Among the many subjects Leonardo studied, the possibility of human mechanical flight held particular fascination. He produced more than 35,000 words and 500 sketches dealing with flying machines, the nature of air, and bird flight. These investigations of flight are scattered throughout the many da Vinci codices and manuscript collections, but he did produce one short codex almost entirely on the subject in 1505-1506, the Codice sul volo degli uccelli (Codex on the Flight of Birds).