The West Virginia Northern Flying Squirrel (Glaucomys sabrinus fuscus) is a 'small, nocturnal, gliding mammal' with a 'long, broad, flattened tail ..., prominent eyes, and dense, silky fur' that lives in West Virginia and Virginia. In 1985, when scientists had documented only ten living squirrels, the Fish and Wildlife Service (FWS) concluded it was endangered. As required by § 4(f) of the Endangered Species Act, FWS created a recovery plan for the 'conservation and survival' of the squirrel, enumerating the following 'criteria which, when met, would result in a determination ... that the species be removed from the list' of endangered species. In 2002, FWS hired a biologist to investigate the possibility of removing the squirrel from the list of endangered species. Whereas only ten squirrels had been sighted at the time of the original listing in 1985, by 2006 scientists had captured 1,063 individual squirrels at 107 sites, which suggested to D the population was robust. FWS proposed to remove the squirrel from the list of endangered species. It was removed from the list. P filed a complaint in the district court claiming (1) promulgation of the Delisting Rule violated the Endangered Species Act by ignoring the objective, measurable criteria in the Recovery Plan and (2) the Rule itself was arbitrary and capricious because it was not based upon the best available science. The district court entered summary judgment for P on the ground D was bound by the criteria in the Recovery Plan and its decision to delist the squirrel without following those criteria, therefore, constituted a revision to that plan, made without going through notice and comment rulemaking as required by the Act. D appealed.