Systematics

Three subspecies recognized:

Eurypyga helias major: Described by Hartlaub in 1844 as Eurypyga major, with a type locality of Colombia. Occurs from Central America south to western Colombia and western Ecuador. Bill is heavier than in nominate helias; upperparts primarily gray, with narrower black bars (Ridgway and Friedmann 1941, Blake 1977).

Eurypyga helias helias: Described by Pallas in 1781, with a type locality of Suriman. The widespread subspecies of lowland South America, found in Colombia, Venezuela and the Guianas south through Brazil, east Bolivia, northeast Peru, and east Ecuador (Haverschmidt 1968). Bill is more slender than the other subspecies, and the back shows wide black barring with buffy interspaces (Ridgway and Friedmann 1941, Blake 1977).

Eurypyga helias meridionalis: Described by Berlepsch and Stolzmann in 1902, with a type locality of La Merced, Chanchamayo, Peru. Occurs on the lower slopes and foothills of the Andes of Peru (Schulenberg et al. 2007), from Amazonas south to Puno. Less barred above than nominate helias, and with grayer upperparts; the colors of the bill and tarsi perhaps also are brighter (Blake 1977, Schulenberg et al. 2007).

Mindell (1997) offers a good summary of theories on the classification within the order Gruiformes before more recent developments in molecular techniques, including Olson's 1985 proposal that the Sunbittern is part of an ancient group including herons and roatelos. Cracraft (1982) placed the Sunbittern closer to the Kagu of New Caledonia (Rhynochetidae) based on cranial osteology, but also claimed that the Kagu is more closely related to the extinct Aptornis. Sibley and Ahlquist (1990) looked at DNA hybridization and did not place the Sunbittern near the Kagu, but rather between rails (Rallidae) and bustards (Otididae).

The method of 12S rDNA analysis employed by Houde et al. (1997) supported a tree which recognized Sunbittern and Kagu as members of the same clade, exluding Aptornis.

More recently, Hackett et al. (2008) analyzed aligned nuclear DNA sequences from 19 independent loci, and, in addition to many other structural rearrangements to the phylogenetic tree of birds, supported the classification of Sunbittern as a sister group to the Kagu, with these two families located outside the core Gruiform group.