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Systematics
Geographic Variation
Three subspecies currently recognized (Amadon 1975) based on morphometrics from small samples (Baird et al. 1874, Friedmann 1933, 1950, Nelson and Goldman 1933). Plumage colors and patterns essentially similar, as is color of soft parts for all forms (Friedmann 1950, Amadon 1975). Brown and Amadon (1976) and Beissinger (1988) questioned the separation of this raptor into 3 subspecies based on size variation. Means from larger samples (see Appendix; PWS, REB), however, indicate that body mass and bill, wing, and tail length are largest for R. s. major; wingspread largest for R. s. plumbeus; bill about the same for R. s. plumbeus and R. s. sociabilis; and wing, tail, and tarsal length, wingspread, and body mass smallest for R. s. sociabilis . Full statistical analysis of geographic variation within and among populations is needed.
Subspecies; Related Species
Much variation among each measured element within each subspecies (Appendix).
R. s. plumbeus Ridgway 1874 (type locality Everglades, FL), peninsular Florida, Cuba (including Isla de la Juventud), and nw. Honduras. Subspecies R. s. levis (Friedmann 1933) of Cuba is now considered a synonym of R. s. plumbeus (Amadon 1975). Also, Monroe (1968) referred birds in Honduras (n = 11 skins) to R. s. plumbeus rather than nominate R. s. sociabilis based on wing and culmen measurements. Diagnosis: large subspecies with small bill similar to R. s. sociabilis; described in Appearance section.
R. s. major Nelson and Goldman 1933 (type locality Catemaco, Veracruz, Mexico), se. Mexico (including states of Veracruz, Oaxaca, Tabasco, Chiapas, Campeche, and Quintana Roo), n. Guatemala and around Panzós, and northern half of Belize. Diagnosis: largest of 3 subspecies, particularly with regard to bill size.
R. s. sociabilis Vieillot 1817 (type locality Rio de la Plata and Corrientes District, Argentina), s. Nicaragua south in suitable habitat through Panama and in South America from Colombia east to Venezuela, Guyana, Surinam, French Guiana, Trinidad, and most of Brazil, south to Ecuador, ne. Peru, e. Bolivia, and n. Argentina (south to Prov. Buenos Aires). Diagnosis: smallest of 3 subspecies.
Closely related to Slender-billed Kite (R. hamatus) of e. Panama (Ridgely and Gwynne 1989), with discontinuous distribution in South America, including n. Colombia, Venezuela, Surinam, e. Peru, and Amazonian Brazil (Meyer de Schauensee 1966). R. hamatus is a swamp or wet-forest species whose diet also consists mainly of snails of genus Pomacea (Beissinger et al. 1988, Ridgely and Gwynne 1989).
Sykes, Jr., P. W., J. A. Rodgers, Jr. and R. E. Bennetts. 1995. Snail Kite (Rostrhamus sociabilis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/171
