Between midnight and dawn, millions of birds cross the French sky during a natural phenomenon called nocturnal migration, a process that most humans never witness. These birds—such as warblers, robins, thrushes, and raptors—travel along ancient routes that have been used for thousands of years. France, located at a key geographical crossroads between Northern Europe and Africa, serves as a major corridor for this migration. The intensity of the movement can vary greatly depending on wind patterns, with radar data showing that bird traffic can nearly double from one night to the next based on weather conditions. Early September is a peak time for this seasonal migration, as northern bird populations begin their journey south toward Africa. However, artificial light from cities can interfere with their navigation, causing them to become disoriented and exhausted. This often results in collisions with illuminated windows of urban buildings, leading to hundreds of thousands of bird deaths each year in France. The true number of casualties is likely much higher, as many birds are quickly consumed by predators or scavengers before their deaths are noticed. To better understand this migration, volunteer naturalists use a technique called "nocmig," which involves recording birds' contact calls during the night. These recordings, combined with data from meteorological radars, help create a detailed map of the migratory paths. In the Vendée region, a single season of monitoring identified 73 different species in migration or local movement over 1,100 hours of recording. In Île-de-France, the French League for the Protection of Birds (LPO) compiles an annual report using data from multiple listening points set up by amateur observers. Nocturnal migrants rely on natural cues such as the landscape, reflections from water, and the stars to navigate. Artificial light can interfere with these signals, causing a behavior known as phototaxis, where birds are attracted to light sources. This can lead them to mistake urban lights for the moon, resulting in prolonged circling and energy loss. In areas like La Défense in Paris, birds are drawn to artificial lights and often collide with glass buildings. To reduce the impact of artificial light on migrating birds, various measures have been introduced. A 2018 French decree encourages the reduction of nighttime lighting, including turning off or dimming lights in the middle of the night. Other strategies include directing lights downward, turning off signs after hours, using amber-colored LEDs, and applying frosted films to large glass windows. These cost-effective solutions can be implemented by building managers or city officials to help protect migratory birds during their critical journeys.