Evolution and Systematics
Discussion of Phylogenetic Relationships
This tree represents a considerable reorganization of avian relationships on the basis of two extensive genetic studies (Hackett et al. 2008, Ericson et al. 2006). Several traditional orders have been rearranged, and relationships among orders have been greatly altered. Orders may not have their traditional contents. Families and genera listed independently here have been removed from their traditional orders.
Some recent studies of morphology (e.g. Livezey and Zusi 2007) have strongly contradicted this arrangement and have supported traditional relationships.
"Land Birds" and "Water Birds" are informal names for two large clades within Neoaves, each encompassing several traditional orders.
Gruiformes, previously a very diverse order, has been considerably reduced: Eurypygidae (sunbittern), Rhynochetidae (kagu), Otididae (bustards), and Mesitornithidae (mesites) are traditionally considered gruiforms but do not belong there, nor do Cariamidae (seriemas), which are placed within "Land Birds", or Turnicidae (buttonquails), which are placed within Charadriiformes. Phaethontidae (tropicbirds) likewise are traditionally considered to belong to Pelecaniformes, within "Water Birds", but do not belong there.
Caprimulgiformes traditionally includes Aegothelidae (owlet-nightjars), but this familly is more closely related to Apodiformes. Relationships among the remaining families of Caprimulgiformes are currently not well resolved, and we don't yet know whether Caprimulgiformes (with owlet-nightjars removed) is monophyletic or paraphyletic.
Fain and Houde (2004) have suggested that Neoaves is divided into two clades, Coronaves (taxa from "Land Birds" through Musophagiformes on this tree) and Metaves (Opisthocomiformes through Phoenicopteriformes). However, support for this hypothesis is so far restricted to a single gene, and confirmation is needed.
The relationship between Podicipediformes (grebes) and Phoenicopteriformes (flamingos), odd as it seems, was first hypothesized by van Tuinen et al. (2001), and has been confirmed by nearly every subsequent molecular analysis.