BLOOMINGTON -- Twenty whooping cranes led by four ultra-light aircraft are poised to fly over Central Illinois as early as Saturday on their way to wintering grounds in Florida.

Weather could stall them in northern Illinois for days, said Liz Condie, spokeswoman for Operation Migration, a Wisconsin-based project to reintroduce the endangered birds to the wild.

They need calm skies to fly daily legs averaging 50 miles, she said. Warm temperatures mean southerly headwinds that stymie their progress, she said.

Ideally, the crew wants cold, crisp mornings with northwest breezes of about 5 mph.

Friday's winds topped 30 mph, so the aerial caravan never left its resting place in Winnebago County. The exact stopover sites are not publicized to avoid too much human contact with the cranes.

The next rest point is scheduled for LaSalle County, where they will pause briefly to celebrate Operation Migration's 10,000th mile leading cranes since 2001. A Livingston County stop is scheduled to follow a day later.

Last year was the first time the birds flew over Central Illinois. They traveled from Necedah National Wildlife Refuge in central Wisconsin, where they are reared by humans wearing white clothes and hoods, to Chassahowitzka and St. Mark's national wildlife refuges on Florida's Gulf Coast.

Over the years, Operation Migration has escorted more than 100 birds on the trip to imprint the route on the birds, taking 23 days in flight to cover 1,285 miles. This year's migration left Oct. 16 after enduring weather delays for six days.

The population of whooping cranes in the United States fell to under 20 birds by World War II. The Wisconsin-to-Florida flock now numbers nearly 80 birds. The goal is to reach the self-sustaining level of 125 individual birds and 25 breeding pairs. A separate flock of cranes numbering about 220 migrates between Texas and Canada.

On the Net: www.operationmigration.org