LITTLE ROCK, Ark.— A federal judge temporarily stopped construction on a $320 million irrigation project yesterday, 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 Wilson said that, for purposes of the lawsuit brought by environmental groups, he had to presume the woodpecker exists. 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 Army Corps of Engineers began building the Grand Prairie Irrigation Project last year, about 14 miles from where the bird was reportedly spotted.

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