More on Metaves
Metaves breaks into two parts. The first contains what looks like a rather heterogeneous collection of families: sandgrouse, pigeons, tropicbirds, mesites, flamingos, grebes. The other group contains kagu and sunbittern in a basal position, followed by the nightjars and allies, hummingbirds, and swifts.
Following Huxley (1867), the nightjars and relatives (potoos, oilbird, frogmouths) and the Apodiform grouping of owlet-nightjars, swifts, treeswifts, and hummingbirds are collectively referred to as the Cypselomorphae. The position of the owlet-nightjars is surprising as they had previously been thought closer to the main nightjar group. The exact relation of the other Cypselomorphae remains uncertain (e.g., Barrowclough et al., 2006; Mariaux and Braun, 1996; Mayr, 2002). In fact, they usually do not form a clade in DNA studies! (The oilbirds are particularly problematic.) Hackett et al. (2008) and Ericson et al. (2006a) are two of the few where they are a clade.
Given the lack of solid information, and uncertainty about the relationships of these birds, I've put the Apodiformes (with owlet-nightjars) in one order, and separated the nightjars, frogmouths, potoos, and oilbird in their own orders Caprimulgiformes, Podargiformes, Nyctibiiformes, and Steatornithiformes. It may be possible to consolidate some of these orders in the future, or perhaps one will move to an entirely different place on the tree.
In comparison, in the Sibley-Monroe list, the families in Metaves are scattered, but many are recognized as problematic. The Sibley-Monroe list has the Podicipediformes (grebes) and Phoenicopteriformes (flamingos) as isolated orders with unclear affinities. The Phaethontidae (tropicbirds) are in the Pelecaniformes (Brown et al. (2008) group them with the hawks). The Rhynochetidae (kagu), Eurypygidae (sunbittern), and Mesitornithidae (mesites) are considered Gruiformes. The Pteroclidae (sandgrouse) are considered Charadriiformes and the Columbiformes (pigeons and doves) are an isolated order.
The Cypselomorphae fare better. Gill (1995) had already noted the Caprimulgiformes may be related to the Apodiformes.
Coronaves
There seem to be three major branches in Coronaves. One is primarily waterbirds (Pelecanae), one includes the shorebirds (Charadriae), and one consisting of land birds (Passerae).
Pelecanae: Waterbirds, Waders, and Cuckoos
The Pelecane consist of mostly aquatic and semi-aquatic bird families (plus hoatzin, cuckoos and turacos). I used the old name ‘Natatores’ for similar groupings in earlier versions of this list. Natatores was a name used in the 19th century for waterbirds; although it has been attributed to Baird 1858, its ornithological use substantially predates him. The terms Conglomerati and Cracrafti have also been used for similar groupings that also include raptors.
The hoatzin is basal in the Pelecanae, so we put it in a separate superorder, Opisthocomimorphae. The remainder then splits into two parts. The first contains the bustards, cuckoos, cranes and rails (Otidiimorphae). The other contains turacos (Musophagimorphae) and seabirds and waders (Pelecanimorphae).
The Pelecanimorphae is an assemblage of water and wading birds. This includes most pelagic species (except tropicbirds) and the large waders (but not sunbittern or flamingos). I break this group into several orders because there is a substantial chance that more rearrangement will be needed in the future. This group is the core of what Sibley and Monroe called the Ciconiiformes. Here, the Ciconiiformes have been completely dismantled, with only Ciconiidae (storks) remaining. The other groupings are the Gaviiformes (loons), Sphenisciformes (penguins), Procellariiformes (seabirds), Suliformes (frigatebirds, boobies, anhingas, and cormorants), Pelecaniformes (hamerkop, shoebill, and pelicans), and Ardeiformes (herons and ibis).
There are also taxonomic issues in the families inside the Pelecaniformes. The Boat-billed Heron was previously considered to be the only member of the Cochlearidae. The status of two other monotypic families, the Shoebill and Hammerkop, has also been a perennial issue. The analyses of Ericson et al. (2006a) and Hackett et al. (2008) indicate that both are relatives of the pelicans. Indeed, the tree allows them to be lumped into the same family. We keep them separate not only because of their uniqueness, but also because the division between them seems to be ancient.
The Gruiformes are another area of major change, with three families lost to Metaves, one lost to Charadriiformes, one lost to Cariamiformes, and one (Otididae, bustards) becoming its own order near the Gruiformes. The flufftails (Sarothruridae) are separated from the Rallidae as they seem more closely related to the finfoots. The Musophagidae (turacos) are now considered sister to the Pelecanimorphae.
Charadriae: Shorebirds, Gulls, Terns, and Alcids
There is a lot of evidence for the Charadriae grouping (e.g., Ericson et al., 2003a; Paton et al., 2003; Cracraft et al., 2004). Other than arguments about the Herring Gull complex, the taxonomy of this clade is now pretty well worked out. Many studies have found that gulls and alcids are closely related to the shorebirds. A recent development is the recognition that the buttonquails are part of it (Paton et al., 2003; Paton and Baker, 2006; Fain and Houde, 2007). The differences from the Sibley-Monroe list are the loss of the sandgrouse (to Metaves), the addition of the buttonquail, splitting the terns as a separate family, and some reordering of the families.
Passerae: Land Birds
The third branch of Coronaves is Passerae. It is probably best thought of consisting of three pieces: Accipitrimorphae (hawks and American vultures), ‘Anomalogonatae’ (the “higher land birds”), and the Passerimorphae (seriemas, falcons, parrots, and passerines).
