WASHINGTON D.C.: The U.S. Fish and Wildlife Service declared the extinction of the ivory-billed woodpecker and 22 other birds, fish and other species, noting its scientists said they exhausted their efforts to find any of the 23 species.

The scientists also warned about the issue of climate change, which could make extinction more common.

The ivory-billed woodpecker is the best known species declared extinct by the U.S. government, while others, such as the flat pigtoe, a freshwater mussel from the southeastern U.S., were only spotted in the wild a few times and never seen again.

“When I see one of those really rare ones, it is always in the back of my mind that I might be the last one to see this animal again,” said Biologist Andy Ford, as quoted by the Associated Press.

All 23 extinct species were added to the endangered species list in the 1960s, when they were thought to have a slim chance of surviving.

In response to the extinctions, wildlife officials stressed they will again enforce the Migratory Bird Treaty Act to punish companies responsible for preventable bird deaths, but prosecutions stopped for several years during the Trump presidency.

Some 902 species have been documented as extinct globally, but the actual number is considered to be much higher.

However, some scientists noted it is possible for one or more of the 23 species to reappear.

Cornell University bird biologist John Fitzpatrick, who claimed the ivory-billed woodpecker was rediscovered in a 2005 study, said it was premature to declare its extinction, adding, “Little is gained and much is lost with an extinction declaration,” as reported by the Associated Press.

In addition, the Switzerland-based International Union for Conservation of Nature has not placed the woodpecker in its extinction column because it could possibly still exist in Cuba, said the union’s Craig Hilton-Taylor.

Since 1975, 54 species have recovered enough to be taken off the endangered list, including the bald eagle and humpback whales.

But climate change, which has caused drought, floods, wildfires and unstable temperatures, is making the recovery of species uncertain.