"The Birdman of Alcatraz" refers to Robert Stroud, a notorious criminal who was imprisoned at Alcatraz Federal Penitentiary in the 20th century. Stroud became famous for his work with birds, particularly canaries, which he raised and studied while serving his life sentence. His passion for ornithology led to the publication of a book on the subject. Despite his reputation as a violent criminal, Stroud's relationship with birds showcased a different side of his personality. He was often depicted as a misunderstood figure, and his life inspired films and books, solidifying his legacy in American criminal history.