A federal judge temporarily stopped construction on a $320 million irrigation project Thursday, ruling the changes could disturb the habitat of a woodpecker that might or might not be extinct.

The first purported sighting of the ivory-billed woodpecker in the area was in 2004, but more than 100 volunteers and researchers who spent weeks last winter trying to find conclusive evidence of its existence came back empty-handed.

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

"When an endangered species is allegedly jeopardized, the balance of hardships and public interest tips in favor of the protected species. Here there is evidence," he wrote, that the ivory-billed woodpecker may be jeopardized.

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

The last confirmed sighting of the woodpecker in North America was in 1944, and scientists had thought the species was extinct until a kayaker said he saw one in early 2004 near the White River in the big woods of eastern Arkansas.

The report caused a sensation in scientific circles and has attracted people from all over the world who hope to see the bird.

Ornithologists caught on tape a flicker of what they believed was the bird but announced this year they couldn't prove conclusively that the woodpecker still lives.

The Army Corps of Engineers began building the Grand Prairie Irrigation Project last year, about 14 miles from where the bird was reportedly spotted.

Aquifers beneath eastern Arkansas soybean, cotton and rice fields are being depleted, and a federal court in a separate case last year said that the main aquifer used for farming will be depleted by 2015 if water isn't conserved and that the region would suffer significant economic hardship if it ran dry.

Workers have already started on a pump station to draw 158 billion gallons from the White River per year. A Justice Department lawyer said this year that a one-month delay would cost the Corps as much as $264,000, and that a six-month wait would cost $3 million.

The judge on Thursday said the Corps and the Interior Department must conduct further studies before proceeding.

The agencies must evaluate any ivory-bill nests and forage sites within 2½ miles of the construction project. They also must identify and inspect for nesting, roosting and foraging all trees a foot or more across in the areas to be affected by eventual changes in water levels, Wilson said.

The Corps had conducted a study showing the project would not significantly harm the woodpecker's habitat, but the environmental groups suing it said the study was too narrow and did not comply with the Endangered Species Act.