Skyscrapers in New York are turning off their lights to stop migrating birds bashing into them.
Around 90,000 birds are killed each year by crashing into buildings while they're flying south for the winter.
The organisers of the 'lights out' project say the skyscrapers' dazzling lights confuse birds and can make them lose their way.
The project is in its fifth year, and runs until 1 November, when most birds have finished migrating.
NYC Audubon, which runs the project, wants buildings to dim their lights in the evenings.
They also think late workers should draw the curtains or use little desk lamps.