Every year from sunset on September 11th to sunrise on September 12th, the lights of the National September 11 Memorial & Museum’s Tribute in Light are turned on in remembrance of the lives lost during the terrorist attacks of September 11th 2001. Beams from eighty-eight 7500-watt bulbs cast light skyward in two towering pillars as high as the eye can see, noticeable for a 100-mile radius around New York City. And it’s not just people that take notice: nocturnally migrating birds are attracted and disoriented by the lights. At times a close look can reveal tens of thousands ceaselessly circling through the beams. In a new study published in Proceedings of the National Academy of Sciences (PNAS), “High-intensity urban light installation dramatically alters nocturnal bird migration,” authors from the Cornell Lab of Ornithology, Oxford University, and New York City Audubon quantify the impact of this light installation on nocturnally migrating birds using radar, acoustics, and visual counts archived on eBird.
Using weather surveillance radar, the authors estimate that ~1.1 million migrating birds were influenced by the lights over a period of 7 nights over 7 years. While the abundance of migrants in the beams is visually apparent at the site, only by using radar were the authors able to view a full “footprint” of the lights’ impacts on birds, which may extend multiple kilometers from and above the lights themselves.
Working closely with the tribute’s organizers (first the Municipal Arts Society, and now the National September 11 Memorial & Museum), New York City Audubon lobbied from the outset for extinguishing the lights for 20-minute periods when more than 1000 birds were present in the beams or birds were flying dangerously low, an agreement they hoped would support the tribute’s mission while allowing migrants to escape the draw of the lights. The authors’ analysis revealed that birds returned to normal behavior almost immediately after the lights were turned off, suggesting that selectively turning out lights is an effective way to reduce threats to birds in urban areas.
The paper highlights this collaborative effort, which serves as a model for bringing multiple stakeholders together to mitigate human impacts on migratory birds. This is especially important in a world where the night sky is becoming ever more polluted by artificial light.
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