"The easy part is identifying them. The hard part is protecting them," Audubon President John Flicker told Reuters.

Among the U.S. sites designated on Wednesday was the San Francisco Bay area, where once extensive wetlands have been overtaken by development.

"Every year one million birds rely on the wetlands and associated habitats of the San Francisco Bay region during their migration along the Pacific Flyway," Flicker said, referring to the coastal strip reaching from Alaska to Chile where many migratory birds travel.

In the Bay Area, Audubon has sought to raise money to restore some 100,000 acres (40,500 hectares) of destroyed wetlands to revive what is the largest estuary along the entire Pacific coast.

Other critical bird habitats in the United States include regions ranging from the Commanche National Grassland in Colorado and Georgia's Okenfenokee National Wildlife Refuge to the Jamaica Bay Complex in New York City's borough of Queens and the C&O Canal running through Washington D.C.

"America's birds are in trouble, and this is a chance for individuals to make a difference," Flicker said.

The global program, which is also under way in Europe, Africa and the Middle East, uses local volunteers to find and identify important bird habitats.