Two North American subspecies of the Yellow-billed Cuckoo (Coccyzus americanus) have been defined by a difference in size: a smaller eastern (C. a. americanus) and a larger western (C. a. occidentalis). This taxonomic treatment was proposed originally by Ridgway (1887) and later followed by other workers (Peters 1940, A.O.U. 1957, Oberholser 1974), though various authors have questioned the validity of this separation (Todd and Carriker 1922, Van Tyne and Sutton 1937, Mees 1970, Banks 1988).