Have you have ever been on an airplane at night and looked down on the illuminated earth? Our various light sources can be seen from miles away and the brightening of the night sky above cities even has a name – urban sky glow. Obviously, light sources at night are needed in modern civilisations, but diminishing darkness can cause severe problems, such as altering wake–sleep rhythms and affecting health. Although we have found ways to live with light pollution, other animals cannot always avoid it.
Zeynep Ulgezen, from Wageningen University in The Netherlands, and an international team of colleagues wanted to know the consequences for birds of sleeping under night illumination. For their study, the researchers caught great tits (Parus major) from an urban and a forested area and tested whether they preferred to sleep under constant darkness or under weak green or white LED light (1.5 lx). Next, they exposed the birds to constant darkness or nightly white or green illumination for 14 days, and monitored the birds’ sleep, activity and how much energy they spent per night. The researchers also took blood samples from the birds before and after the treatment to determine the animals’ oxalic acid levels, a biomarker for sleep disruption, and compared the memory and learning skills of the great tits exposed to the different light conditions, by training them to differentiate between colours and select a certain colour in return for a reward.
To their surprise, Ulgezen and her colleagues found that all of the birds preferred to sleep under light instead of in constant darkness, and – when given the choice – the birds preferred green over white light sources. When the researchers looked into the consequences of sleeping under light, they found no effect of nightly illumination on the birds’ memory and learning, and their blood levels did not show any indication of sleep disruption. However, both light sources caused the birds to be more active at night. The great tits that slept under white light became active almost 3 h earlier than those sleeping in constant darkness and half an hour earlier than those that slept under green light. Consequently, the birds facing nightly light pollution spent more energy than the birds that slept in complete darkness. But the effect of light also depended on the origin of the birds. The animals that were captured in a forest were most active under nocturnal white light, while urban birds showed the same amount of activity under green and white light.
This study suggests not only that urban birds can get used to the lights of the city, but also that birds actually prefer to have – at least a little – light at night. While the researchers have used light intensities that mimic realistic light pollution, the illumination was still relatively weak, and birds nesting under street lamps would experience light that was up to 13-fold more intense. But why do birds prefer light over darkness? Ulgezen and her colleagues suggest that the birds chose the illuminated areas for a simple reason: more light allows them more time to forage and enchant members of the opposite sex.