The only whooping cranes in the world that migrate without human help died at twice the rate they did last year, according to a story by the Associated Press.
At 5.5 feet tall, the whooping crane is the tallest bird in North America and once faced extinction, with just 15 left in 1941. They’ve since recovered to number 539. There are three North American flocks, but only one naturally migrates. Ten percent of that flock typically dies each year, but 21 percent died last year. When the birds return to Aransas National Wildlife Refuge this fall, 250—20 fewer than last year—are expected.
The cause of their decline is unknown, though habitat change and a Texas drought that reduced their food supply are possibilities suggested by Tom Stehn of the U.S. Fish and Wildlife Service. Roughly 20 of the Aransas birds are to fly off with radio transmitters next spring.