1989. 35 pp. Paperback in near fine condition. Light shelf wear. The idea of coding Mexican birds according to the degree of difficulty of observing them, originated with the similar listings by James Vardaman and Benton Basham for their respective Big Year efforts for N. American birds. The author believes that a similar classification for Mexico will help birders and increase the knowledge of the avifauna of this country. These codes reflect the degree of difficulty of observing the species in the country as a whole.