By Carolyn Burt, Ph.D
In honor of World Migratory Bird Day on May 13, Carolyn Burt, a scientist with the National Science Foundation’s Convergence Research program, provides us some insight on the importance of turning our lights out at night for migrating birds.
Birds migrate, sometimes across hemispheres, every spring and fall. Thanks to these journeys, spring is characterized by the return of birdsong and the delight of seeing bright yellow, blue, or red songbirds visit our cities and neighborhoods. However, during this awe-inspiring journey, birds face many human-caused threats – one being artificial light at night.1
In North America, 70% of birds are migratory and, of those that migrate, a whopping 80% do so under the cover of darkness.2 The most direct way that artificial light threatens both migrant and resident birds is attraction to light sources, often resulting in disorientation and, at times, collisions with lit structures. Coordinating Lights Out campaigns and partnerships across cities can be difficult, as artificial lights at night have become integral in our society due to perceived safety, function, and aesthetics. I am a Convergence Coordinator for a National Science Foundation (NSF) funded project through NSF’s Growing Convergence Research program – a team of ecologists, political scientists, and sociologists at Colorado State University and the University of Oklahoma working toward sustainable night skies. A transdisciplinary approach is needed to begin to address this large, societal issue, as we cannot solve it through a biological lens alone.3
The effects of light pollution can impact migratory birds on a variety of spatial scales.4 At the macroscale, birds are attracted to the sky glow from illuminated urban or suburban areas and can be drawn into heavily urbanized areas, disrupting and even reshaping their migratory routes.5 We also know that sky glow has been increasing by up to 10% per year in North America.6 Exposure to light pollution across migratory routes can even lead to phenological shifts, whereby birds may differentially depart or arrive to areas because of light pollution exposure. At more regional scales, light pollution can alter where birds stop over during their migration. These are critical habitats to sustain migrants on their journey, allowing birds to rest, refuel, and seek refuge during inclement weather.7
“Monitoring efforts at Chicago’s McCormick Place convention center revealed more than 40,000 fatal bird collisions across 40 years, with illuminated windows on the building seeing increased collision rates.”
At the most local scale, light pollution disorients birds and elevates the risk of fatal collisions with buildings. For example, monitoring efforts at Chicago’s McCormick Place convention center revealed more than 40,000 fatal bird collisions across 40 years, with illuminated windows on the building seeing increased collision rates.8 While it’s challenging to know how different age classes of birds (e.g., juvenile versus adult) respond to lights, attraction to lights appears higher during the fall migration season. During the fall, populations of migrants are heavily skewed towards juveniles, often making up the majority of birds. While it’s still an open question, there are indications that younger birds may have a greater attraction to light.
To help mitigate these harmful effects, we suggest dimming or extinguishing any unused and unnecessary lights – no matter the type – during Lights Out or peak migration periods. And it’s not just exterior lights that can increase the risk of fatal bird collisions. Turning off interior lights on a heavily windowed building has been shown to reduce fatal collisions by up to 60%.8
There are several additional ways building managers and others can help reduce harm, including implementing motion sensors for some areas so that lights are only turned on when needed. Shielding exterior lights so that the light is directed downward also helps by decreasing the upward radiance that contributes to sky glow. Finally, using warm and low color temperature light could reduce some risk to migratory birds.
A straightforward way that we can help the birds along their journeys is to turn off outside lights during peak migration where the highest numbers of birds are in the air. This can be as few as ten days each spring and fall9 with those days varying year-to-year. Local bird or Audubon groups and BirdCast provide forecasts and warnings for when birds will be moving through specific areas.10 Across the U.S., some areas see significantly greater numbers of birds, making it especially imperative to turn out lights in those places. Right now, and every spring, billions of birds are crossing the Gulf of Mexico, landing along the coastlines of Texas, and settling into breeding grounds across North America. Critical cities for Lights Out during spring migration include Chicago, Houston, Dallas, Los Angeles, and St. Louis, while the critical cities for fall migration include Chicago, Houston, Dallas, Atlanta, and New York City.2
Every little bit will help – even spreading this awareness in your community. To learn when birds will be in your airspace and for Lights Out warnings for your area, visit AeroEcoLab.com/uslights and BirdCast.info.
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(2) Horton, K. G., C. Nilsson, B. M. Van Doren, F. A. La Sorte, A. M. Dokter, and A. Farnsworth (2019) Bright lights in the big cities: migratory birds’ exposure to artificial light. Frontiers in Ecology and the Environment 17, 209–214.
(3) Burt, C. S., J. F. Kelly, A. S. Fox, H. C. Jenkins-Smith, M. Leon-Corwin, A. Khalighifar, G. E. Trankina, C. L. Silva, and K. G. Horton (2023b) Can ecological forecasting lead to convergence on sustainable lighting policies? Conservation Science and Practice 5, e12920.
(4) Burt, C. S., J. F. Kelly, G. E. Trankina, C. L. Silva, A. Khalighifar, H. C. Jenkins-Smith, A. S. Fox, K. M. Fristrup, and K. G. Horton (2023a) The effects of light pollution on migratory animal behavior. Trends in Ecology & Evolution 38, 355– 368.
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(7) McLaren, J.D., J. J. Buler, T. Schreckengost, J. A. Smolinsky, M. Boone, E. E. van Loon, D. K. Dawson, and E. L. Walters (2018) Artificial light at night confounds broad-scale habitat use by migrating birds. Ecology Letters 21, 356–364.
(8) Van Doren, B.M., D.E. Willard, M. Hennen, K.G. Horton, E.F. Stuber, D. Sheldon, A. H Sivakumar, J. Wang, A. Farnsworth, and B. M. Winger (2021) Drivers of fatal bird collisions in an urban center. Proceedings of the National Academy of Sciences 114, 11175–11180.
(9) Horton, K. G., B. M. Van Doren, H. J. Albers, A. Farnsworth, and D. Sheldon (2021) Near-term ecological forecasting for dynamic aeroconservation of migratory birds. Conservation Biology 35, 1777–1786.
(10) Van Doren, B. M. and K. G. Horton (2018) A continental system for forecasting bird migration. Science 361, 1115-1118.