Perching birds

A passerine is a bird of the order Passeriformes, which includes more than half of all bird species. Sometimes known as perching birds or, less accurately, as songbirds, the passerines form one of the most diverse terrestrial vertebrate orders: with around 5,093 species^[1], it is roughly twice as diverse as the largest of the mammal orders, the Rodentia.

The names "passerines" and "Passeriformes" are derived from Passer domesticus, the scientific name of the type species – the House Sparrow – and ultimately from the Latin term passer for true sparrows and similar small birds.

Origin and evolution

The evolutionary history of and relationships among the passerine families remained rather mysterious until around the end of the 20th century. Many passerine families were grouped together on the basis of morphological similarities that, it is now believed, are the result of convergent evolution, not a close genetic relationship. For example, the "wrens" of the northern hemisphere, those of Australia, and those of New Zealand look very similar and behave in similar ways, and yet belong to three far-flung branches of the passerine family tree; they are as unrelated as it is possible to be while remaining Passeriformes.

A little later, a great radiation of forms took place out of Australia-New Guinea: the Passeri or songbirds. A major branch of the Passeri, "Parvorder Passerida", emerged either as the sister group to the basal lineages and corvoids ("Parvorder Corvida"), or more likely as a subgroup of it, and expanded deep into Eurasia and Africa, where there was a further explosive radiation of new lineages. This eventually led to three major passeridan lineages comprising about 4,000 species, which in addition to the corvoidan clade and numerous minor lineages make up songbird diversity today. There has been extensive biogeographical mixing, with northern forms returning to the south, southern forms moving north, and so on.

Fossil record

Earliest passerines
Perching bird osteology, especially of the limb bones, is rather diagnostic.^[3] However, the early fossil record is poor because the first Passeriformes were apparently on the small side of the present size range, and their delicate bones did not preserve well. QM specimens F20688 (carpometacarpus) and F24685 (tibiotarsus) from Murgon, Queensland are fossil bone fragments clearly recognizable as passeriform; they represent two species of approximately some 10 and some 20 cm in overall length and prove that some 55 mya, barely into the Early Eocene, early perching birds were recognizably distinct.^[4] A quite similar group, the Zygodactylidae (named for their zygodactylous approach to perching) independently arose at much the same time – and possibly from closely related ancestors – in the landmasses bordering the North Atlantic, which at that time was only some two-thirds of its present width.

Modern knowledge about the living passerines' interrelationships (see the list of families below) suggests that the last common ancestor of all living Passeriformes was a small forest bird, probably with a stubby tail^[6] and an overall drab coloration, but possibly with marked sexual dimorphism. The latter trait seems to have been lost and re-evolved multiple times in songbird evolution alone, judging from its distribution among the extant lineages: the common ancestor of Passerida for example was almost certainly not markedly dimorphic considering the trait is very rare among the basal lineages of these, but very common among the youngest passerid clade, the Passeroidea; on the other hand among the basalmost Passeri there are a considerable number of strongly dimorphic lineages such as the very ancient Menuridae as well as many Meliphagoidea and Corvoidea. Sexual dimorphism is also not uncommon in the Acanthisittidae and prominent in some suboscines such as the Pipridae and Cotingidae.

Early European passerines
In Europe, perching birds are not too uncommon in the fossil record from the Oligocene onwards, but most are too fragmentary for a more definite placement:

Wieslochia was possibly not a member of any extant suborder. That not only the Passeri expanded much beyond their region of origin is proven by an undetermined broadbill (Eurylaimidae) from the Early Miocene (roughly 20 mya) of Wintershof, Germany, and the indeterminate Late Oligocene suboscine from France listed above. Even very basal Passeriformes might have been common in Europe until the Middle Miocene, some 12 mya.^[11] Extant Passeri superfamilies were quite distinct by that time and are known since about 12–13 mya when modern genera were present in the corvoidean and basal songbirds. The modern diversity of Passerida genera is known mostly from the Late Miocene onwards and into the Pliocene (about 10–2 mya). Pleistocene and early Holocene lagerstätten (<1.8 mya) yield numerous extant species, and many yield almost nothing but extant species or their chronospecies and paleosubspecies.

American fossils
In the Americas, the fossil record is more scant before the Pleistocene, from which several still-existing suboscine families are documented. Apart from the indeterminable MACN-SC-1411 (Pinturas Early/Middle Miocene of Santa Cruz Province, Argentina),^[12] an extinct lineage of perching birds has been described from the Late Miocene of California, USA: the Palaeoscinidae with the single genus Paleoscinis. "Palaeostruthus" eurius (Pliocene of Florida) probably belongs to an extant family, most likely passeroidean.

