Billions of birds undertake migratory journeys each spring and autumn. Most of these spectacular movements go unseen, occurring under the cover of darkness. A new study provides some of the most compelling evidence yet that artificial light at night causes radical changes in the behaviour of migrating birds.
“We found that migrating birds gather in large numbers because they’re attracted to the light,” says Benjamin Van Doren of Oxford University, a lead author of the study. “They slow down, start circling, and call more frequently. They end up burning energy without making any progress and risk colliding with nearby buildings or being caught by predators.”
Scientists from the University of Oxford, the Cornell Lab of Ornithology, and New York City Audubon studied migrant bird behaviour over seven years in a truly unique setting—Tribute in Light in New York City, held to commemorate the September 11, 2001, terrorist attack. Two beams of light–each with 44 xenon bulbs of 7,000 watts—rise into the night sky, mirroring the twin towers of the World Trade Center where nearly 3,000 lives were lost.
“This was a rare opportunity to witness the impact of powerful ground-based lights on nocturnally migrating birds,” says co-lead author Kyle Horton, Cornell Lab, but working at the University of Oklahoma during the study. “This analysis would not have been possible without the help of tribute organizers.”
Well before the results of the study highlighted the effects of the installation, New York City Audubon reached out to the original tribute organizers, the Municipal Art Society, to let them know about the impacts of artificial light on migratory birds. In 2002, the two organisations developed a protocol to save the affected birds. The tribute lights are turned off for approximately 20 minutes when more than 1,000 birds are seen circling in the beams or flying dangerously low with frequent calling. The National September 11 Memorial & Museum, which took over as tribute organisers in 2012, continued this practice.
These brief interludes of darkness during the nightlong tribute provide a unique opportunity for the scientists to quantify changes in bird behaviour in several ways during the alternating periods of light and darkness.
“We had more than 20 wonderful volunteers on the ground counting birds to confirm what was happening in the beams,” says co-author Andrew Farnsworth at the Cornell Lab. “We also used remote sensing tools, including data from local National Weather Service radars, to understand the density and movements of the birds. Acoustic monitors recorded call notes and captured the vocal behavior of birds in the beams. And we ran computer simulations to try to better understand the dynamics of the patterns we were observing.”
When the tribute was illuminated, the study’s authors found that densities of birds over lower Manhattan could reach 60 to 150 times the number that would typically be found in the area at that time. The concentrating effects of the intense light on the birds reached as high as 4 kilometres (2.5 miles). The impact on birds was consistent even on clear nights. (Many previous artificial-light studies focused on nights with poor visibility.) When the light beams were turned off, the birds dispersed within minutes to continue their migrations.
Although Tribute in Light provided a way to measure a specific instance of light attraction, bright nighttime lighting poses problems for birds, even in rural areas. The study’s authors point out that one solution is relatively simple.
“We recommend building lights be turned off for as much of the night as possible, but at least from midnight to dawn during migration season,” says study co-author Susan Elbin of New York City Audubon. “This is true for areas around homes as well as other brightly lit areas such as sports stadiums, construction sites, offshore oil rigs, and large buildings. Migrating is already hard enough for birds without this added danger from artificial light at night.”
The full paper High-intensity urban light installation dramatically alters nocturnal bird migration can be accessed here