Singing under a streetlamp
Artificial light rather than traffic noise has been pinpointed as the main modifier of timing of singing in songbirds
Dawn and dusk are two critical periods were male songbirds sing to defend their territory and their mate. The timing of dawn song is frequently regarded as a reliable indicator of male quality because in some cases it is age-related and condition-dependent. Previous work showed that artificial night lighting could modify this timing but no studies have ever investigated the influence of light pollution in parallel with another stressor associated with urbanization, i.e. noise pollution, and in comparison with natural conditions.
In this study, researchers from the Max Planck Institute for Ornithology have recorded 6 common European songbird species from winter to the peak of breeding season and have compared the effects of artificial light and traffic noise, not only on the onset of their dawn singing but also on the cessation of their dusk singing. The researchers found that artificial night lighting, but not noise, led to an earlier start of dawn singing in 5 out of 6 species, and to a latter cessation of dusk singing in 3 species.
This effect was strongest at higher light intensities. Earliest singers, i.e. the European robin (Erithacus rubecula), the common blackbird (Turdus merula) and the great tit (Parus major) were the most affected, although the effect was smaller for the dusk song than for the dawn song (up to 1.5 hours earlier than sunrise for robins and blackbirds, up to an hour earlier than normal for great tits). This highlights the importance of natural light as an environmental cue used by territorial males to time their first and last song of the day. Artificial light glow alters the perception of this cue, with birds perceiving the day as longer, whereas traffic noise alone doesn’t interfere with timing of singing. Interestingly, the importance of natural light is emphasized by the finding that rain and cloud coverage delayed the onset of singing at dawn and advanced the cessation at dusk for all the species. Last but not least, onset and cessation of singing changed relative to sunrise and sunset with the progress of the season. Whether the shift of timing due to light pollution is beneficial or costly remains to be fully investigated but males could benefit from early singing by attracting more extra-pair females.