The cranes are starting to dance.

Biologists say the two surviving whooping cranes that migrated to New Mexico with an ultralight plane last fall are healthy, socializing with fellow cranes and are starting a bouncy behavior that sometimes is a precursor to migration.But the bouncing up and down on their long, spindly legs could also mean they're noticing the dancing of other birds, are maturing - or just playing.

Only two whoopers remain from the small flock of four that came in last fall from Idaho with seven sandhill cranes and researcher Kent Clegg, the ultralight pilot. The whoopers are wintering at Bosque del Apache National Wildlife Refuge south of here with nearly 150,000 other birds of varying species.

"The two whooping cranes that are left are doing well. They're healthy and associating with each other and two older whooping cranes," said Hans Stuart of the U.S. Fish and Wildlife Service.

"Starting about a week ago, the two `ultralight' whooping cranes have started showing signs of `dancing,' a pre-migratory behavior. We're very encouraged by that," Stuart said. "Starting now, lots of cranes do this little bit of dancing. It becomes more and more frequent."

It is common among whoopers and sandhill cranes, he said.

Tom Stehn, the agency's director of whooping crane recovery, said the dancing might just as easily be unrelated to migration.

"They can dance for lots of reasons, including just excitement," Stehn said by phone from his home in Aransas Pass, Texas, near the Aransas National Wildlife Refuge. "Dancing at their age would probably be just like playing.

"They've been watching other cranes starting to dance, so they're probably imitating the other cranes," he said.

Besides the ultralight birds, two adult whooping cranes transplanted in an earlier federal program have been wintering at the Bosque and migrating with sandhills. The juveniles are socializing with the two adult whoopers and with each other, Stuart said.

At Aransas, on Texas' Gulf Coast, the Fish and Wildlife Service oversees the only migratory flock of whooping cranes in the world. The whooper population there numbers 180. Overall there are about 370 whoopers, including nonmigratory birds. That represents a major recovery since the mid-20th century, when there were just 22 whoopers, including 16 at Aransas. The birds had been depleted mainly by a loss of wetland habitats to agriculture.

Stehn says the recovery won't be complete until there is at least one additional migratory flock.

That's where ultralight planes come in.

If scientists can teach whooping cranes to migrate from summer habitats in Canada to a proposed new site in the southeastern United States - probably Florida or Louisiana - then they hope to establish that second flock as a hedge against any catastrophic event that might decimate the Aransas flock.

Until the migratory flight with Clegg, who has worked with cranes at his Idaho ranch for some 20 years, scientists did not know if the technique would work with whooping cranes.

Four of eight whooper hatchlings died even before last fall's migratory flight, and two others were victims of predators - coyote and bobcat - in November and December at Bosque del Apache.

Stuart says the surviving whoopers are now very much aware of predators, especially coyotes.

"During times when coyotes are in the area, they do recognize them and move away. So it's good they've learned that about predators," he said.

But even though only two survive, Stehn said Clegg's 800-mile odyssey is more than a milestone.

"It's very much a success - he made it. We've got two birds alive when we didn't even know if this was possible," Stehn said.