http://green.blogs.nytimes.com/2012/02/02/a-whooping-crane-migration-will-finish-by-truck/?src=rechp

An effort using ultralight planes to coax essentially orphaned
whooping cranes all the way from Wisconsin to their wintering grounds in Florida
has ended for the season. The problem this time isn’t a possible violation of
Federal Aviation Administration rules, but rather the birds themselves.

"The birds haven’t been cooperative," said Liz Conde, a spokeswoman for
Operation Migration, the group that organizes the flights. "The birds are
refusing to follow the ultralights any further."