A notable feature of this taxonomy is that the American Vultures are classified in their own order, Cathartiformes (the SACC has now also adopted this classification). I have done this partly to reflect the considerable uncertainty about where they fit in. Contrary to Sibley-Monroe and 1990's-style checklists, they are not closely related to storks (see Cracraft et al., 2004; Gibb et al., 2007; Slack et al., 2007). Keep in mind that the vultures also appear no more related to the Accipitriformes than they are to anything else in this superorder.
I have followed Hackett et al. (2008) concerning the placement of the hawks, falcons, and American vultures (Accipitriformes, Falconiformes, and Cathartiformes). However, this is controversial and several alternatives tree should be considered. Slack et al. (2007) put all three orders in Pelecanae. Gibb et al's (2007) results are more ambiguous. They present a tree showing the all three in a group that includes other members of both parts of Coronaves. They also present a network diagram that could be consistent with placing the hawks and falcons between Pelecanae and Passerae. The fact that Gibb et al. include more birds from Passerae (an owl and parrot) in their analysis than Slack et al. did may account for this.
One difference from recent AOU lists is the treatment of the Falconiformes, Accipitriformes, and Cathartiformes. Also, the Cariamidae (seriemas) are moved from the Gruiformes to the Falconiformes. The “terror birds” of ancient South America (Phorusrhacidae) are thought to be related to the seriemas (Alvarenga and Höfling, 2003).
The original Anomalogonatae were named by Garrod (1874), with the Strigiformes added later by Beddard (1898). Most of them have remained together in most taxonomic lists since then. One important defining character was the lack of an ambiens muscle (also lost by some unrelated birds). The ‘true Anomalogonatae’ are Beddard's Anomalogonatae, minus the Passeriformes and Cypselomorphae. This leaves a core group consisting of the Leptosomiformes, Strigiformes, Coliiformes, Trogoniformes, Bucerotiformes, Coraciiformes, and Piciformes. Ericson et al. (2006a) and Hackett et al. (2008) found it a monophyletic group, a group that includes a common ancestor and all descendants.
Other than their ordering and placement in the ‘Anomalogonatae’, the treatment of much of the remaining non-passerine families is close to that of the Sibley-Monroe list. Changes over time have mostly involved whether to consider certain groups families or sub-families. One interesting case is the Cuckoo Roller. It was originally considered a cuckoo, some affinities with the rollers were noted, and it has more recently been considered its own family, Leptosomidae. Although it belongs in ‘Anomalogonatae’, its exact position is still unsettled.
The owls, mousebirds, trogons are also placed in separate orders. The hornbills, which are split into Bucorvidae (ground-hornbills) and Bucerotidae (hornbills) form a grouping with the hoopoes and woodhoopoes. The rollers, bee-eaters, todies, motmots, and kingfishers form the Coraciiformes. The consensus seems to be to leave the three kingfisher subfamilies as subfamilies, and I follow that. That brings us to the Piciformes.
The classification of the Piciformes follows the AOU's South American Classification Committee rather than the AOU checklist. This means the Capitonidae (American barbets), Semnornithidae (toucan-barbets), and Ramphastidae (toucans) have family status and that requires the Asian and African barbets each have their own family (Moyle, 2004; Johansson and Ericson, 2003). All of the new world forms are more closely related to each other than to the old world barbets. The tree makes this clear.
This brings us to the Passerimorphae, which includes the seriemas, falcons, parrots, and passerines. All of the non-passerines together amount to about 4000 species. The rest of the birds, almost 6000 species, are Passerines, songbirds. They are all members of a single order—Passeriformes.
Passeriformes: The Songbirds
Although we have long known which birds are passerines and which are not, their relationships have been poorly understood. A comparison of Clements 5th edition (which uses an old taxonomy) and Howard-Moore 3rd edition (more recent, but not current) shows how much revision has been necessary. There are still many passerines that are classified in the wrong family (and genus) which makes it harder to determine proper family boundaries and relations. Recent work on passerine taxonomy has done much to clarify the situation, and I've interspersed discussion of that work in passerine portion of the annotated checklist.
The overall organization of the Passeriformes is fairly well understood now. After a basal split between the New Zealand wrens and all other songbirds (Barker et al., 2002; Barker et al., 2004), the big division is between the suboscines and the oscines. Like the passerines as a whole, the suboscines have generally been identifiable as suboscine, but teasing out the relationships between the suboscines has been difficult. The suboscine branch is comprised of two groups, an old world group (Eurylaimides) and a new world group (Tyrannides). The latter contains the ovenbirds (Furnariida) and a group consisting of the tyrant flycatchers and allies (Tyrannida).
Most of the passerines are in the oscines. Sibley and Ahlquist's view was that the remaining passerines split cleanly into a corvid group and a group containing everything else (Passerida). Further study of these families has shown that reality is more complex. Unlike the Passerida, the SAM corvids are not a monophyletic group. A number of groups split off separately before we come to the big split between the Corvida and Passerida. There are also some smaller groups in the Passerida, but it mostly splits into 3-4 major groups. I compromised on 4 for these web pages. The groups are: Sylvioidea (babblers and Old World warblers), Certhioidea (nuthatches, wrens), Muscicapoidea (thrushes, starlings), and Passeroidea (finches, sparrows, tanagers).