects of noise that cause the change in behaviour. Overall, our results show that there are noisy conditions that do not affect detection but still affect perception of information in the detailed structure of songs. Not being able to respond appropriately to songs that differ in performance level is likely to have negative fitness consequences and contribute to a detrimental impact of anthropogenic noise on individuals and populations.