MADISON, Wis. — A conservation organization that uses ultralight planes to lead endangered whooping cranes from Wisconsin to Florida for the winter has to replace its aircraft by next spring to comply with Federal Aviation Administration regulations.

Operation Migration ran into trouble with the FAA because it pays salaries to pilots. FAA regulations say sport planes — a category that sometimes includes aircraft of exotic design — can only be flown for personal use.

In addition to buying three new $20,000 aircraft with support from donors, pilots were required to obtain private pilot licenses, co-founder and pilot Joe Duff told the Wisconsin State Journal (http://bit.ly/157jF0u ).

The FAA allowed Operation Migration an exemption from the ultralight rules until April 30, 2014. The Canada-based conservation nonprofit hopes to have its new planes by January or February.

"It was definitely a stressful time for us," Duff said. "We're fortunate that the FAA wants to work with us and wants us to continue what we were doing. There was just no space for us in the rules."

Duff said to successfully guide the whooping cranes, Operation Migration's aircraft must have small 50-horsepower engines and travel 30 to 50 mph so the birds can keep up.

The new fleet also must have bird-friendly propeller guards and eventually will have speakers that play a louder version of the comforting, brooding sound a mother whooping crane makes to her chicks.

"Even though we're the regulators, we believe what they do is a good thing and we want to help them achieve their mission," FAA spokeswoman Elizabeth Cory said.

Operation Migration pilots have successfully taught new generations of Wisconsin-raised whooping cranes to migrate south for the long Midwestern winter months. After being nearly wiped out in the 1940s, there are now 600 of the whooping cranes nationwide.