Systematics and taxonomy

Initially^{citation needed}, the Corvida and Passerida were classified as "parvorders" in the suborder Passeri; in accord with the usual taxonomic practice, they would probably be ranked as infraorders. As originally envisioned in the Sibley-Ahlquist taxonomy, they contained, respectively, the large superfamilies Corvoidea and Meliphagoidea as well as minor lineages, and the superfamilies Sylvioidea, Muscicapoidea and Passeroidea.

This arrangement has been found to be overly simplified by more recent research. Since the mid 2000s, literally dozens of studies are being published which try rather successfully to resolve the phylogeny of the passeriform radiation. For example, the Corvida in the traditional sense were a rather arbitrary assemblage of early and/or minor lineages of passeriform birds of Old World origin, generally from the region of Australia, New Zealand, and Wallacea. The Passeri on the other hand can be made monophyletic by moving some families about, but the "clean" three-superfamily-arrangement has turned out to be far more complex and it is uncertain whether future authors will stick to it.

Major "wastebin" families such as the Old World warblers and Old World babblers have turned out to be paraphyletic and are being rearranged. Several taxa turned out to represent highly distinct species-poor lineages and consequently new families had to be established, some of them – like the Stitchbird of New Zealand and the Eurasian Bearded Reedling – monotypic with only one living species.^[13]. It seems likely that in the Passeri alone, a number of minor lineages will eventually be recognized as distinct superfamilies. For example, the kinglets constitute a single genus with less than 10 species today, but seem to have been among the first perching bird lineages to diverge as the group spread across Eurasia. No particularly close relatives of them have been found among comprehensive studies of the living Passeri, though it is suspected that they might be fairly close to some little-studied tropical Asian groups. Treatment of the nuthatches, wrens, and their closest relatives as a distinct subfamily Certioidea is increasingly considered justified; the same might eventually apply to the titmice and their closest relatives.

This process is still continuing. Therefore, the arrangement as presented here is subject to change. However, it should take precedence over unreferenced conflicting treatments in family, genus and species articles here; see the next section for default sources.

Taxonomic list of Passeriformes families

This list is in taxonomic order, placing related species/groups next to each other. The Passerida subdivisions are updated as needed from the default sequence of the Handbook of the Birds of the World,^[14] based on the most modern and comprehensive studies.^[15] Updates are added as necessary.

Regarding arrangement of families

The families are sorted into a somewhat unusual sequence. This is because so many reallocations have taken place since about 2005 that a definite arrangement has not been established yet. The present sequence is an attempt to preserve as much of the traditional sequence while giving priority to adequately addressing the relationships between the families.

Suborder Acanthisitti

Suborder Tyranni

Suborder Passeri

Infraorder Passerida

Footnotes

^ Ernst Mayr, "The Number of Species of Birds", The Auk, Volume 63, Number 1 (January, 1946), p.67
^ Johansson & Ericson (2003)
^ See e.g. Boles (1997), Manegold et al. (2004), Mayr & Manegold (2006)
^ Boles (1997)
^ Worthy et al. (2007)
^ The last common ancestor of all songbirds most likely had a decidedly longer tail. See del Hoyo et al. (2003, 2004).
^ Specimen SMF Av 504. A flattened right hand of a passerine perhaps 10 cm long overall. If suboscine, perhaps closer to Cotingidae than to Eurylaimides: Roux (2002), Mayr & Manegold (2006)
^ Huguenet et al. (2003), Mayr & Manegold (2006)
^ Specimens SMF Av 487-496; SMNS 86822, 86825-86826; MNHN SA 1259–1263: tibiotarsus remains of small, possibly basal Passeriformes: Manegold et al. (2004)
^ A partial coracoid of a probably Muscicapoidea, possibly Turdidae; distal tibiotarsus and tarsometatarsus of a smallish to mid-sized passerine which may be the same as the preceding; proximal ulna and tarsometatarsus of a Paridae-sized passerine: Gál et al. (1998-1999, 2000)
^ Manegold et al. (2004)
^ Distal right humerus, possibly suboscine: Noriega & Chiappe (1991, 1993)
^ The former does not even have recognized subspecies, while the latter is one of the most singular birds alive today. Good photos of a Bearded Reedling are for example here and here.
^ del Hoyo et al. (2003-)
^ Lovette & Bermingham (2000), Cibois et al. (2001), Barker et al. (2002, 2004), Ericson & Johansson (2003), Beresford et al. (2005), Alström et al. (2006), Jønsson & Fjeldså (2006)
^ Gill, F., Wright, M. & Donsker, D. (2008). IOC World Bird Names (version 1.6). Available at http://www.worldbirdnames.org/

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