File:Sunbittern (Eurypyga helias) (3419492379).jpg

The sunbittern is usually placed in the Gruiformes, but this was always considered preliminary. In some aspects of its morphology the Sunbittern is close to the herons and their relatives (which include bitterns), but this appears to be convergent evolution due to similar lifestyles. Altogether, the bird is most similar to another enigmatic bird provisionally placed in the Gruiformes, the Kagu. Molecular studies (e.g. Fain & Houde 2004) seem to confirm that the Kagu and Sunbittern are each other's closest living relatives. 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 possibly also includes 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.

The rather comprehensive DNA study by Hackett et al confirms that the sunbittern and the Kagu are close relatives but rather surprisingly it puts these two as a sister group of the swifts, nightjars and hummingbirds.