The sunbittern was traditionally placed in the
Gruiformes, but this was always considered preliminary. Altogether, the bird is most similar to another bird that was provisionally placed in the Gruiformes, the
kagu (
Rhynochetos jubatus). Molecular studies seem to confirm that the kagu and sunbittern are each other's closest living relatives and have a similar wing display. They are probably not Gruiformes (though the proposed
Metaves are just as weakly supported). Altogether, the two species seem to form a minor
Gondwanan lineage which could also include the extinct
adzebills and/or the
mesites, and is of unclear relation to the Gruiformes proper. Notably, the kagu and mesites also have
powder down. An indeterminate fossil eurypygid has been documented from the
Green River Formation of
Wyoming, USA. This specimen, known from a full skeleton, is the oldest and only known fossil of the group, and suggests that eurypygids had a much more northernly range in the past. This specimen has been figured in several studies, and was given the unofficial name "
Eoeurypyga olsoni" in a 2003 dissertation, but as-of-yet remains unnamed.
Subspecies The sunbittern was formerly treated as two species (
E. helias and
E. major), but now they are treated as a single species with considerable variation between the
subspecies. The three subspecies are recognised on the basis of plumage characters and size. The three subspecies are
allopatric. •
E. h. helias – Amazonian sunbittern •
E. h. major – northern sunbittern •
E. h. meridionalis – foothill sunbittern ==Distribution and habitat==