Sleepless in the city: Drivers of the shift in dawn song of urban dwelling European blackbirds

Background/Question/Methods

Urban living organisms are exposed to completely different environmental conditions compared to their rural conspecifics. Especially anthropogenic noise and artificial night light are closely linked to urbanization and pose new challenges to urban species. Songbirds are particularly prone to these factors since they rely on the spread of acoustic information and adjust their behaviour to the rhythm of night and day, e.g. time their dawn song due to changing light intensities.

In this study we focused not only on one of the urban factors, but included several types of ambient noise, weather as well as artificial night light to clarify their impact on the onset of dawn song of urban dwelling blackbirds (Turdus merula). We investigated the song activity along a steep urban gradient ranging from an urban forest to the city centre of Leipzig, Germany, over a 9 week period.

Results/Conclusions

The blackbirds start their dawn song by up to 4 hours significantly earlier in the city centre compared to semi-natural habitats. Overall artificial night light is the driving factor of the shift of dawn song into true night, although it is not completely separable from the effects of ambient noise. Our investigations reveal that when morning rush hour lay close to twilight, ambient noise and night light equally affected the onset of dawn song. Weather affected the dawn song in a way that windy and cold conditions delayed the onset, while cloud coverage reflects night light emissions and thus amplifies sky luminance and the effect of artificial light. Beside these temporal effects we found also differences in the spatial autocorrelation of dawn song onset showing a much higher variability in noisy city areas than rural parks and forests.

These findings suggest that urban hazards as ambient noise and light pollution interfere manifold with naturally evolved cycles and have immense effects on the activity patterns of urban blackbirds.

The study was supported by the German Federal Ministry of Education and Research (Grant 033L038E), the EU FP7 project SCALES (Grant 226852) and a travel grant of Helmholtz Interdisciplinary Graduate School for Environmental Research (HiGrade).