Classification Hackett et al (2008) have established that the Kagu and Sunbittern (Eurypygiformes) have a sister relationship, quite outside the core Gruiforme clade. In turn, the Eurypygiformes are sisters to the Caprimulgiformes.

The Sunbittern's range extends from Guatemala to Brazil, in the humid neotropical forests. Sunbitterns have a generally subdued coloration, with fine linear patterns of black, grey and brown. Their remiges however have vividly-colored middle webs, which with wings fully spread show bright eyespots in red, yellow, and black. These are shown to other sunbitterns in courtship and threat displays, or used to startle potential predators.

This bird is most similar to another enigmatic bird that had been provisionally placed in the Gruiformes, the Kagu of New Caledonia. The Kagu is a grey-blue bird with long legs, nearly unable to fly and threatened of extinction.

The close genetic relationship between these two monospecific families, Sunbittern (Eurypygidae) and Kagu (Rhinochetidae), on both sides of the Pacific ocean, has suggested that they form a minor Gondwanan lineage which could also include the extinct adzebills and/or the mesites. Notably, the Sunbittern, Kagu and Mesites all have powder down, a feature that is present in a few groups of apparently unrelated birds, but not in core Gruiformes.

Hackett SJ et al (2008): Phylogenomic Study of Birds