Two subspecies usually are recognized (Humphrey and Parkes 1963, Blake 1977, Dickinson 2003), but some authors consider the species to be monotypic (Payne 1979).
fostersmithi Friedmann 1949; type locality Caicara, Monaguas, Venezuela
Occurs in eastern Colombia and in Venezuela.
Similar to nominate sibilatrix, but crown is more bluish slate, less blackish; the wing coverts are more yellowish, less pinkish, cinnamon, with narrower black streaks; neck and breast light honey yellow, not light buffy olive; and the bill averages longer (see Measurements) (Blake 1977).
sibilatrix (Temminck 1824); type locality Brazil and Paraguay
Occurs from Bolivia south to southeastern Brazil, Uruguay and northeastern Argentina. See Detailed Description.
This species was described in the genus Ardea (Temminck 1824). Ridgway (1878) erected the genus Syrigma, with type species sibilatrix, and most authorities have classified it as Syrigma sibilatrix. The genus Syrigma is monotypic.
The affinties of sibilatrix long were unclear, however. It often was considered to be related to night-herons, and some authors even merged Syrigma in the genus Nycticorax (e.g. Bock 1956). Payne and Risley (1976), using a phenetic analysis of morphological characters, concluded that Syrigma was not related to night-herons, and instead was a basal member of the day herons (Ardeinae). Investigations of the systematics of herons based on DNA hybridization (Sheldon 1987, McCracken and Sheldon 1997, Sheldon and Slikas 1997) and on phylogenetic analysis of mitochondrial DNA sequence data (Sheldon et al. 2000) also classify Syrigma among the day herons, and suggest that Syrigma is sister to Egretta.