Geographic variation represented by seven described subspecies (Blake 1968). Among these M. b. occidentalis, of western South America, is palest; M. b. cabanisii, of northwestern South America, is the largest, M. b. minimus, of northeastern South America north through the Antilles to south Florida, is the smallest.
Characters used to differentiate subspecies include the coloration of both male and female, overall size based on wing and tail length, and bill size and shape. Males of nominate M. b. bonariensis of eastern and southern Brazil and M. b. minimus which has invaded North America have similar plumage but differ in size; females of these two subspecies generally similar, excepting size, but M. b. bonariensis with top of head lighter and scapulars without distinct streaking.
Molothrus bonariensis cabanisii Cassin, 1866: Eastern Panama, tropical and lower subtropical zones of Colombia west of the eastern Andes, and eastern slope of eastern Andes; intergrades with bonariensis in southeastern Colombia. The largest subspecies. Male plumage similar to M. b. bonariensis; female slightly paler.
Molothrus bonariensis aequatorialis Chapman, 1915: Tropical zone of southwestern Colombia, south of Río Patia, and western Ecuador south to Guayaquil and Puná Island. Larger than M. b. bonariensis. Males with more violet and less blue iridescence; female darker and without buffy whitish postocular streak.
Molothrus bonariensis occidentalis Berlepsch and Stolzmann, 1892: Extreme southwestern Ecuador and western Peru east to province of Jaén in Cajamarca and south to Lima and to Ica (Schulenberg et al. 2006). Male possibly more bluish than M. b. bonariensis, similar to M. b. venezuelensis. Female differs from all other subspecies by pale upperparts, very pale and streaked underparts and conspicuous buffy postocular stripe.
Molothrus bonariensis venezuelensis Stone 1891: Tropical zone of eastern Colombia from Zulia Valley south to the eastern Llanos; northern Venezuela south in Llanos to Apure and Orinoco rivers and south of Orinoco in northwestern Amazonas and northern Bolívar. Male more richly glossed with purple than M. b. bonariensis, similar to M. b. occidentalis. Female darker than M. b. bonariensis, like M. b. minimus.
Molothrus bonariensis minimus, Dalmas 1900: Lesser Antilles north to Martinique; Tobago; Trinidad; Guianas and extreme northern Brazil in region of upper Rio Branco; expanded range through West Indies since about 1900 (Cruz et al. 1985, Post et al. 1993): Carriacou in 1899, Puerto Rico by 1955, Hispaniola by 1972, Cuba in 1982. First appeared in Florida Keys in 1985, and Florida mainland in 1987. Smallest subspecies. Male similar to M. b. bonariensis. Female with forehead and crown darker than M. b. bonariensis and with distinct streaks on scapulars and interscapulars.
Molothrus bonariensis riparius Griscom and Greenway, 1937: Lower Amazon Valley (to Obidos in northern Brazil on north bank of Amazon) west to the Río Ucayali, eastern Peru. Male similar to M. b. bonariensis. Female slightly blacker on upperparts and paler on underparts than M. b. bonariensis.
Molothrus bonariensis bonariensis Gmelin 1789: Eastern and southern Brazil, north to Mato Grosso, Maranhão, Piauí, and Ceará; eastern Bolivia, Paraguay, Uruguay and Argentina south to Chubut. Introduced into Chile and established from Coquimbo Province south to Valdivia in Los Lagos Province. Male with head, back, breast and upper abdomen silky violet-black; rest of body blue-black; wings glossy greenish black. Female with head and back grayish olive-brown; rump and tail coverts grayer; wings and tail dusky brown; underparts hair-brown.