Hundreds of millions of birds are currently on the move every night across North America as they wing their way south during fall migration. Chicago is under a high alert Sunday, with a massive number of birds expected to pass overhead.

The timing coincides with the annual World Migratory Bird Day. The best way to celebrate? Dim external lights to help out the millions of migrating birds passing over Cook County.

The community science project, held over four days every February, collects data that provides scientists with a long-term record of bird distribution across the globe, helping to identify trends that might be associated with urbanization or climate change.

This weekend, people around the world will report their bird sightings as part of a massive citizen-science project. Here’s how it works.