Does lighting a forest all night change when four songbird species start their dawn songs?

Details

Earlier observational studies found that several songbird species began singing earlier in light-polluted areas, but the reasons were unclear. To test whether birds respond directly to nighttime light, researchers repeatedly switched all-night lighting on and off in several naturally undisturbed forest patches over two consecutive breeding seasons.

Males of all four species immediately adjusted when they began their dawn songs, and the changes reversed when the lighting was removed. The response was strongest in European robins and relatively small in blue tits, great tits and blackbirds. Robins also began singing progressively earlier during the lighting treatment. The researchers found no evidence that the treatment's effects carried over into the following dark period.

The responses of great tits and blackbirds were smaller than expected from previous observational studies. The small sample could explain this difference. Longer-term effects of living in light-polluted urban areas or birds avoiding light are other possible explanations, not findings established by this experiment. The experiment supports a direct, reversible response to nighttime light, but it does not establish that this response fully explains the earlier singing observed in light-polluted areas.