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Why do migratory birds crash into buildings and how can it be prevented?

Birds fly across the moon against a purple sky.
Migrating Whimbrels by John Mueller via Birdshare.

Even in the best of circumstances, spring and fall are dangerous times for migrating birds. Most birds migrate at night, and major cities full of brightly lit buildings can draw the birds off course and increase the risk of striking windows. 

The reasons are not entirely understood, but nocturnal migrants often navigate by the stars. Illuminated windows and other night lights may divert them from their original flight paths, especially in low-ceiling or foggy conditions. The birds mill about in the lighted area, where they collide with the lighted structure or with reflective windows the next day. In addition to buildings, communications towers and radio antennas pose similar threats. Hundreds or thousands of dead birds may litter city streets after a wave of migrants has passed through. In all, up to 1 billion birds die each year in North America from colliding with structures.

As people learn more about where, when and how these nighttime collisions happen, they are also learning how to prevent them. Since 1993, the Fatal Light Awareness Program (FLAP), a Toronto-based conservation society, has spearheaded “Lights Out” programs across North America, calling for buildings in major cities to turn out all of their lights each night during peak migration times. One study conducted by the Field Museum in Chicago showed that in one building, turning the lights off reduced the number of bird kills by an average of 83%. Birdcast a joint project of the Cornell Lab of Ornithology, Colorado State University, and the University of Massachusetts Amherst analyzes weather surveillance radar to estimate the numbers, flight directions, speeds, and altitudes of birds aloft across the contiguous United States. See Birdcast migration dashboard for more information on birds passing over any counties in the Lower 48. BirdCast also offers resources on how to go Lights Out, and you can sign up for alerts when particularly heavy migration is expected in your area and turning out lights is most beneficial.

In addition to turning out lights, it’s important to make windows safer for birds. And it’s not just skyscrapers that are a problem: millions of birds die in collisions at residences as well. Visit our Window Collisions page for more about window treatments that help prevent collisions in residential areas.

More Information:

The Cornell Lab

All About Birds
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American Kestrel by Blair Dudeck / Macaulay Library