Whooping cranes declined through the nineteenth and early twentieth centuries as prairie wetlands were drained and birds were shot. By 1941 the surviving migratory flock numbered roughly fifteen individuals, wintering at what is now Aransas National Wildlife Refuge in Texas. The breeding grounds were unknown until 1954, when nests were found in Wood Buffalo National Park in northern Canada.
Recovery has been slow because the biology is slow. Cranes take several years to reach breeding age, typically lay two eggs and usually raise one chick, and pairs nest once a year. Protection of both ends of the migration, captive breeding, and reintroduction of separate flocks have raised the total to several hundred birds, with the original wild flock the largest.
The bottleneck left a mark that protection cannot undo. Genetic diversity is low and cannot be restored from anywhere, so the species remains vulnerable to disease and to a single catastrophic event on its concentrated wintering grounds.