The slight variation in size and in plumage iridescence of males has been represented by 3 or by 2 subspecies; or, the variation observed has been considered insufficient for subspecific recognition. The pattern of variation intended to be described by subspecies is that populations of largest the Giant Cowbirds are centrally located within the species' distribution (in Panama and Colombia), with smaller forms to the north and to south; males of more northern populations had been described as more violaceous while central and southern populations more bronzy. Boundaries for subspecies distributions were in the areas of Belize - s. Mexico and s. Panama - Colombia; this second boundary is, perhaps, more real.
Three subspecies, if recognized (e.g., Ridgway 1902, Friedmann 1929), consist of (from north to south): M. o. impacifus (Peters 1929), M. o. oryzivorus (Gmelin 1788), and M. o. violeus (Bangs 1900). With more and better material available, Hellmayr (1937) and Blake (1968) saw no distinction between M. o. oryzivorus and M. o. violeus. Wetmore et al. (1984) argued that the amount of overlap between different populations was too great to justify any subspecific distinctions [e.g., wing measures of males: São Paulo, Brazil, 176 - 200 mm (n = 10); Santa Marta, Colombia, and w. Ecuador, 186 - 210 mm (n = 10; Hellmayr 1937)].
The genus Cassidix had been mis-applied to the Giant Cowbird until Peters (1929) supplied the name Psomocolax for the genus; however, that generic name was shown to be unnecessary since Scaphidura had been valid and available (Parkes 1954). Genetic work by Lanyon (1992; see Related species), indicate that generic allocation for Giant Cowbird should be as part of the genus Molothrus defined as brood parasitic cowbirds (see Am. Ornithol. Union 2000).