Two young whooping cranes that trailed an ultralight plane from Idaho to winter grounds in central New Mexico continue their trek north for spring, a sign they are catching on to the ways of the wild.

The birds left Monte Vista National Wildlife Refuge in southern Colorado this weekend along with some young sandhill cranes, said Hans Stuart, a spokesman for the U.S. Fish and Wildlife Service.The two endangered birds - which spent the winter at Bosque del Apache National Wildlife Refuge near Socorro - are believed to be out of the San Luis Valley in southern Colorado, but still in that state, Stuart said.

"It's very encouraging and good that the birds are doing what wild cranes do," he said. "They are flying north like wild birds do. We just don't know where they're going."

They could end up anywhere from northern Utah to western Wyoming to southeast Idaho, where the flock of intermountain sandhill cranes spends the summer, Stuart said.

People in Colorado are watching for the rare birds, particularly at a stopover point between Delta and Montrose, Colo., used by some of the intermountain flock, he said.

Stuart said the two whoopers were among the last birds to leave the San Luis Valley, which is normal for young cranes.

The birds were part of an experiment in which Idaho rancher and researcher Kent Clegg, flying an ultralight plane, led whoopers south to New Mexico last fall.

The young whoopers began their migration north last month without human intervention.

Clegg now is on the road in Colorado, trying to locate the two whoopers, one of whom has a satellite transmitter, Stuart said.

Researchers for years have tried to get whooping cranes to migrate to new habitats to help establish a second migratory flock. The only existing migratory flock spends winters at Aransas National Wildlife Refuge on the Texas Gulf Coast and summers in Canada.