Around 200 years ago, the passenger pigeon (Ectopistes migratorius) was the world’s most abundant bird. Although the species was found only in eastern North America, it had a population of between three and five billion (25 percent of the North American land bird population). Overhunting caused a chain of events that reduced their numbers below minimum threshold, causing them to go extinct. In the 1890s, several states passed laws to protect the pigeon, but it was too late. The last known wild bird was shot in 1900; the last passenger pigeon in captivity, named Martha, died on September 1, 1914, in the Cincinnati Zoo.

Not all animals have just an “endangered” or “extinct” label. Over time, scientists have determined different levels of how an animal species is either thriving or becoming dangerously close to extinction. But extinction and how scientists categorize the status of a species happen in steps. According to the International Union for Conservation of Nature and Natural Resources, the following is a breakdown of some of these levels toward extinction: Not evaluated; Data deficient; Least Concern; Near threatened; Vulnerable; Endangered; Critically endangered; Extinct in the wild; and Extinct. For example, endangered animals in the United States include the Red-Cockaded woodpecker (Picoides borealis), a species in the southern United States; the Alabama Red-Belly turtle (Pseudemys alabamensis), a reptile that feeds almost entirely on aquatic plants; and, believed to be extinct in the 1950s but “rediscovered” in the 1970s, the 8-ounce (224-gram) Mount Graham Red Squirrel (Tamiasciurus hudsonicus grahamensis), found only in the area of Mount Graham in Arizona.