This block of four stamps was issued to raise awareness of the plight of endangered species. The polar bear, the California Condor (one of the rarest birds in the world) and the alligator have all been threatened with extinction. Increased water pollution has also put several species of trout in jeopardy. Wildlife Conservation Series On May 5, 1956, the US Post Office issued the first stamp in a 22-year series calling attention to the importance of wildlife conservation. Initially, the series began in 1956 with three separate stamps honoring examples of successful conservation work carried out by the federal and state governments. The first stamp in the series honored the wild turkey. It was issued on May 5, 1956, at the convention of the Wisconsin Federation of Stamp Clubs, in Fond du Lac, Wisconsin. The wild turkey, the largest and fastest of the game birds, was selected for this stamp because of the successful work in restoring the birds to their native homes in eastern, southwestern, and middle western states. The second stamp in the series was issued on June 22, 1956, at the annual convention of the Izaak Walton League in Gunnison, Colorado, an environmental organization founded in 1922. The league promotes the protection of natural resources, as well as outdoor recreation. The pronghorn antelope was selected for this stamp because it too has benefitted from conservation efforts. Before the arrival of European settlers, the pronghorn population was an estimated 25 million. By 1920, that number had fallen to about 17,000. Conservation efforts helped the pronghorn antelope increase to approximately half a million today.