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Tonight from 8pm to 9pm your local time is Earth Hour, an global exercise to switch of lights everywhere. The effort promises to save gobs of power and mountains of co2 emissions. The American Bird Conservancy points out another benefit: the effort is likely to save birds in the midst of their spring migration. The flight north peaks during mid-March into June, and many birds fly at night. Avian navigation systems evolved over millions of years to track dimmer celestial bodies: the moon and stars. Man-made lights and pollution overwhelm and block out these heavenly lights and can disorient birds, leading them to collide with buildings and other tall structures. The map above shows the distribution of such “light pollution” across North America. As many as 900 million birds are killed in collisions each year, according to the U.S. Fish and Wildlife Service. Turning lights out — especially on tall structures, such as office towers, apartment buildings, bridges and monuments — can make a difference. Below, before and after photos show the Chicago skyline as downtown skyscrapers dim their lights for Lights Out Chicago last year. Lights go out from 8pm to 9pm tonight.