Standing in a northern Everglades marsh in 2015, biologist Corey Callaghan watched as an enormous flock of tree swallows wheeled in the morning sun. As the mass of birds passed overhead, Callaghan and his partner stood in astonishment. Just how many tree swallows were in the flock, he wondered—for that matter, how many birds are in the entire world? 

“It was an awesome experience,” Callaghan says. Inspired, he began by counting the birds in the flock he had just witnessed: more than half a million. He came up with that number by taking photographs, counting birds in different segments of the image, and scaling up. 

Counting all the world’s birds would be much trickier, for obvious reasons, but