At night, it is artificial light from our buildings and cityscapes that endangers birds. Many species of birds migrate at night, using light from the moon, the stars, and the setting sun to navigate. The bright lights of our urban areas confuse these birds and pull them out of their way.
Especially on foggy or rainy nights when the cloud cover is low, birds fly at lower altitudes and are more likely to be disoriented by city lights. They may be pulled down into downtown mazes, where they often collide with buildings.
Also dangerous are floodlights, lighthouses, and airport ceilometers (light beams for measuring cloud altitude). Studies using radar have shown that, once birds are attracted to a light source, they tend not to want to leave it. Birds may become trapped inside beams of light, flying around inside them until they drop from exhaustion.