BILLINGS, Montana (AP) – Death has come knocking a last time for the splendid ivory-billed woodpecker and 22 more birds, fish and other species: The U.S. government on Wednesday declared them extinct.

It is a rare move for wildlife officials to give up hope on a plant or animal, but government scientists say they have exhausted to find these 23.

The ivory-billed woodpecker was perhaps the best-known species the U.S. Fish and Wildlife Service declared extinct. The woodpecker went out stubbornly and with fanfare, making unconfirmed appearances in recent decades that ignited a frenzy of ultimately fruitless searches in the swamps of Arkansas, Louisiana, Mississippi and Florida.

Others such as the flat pigtoe, a freshwater mussel in the southeastern U.S., were identified in the wild only a few times and never seen again, meaning by the time they got a name they were fading from existence.

“When I see one of those really rare ones, it’s always in the back of my mind that I might be the last one to see this animal again,” said Anthony “Andy” Ford, a U.S. Fish and Wildlife Service biologist in Tennessee who specializes in freshwater mussels.

The factors behind the disappearances vary—too much development, water pollution, logging, competition from invasive species, birds killed for feathers and animals captured by private collectors.

But all 23 have one thing in common: they were thought to have at least a slim chance of survival when added to the endangered species list beginning in the 1960s. Only 11 species previously have been removed due to extinction in the almost half-century since the Endangered Species Act was signed into law.

The announcement kicks off a 60-day comment period before the species status changes become final. Wildlife officials citing the extinctions also said they would resume criminal enforcement of the Migratory Bird Treaty Act to punish companies responsible for preventable bird deaths.

Around the globe, some 902 species have been documented as extinct. The actual number is thought to be much higher because some are never formally identified, and many scientists warn the earth is in an “extinction crisis” with flora and fauna now disappearing at 1,000 times the historical rate.

It is possible one or more of the 23 species named Wednesday could reappear, several scientists said.

A leading figure in the hunt for the ivory-billed woodpecker said it was premature to call off the effort, after millions of dollars spent on searches and habitat preservation efforts.

“Little is gained and much is lost” with an extinction declaration, said Cornell University bird biologist John Fitzpatrick, lead author of a 2005 study that claimed the woodpecker had been rediscovered in eastern Arkansas.

“A bird this iconic, and this representative of the major old-growth forests of the southeast, keeping it on the list of endangered species keeps attention on it, keeps states thinking about managing habitat on the off chance it still exists,” he said.

Since 1975, 54 species have left the endangered list after recovering, including the bald eagle, brown pelican and most humpback whales.