The early bird gets the worm, as the old saying goes. And now a lot of birds around the globe are starting their days earlier than ever, because of unnaturally bright skies caused by light pollution.
"For these birds, effectively their day is almost an hour longer. They start vocalizing about 20 minutes earlier in the morning and they stop vocalizing about 30 minutes later in the evening," says Neil Gilbert, a wildlife ecologist with Oklahoma State University.
That's the conclusion of a sweeping study that analyzed bird calls from over 500 bird species in multiple continents, giving researchers an unprecedented look at how human-created lights are affecting the daily lives of birds worldwide.
Scientists already knew that light pollution affects birds. It can send migrating birds off course, and some observations have linked artificial lighting to unusual bird activity, including one recent report of American Robins feeding their babies in their nest at night.
But Gilbert and Brent Pease, with Southern Illinois University, took a more comprehensive view, by analyzing millions of recordings of birdsong.
The audio was collected by thousands of devices installed in backyards and other locations, mostly by birdwatchers and other wildlife enthusiasts, as part of a program called BirdWeather. The BirdWeather devices automatically register bird calls and use them to identify the species, mostly to let bird fans know what's flitting through their yards.
At the same time, however, the project is also accumulating a huge set of basic data on bird activity. Gilbert and Pease realized that this data could be combined with satellite information about light pollution, as well as information about sunrise and sunset times.
In the journal Science, they say that light pollution is extending birds' daily routines by about 50 minutes on average.
What exactly that means for the birds isn't known. Maybe more light means birds have more time to forage, resulting in more food to nourish offspring, says Pease. Or, maybe the light has a detrimental effect by interfering with sleep or seasonal hormonal cycles.
Some species extended their day more than others in response to artificial light. American Robins, Northern Cardinals, and Northern Mockingbirds, for example, all had strong responses to light pollution.
"American Robins will sometimes be singing, you know, two hours before sunrise in some of these polluted areas," says Gilbert.
Overall, the birds that were most sensitive to light pollution in this study were the ones that had larger eyes on average for their body size.
The Tufted Titmouse, for example, has relatively small eyes. "It didn't seem to have a strong response to light pollution at all," says Pease. Neither did the Eastern Bluebird, White Breasted Nuthatch, or Carolina Chickadee.
The researchers also noticed that birds that roost in enclosed spaces, like a tree cavity, seemed less affected. They think that's because the opaque walls of the cavity or nesting box act as a barrier to artificial light.
Jeff Buler, a wildlife ecologist with the University of Delaware who wasn't part of the research team, says this work is notable because it took advantage of both machine learning, which automatically identified the bird calls, as well as the participation of everyday people in science.
"That's what's really cool about this study," says Buler. "They've leveraged this growing global dataset of automated recordings of birds that have been collected essentially by the public."
Compared to what's been done in the past, he says, "it's unprecedented in the scope and extent of the dataset they've been able to use."
He says there's a growing interest among bird researchers in using these new tools to collect and analyze audio recordings of bird calls. This should allow them to ask and answer questions at a far larger scale than ever before — as this study demonstrates.
"This response by some birds, to prolonging their activity when there's artificial light around, seems to be pervasive," Buler says. "I think it's happening everywhere."
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