Little Rock. Ark. ? A federal judge halted a $320 million irrigation project Thursday for fear it could disturb the habitat of a woodpecker that may or may not be extinct.

The dispute involves the ivory-billed woodpecker, seen above in a 2005 illustration. The last confirmed sighting of the bird in North America was in 1944, and scientists had thought the species was extinct until 2004, when a kayaker claimed to have spotted one in the area. But scientists have been unable to confirm the sighting.

Still, U.S. District Judge William R. Wilson said that for purposes of the lawsuit brought by environmental groups, he had to assume the woodpecker exists in the area. And he ruled that federal agencies may have violated the Endangered Species Act by not studying the risks fully.

The National Wildlife Federation and the Arkansas Wildlife Federation had sued the Army Corps of Engineers, arguing that the project on the White River would kill trees that house the birds and that noise from the station would cause the woodpeckers stress.