Unique to North America, the five-foot-high whooping crane is the tallest bird of the continent. By 1941 there were less than 24 of the birds remaining. U.S. expansion, particularly land development and rural electrification, played havoc with the cranes' migration route: tidal flats and salt marshes of Texas to previously unknown spots in northern Canada. Wetlands used by migrating birds and wintering cranes for food and nesting grounds were being converted to dry farmland. As the situation continued to worsen, a bush pilot flying over a remote part of Alberta in 1954 discovered their summer breeding grounds. Efforts to preserve the cranes intensified, with U.S. government scientists flying eggs from Canada to the United States to build "backup" flocks. Even with some setbacks, the number of whooping cranes has increased to the point where the species has been upgraded from "endangered" to "threatened."