Light pollution makes birds work overtime.
A behavioral analysis of nearly 600 bird species suggests that light pollution from human development can lengthen the time birds spend singing by nearly an hour per day, researchers report August 21 in Science.
The extension’s magnitude took the researchers by surprise. “While we expected some behavioral adjustment to the lights at night, we didn’t anticipate that it would be this impactful,” says Neil Gilbert, an ecologist at Oklahoma State University in Stillwater.
Past research has repeatedly shown that the light from human habitation and infrastructure has substantial effects on wildlife. Birds that migrate at night can become disoriented by the lights and fatally crash into buildings. Lights can also perturb the light-dark cues that regulate hormonal and behavioral rhythms. Some studies on small numbers of bird species have shown they’re active earlier in the day in light-polluted areas.
Gilbert and Brent Pease, a wildlife ecologist at Southern Illinois University in Carbondale, wanted to know about light pollution’s effects on birds on a much larger scale. The researchers used data from BirdWeather, a global citizen science project. Volunteers deploy audio sensors to record bird sounds that machine learning then analyzes to identify the species making each sound. After filtering the dataset for 4.4 million calls, the team studied the daily start and end times of the singing activity of 583 bird species, comparing those times to the local light pollution levels.
In the brightest places, birds extended their singing time by an average of 50 minutes compared with the darkest — by 18 minutes in the morning and 32 minutes in the evening. The effect was particularly strong among bird species with larger eyes, such as a killdeer (Charadrius vociferus), possibly because they’re more sensitive to light overall. The effect was also greater during the breeding season, possibly because this is when birds naturally start singing earlier in what are normally darker morning hours. If they live in light-polluted areas, streetlamps and artificial light in the early morning might fool them into thinking it’s later than it is, further encouraging them to sing earlier.
It’s still unclear whether the compulsory encore is harmful to birds. It might disrupt their sleep, though they might compensate by sleeping during the day, Gilbert says. Extra activity may even be helpful, giving birds more foraging time to feed young.
For Pease, the findings help illustrate just how extensive even passive influences from humanity are upon wildlife.
“Our lights, which we cast more or less mindlessly into the night, are having widespread and often subtle effects on the lives of animals all around us.”