Each December, thousands swap wrapping paper for binoculars and step outside for a different kind of holiday tradition: the Christmas Bird Count.

Started in 1900 by ornithologist Frank M. Chapman, the count offered a peaceful alternative to the Christmas 'side hunts,' where people competed to shoot the most animals.

Chapman had a better idea: to count them instead.

Over a century later, that simple shift has grown into the world's longest-running citizen-science project.