From International Dark Sky website
Many songbirds like warblers, thrushes, and sparrows are nocturnal migrators. Without the light of the sun, these travelers use the moon and the stars to navigate during their long journeys, which can span thousands of miles and traverse continents. Artificial light at night (ALAN) can disrupt bird migration in a variety of ways, including disorienting birds from their routes and causing collisions with buildings, resulting in millions of bird fatalities each year.
