Migratory Birds Are Distracted by Porch and Streetlights

Birds distracted by light
Dan Mennill at the University of Windsor holds a nocturnal flight call microphone used to survey avian biodiversity during migration. Recent research showed that nocturnal migratory birds called more in areas lit by ground-level light than in dark areas. This shows they could be thrown off course as they navigate. C. Sanders

Lights from inside skyscrapers make or break a city's skyline. They also kill as many as 600 million migrating birds each year by throwing them off track as they try to navigate by the moon and stars. Skyscrapers from the Chrysler Building to the Sears Tower, and hundreds of buildings in between, take part in the National Audubon Society's Lights Out campaigns during spring and fall bird migrations by switching off all unnecessary lights or drawing the shades. In one study, bird fatalities in Chicago decreased by 80 percent when building lights there were off.

Most artificial light isn't shining from the top of the Chrysler Building, but rather from hundreds of millions of low-level porch and street lights across North America, according to Dan Mennill, associate professor in the department of biological sciences at Canada's University of Windsor. New research using sound technology shows that this ground-level artificial light also does its part to distract migratory songbirds and could inform efforts to keep birds from colliding with buildings, which in North America is the second-leading cause of death to the animals behind cats.

Three hundred percent more birds were detected passing over lit areas than dark areas in their study during the fall 2013 migration season in southern Ontario. The team positioned microphones pointed at the night sky in 16 locations containing a ''light site,'' which had a streetlight or building light nearby, and an adjacent ''dark site,'' which had no artificial light nearby. So that their research wasn't compromised by urban noise, the team chose parklands, low-density residential communities and other semi-rural areas for their studies. Researchers analyzed 352 hours of recordings taken from sunset to sunrise that yielded 1,913 nocturnal flight calls, including calls of at least 15 unique species of birds. Each night, a median of 31 calls were recorded at light sites, compared to a median of 10.5 calls recorded at dark sites.

"What's clear from the study is that more birds are calling from above lit sites than from above dark sites. What's not clear is whether the light is causing them to change their routes," says Mennill. He can think of se veral possibilities for the greater number of calls over lit sites. "It could be that birds are changing their migratory route to fly more often over lit sites. It could be they are flying lower and therefore we can hear them more, or it could be that birds are disoriented by the light."

In any case, the increase in calls in lit areas shows low-level manmade light has effects on migratory birds similar to light coming from tall buildings. Mennill believes artificial lights may lead birds to migrate inefficiently, increasing the time and energy needed for their journeys, which could decrease the likelihood individual birds will survive migration and arrive at wintering or breeding grounds unharmed.

With more and more man-made changes to the natural environment, Mennill believes humans have a responsibility to see how these factors impact wildlife. "Understanding that light changes the behavior of migratory birds can help guide how we approach conservation efforts," he says. "If we understand that light is disruptive, we can simply flick out the backyard light to protect that bird passing overhead."

Birding is a multibillion dollar industry in North America, but birds aren't just beautiful, they are beneficial. By eating insects, they protect fruit orchards and safeguard trees used in the furniture industry. They even reduce pest levels at organic wineries, according to the National Audubon Society. Across the world, birds disperse seeds of plants that provide us with food, medicine, timber and recreation. Scientists rely on birds to study the impact of toxins and foretell environmental degradation. And so with the flip of that porch light switch, we all benefit from birds arriving safely at their breeding grounds so that they and their progeny can go on to disperse seeds or gobble up pesky mosquitoes, just as Mother Nature always intended.