Arthur B. Singer, 72, a naturalist and painter whose watercolors of state birds became best-selling postage stamps. The 1982 stamp series featuring Singer's paintings of birds and flowers of the 50 states is believed to have been the largest-selling special issue in the history of U.S. postage. He received the Augustus St. Gaudens Medal from his alma mater, the Cooper Union Art School, after 750 of his paintings appeared as illustrations in the book "Birds of the World." He was also recognized by the National Audubon Society. In Jericho on Long Island, N.Y., on Saturday of cancer of the esophagus.