Why are they so scarce? Evolutionary pigeonholed into a no-return extinction vortex? It happens.
I would guess it is global warming.
From what I have read there never were many.
For the first time since the late 1800s, there are more than 500 Whooping Cranes in the population that winters in south Texas.
U.S. Fish and Wildlife Service aerial surveys counted 505 cranes in and around the Aransas National Wildlife Refuge in 2018 as a part of their annual winter survey, a 17 percent increase from the previous year. These cranes—which migrate to Aransas from breeding grounds in Canada’s Wood Buffalo National Park—comprise the largest of four populations of endangered Whooping Cranes left in North America. The Aransas–Wood Buffalo group is the only self-sustaining Whooping Crane population on the continent, meaning they breed and migrate without human assistance.
There were concerns for these cranes after Aransas took a direct hit from Hurricane Harvey in September 2017. But the refuge’s marshes weathered the storm just fine and continued to provide excellent habitat for Whooping Cranes, says Wade Harrell, U.S. Whooping Crane recovery coordinator for the USFWS.
By 1941, there were only 15 Whooping Cranes left in the wild, all of which wintered at Aransas. Whooping Cranes were charter members of the Endangered Species Act when it was signed in 1973, and for years the cranes’ recovery was slow but steady. But now the Aransas–Wood Buffalo flock (which contains about 80 percent of all wild Whooping Cranes) has doubled since 2010.