Fringilla coelebs (chaffinch), male
A passerine or passeriform is a member of the order Passeriformes, the largest order of birds, containing more than half of all species. They are also known as perching birds or, less accurately, as songbirds (which actually compose a suborder of Passeriformes). The passerines form one of the most spectacularly diverse terrestrial vertebrate orders; with around 5,400 known species, it is roughly twice as diverse as the largest of the mammal orders, Rodentia.
The passerines, with their diverse forms, colors, and behaviors, and with many adding song to the sounds around us, add an important component to the joy that humans feel in experiencing nature.
The passerines are true perching birds, having four toes, with three directed forward and one backward. The order includes such birds as finches, warblers, and jays. The group gets its name from the Latin name for the house sparrow, Passer domesticus.
The foot of a passerine is specialized for holding onto a branch, with three toes directed forward without any webbing or joining, and one toe directed backward. The hind toe joins the leg at the same level as the front toes. In other orders of birds, the toe arrangement is different. Passerines generally have sharp, curved claws.
Passerines are all terrestrial, found on all continents except Antarctica.
Most passerines are smaller than typical members of other avian orders. The heaviest passerines are the thick-billed raven and the common raven (1.5 to 3.6 pounds). Two species of lyrebird are longer overall. The superb lyrebird or Weringerong, (Menura novaehollandiae), reaches 31-39 inches long in males and 29-33 inches long in females, and the Albert's lyrebird (Menura alberti) reaches a maximum of about 35 inches in males and 33 inches in females.
Most passerines lay colored eggs, in contrast to non-passerines, who lay mostly white eggs, except in some ground nesting groups such as Charadriiformes and nightjars, where camouflage is necessary, and some parasitic cuckoos, who have to match the passerine host's egg. The pigments biliverdin and its zinc chelate provide a green or blue ground color for passerine eggs, while protoporphyrin produces reds and browns used as a base color or as spotting.
The order typically is divided into two suborders, Tyranni (suboscines), and Passeri (oscines).
Some other classifications may list the broadbills as their own suborder, Eurylaimi, and the lyrebirds as their own suborder Menurae, both being listed with Tyranni as suboscines.
The evolutionary history of and relationships among the passerine families remained rather mysterious until around the end of the twentieth century. Many passerine families were grouped together on the basis of morphological similarities that, it is now believed, are the result of convergent evolution, rather than a close genetic relationship. For example, the "wrens" of the northern hemisphere, Australia, and New Zealand, all look very similar and behave in similar ways, but belong to three far-flung branches of the passerine family tree; they are as unrelated as it is possible to be while yet remaining Passeriformes.
Much research remains to be done, but advances in molecular biology and improved paleobiogeographical data are gradually revealing a clearer picture of passerine origins and evolution. It is now thought that the early passerines evolved in Gondwana at some time in the Paleogene, maybe around the Late Paleocene (about 60-55 million years ago). The initial split was between the Tyranni (the ancestors of the songbirds), the Eurylaimides, and the New Zealand "wrens", which must have diverged during a short period of time (some million years at best). The Passeriformes apparently evolved out of a fairly close-knit clade of "near passerines" which contains such birds as the Piciformes, Coraciiformes, and Cuculiformes (Johnasson and Ericson, 2003).
A little later, a great radiation of forms took place out of Australia-New Guinea: the Passeri or songbirds. A major branch of the passerine tree, the 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 the corvoidan clade, three major passeridan lineages, and numerous minor lineages making up songbird diversity today. There has been extensive biogeographical mixing, with northern forms returning to the south, southern forms moving north, and so on.
Perching bird osteology, especially of the leg bones, is rather diagnostic. Still, the first Passeriformes were apparently forms 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 support the view that some 55 million years ago, early perching birds were recognizably distinct. The Murgon fossils represent two species of approximately ten to 20 centimeters overall length. 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.
Until the discovery of the Australian fossils, it was believed for some time that Palaeospiza bella from the Priabonian Florissant Fossil Beds (Late Eocene, around 35 million years ago) was the oldest known passeriform. However, it is now considered a non-passeriform near passerine.
In Europe, perching birds are not too uncommon in the fossil record from the Oligocene onward, but most are too fragmentary for a more definite placement:
A set of Early/Middle Miocene (approx. 15 million years ago) fossils from Sansan, France (MNHN SA 1259-1263) is doubtfully passeriform. The fact that the Passeri expanded much beyond their region of origin is proven by an undetermined broadbill (Eurylaimidae) from the Early Miocene (roughly 20 million years ago) of Wintershof, Germany, and the indeterminate Late Oligocene suboscine from France listed above. Wieslochia was apparently not a member of any extant suborder either. Extant Passeri superfamilies were well distinct by about 13-12 million years ago and modern genera were present in the corvoidean and basal songbirds. The modern diversity of Passerida genera evolved mostly from the Late Miocene onward and into the Pliocene (about 10-2 million years ago). Pleistocene lagerstätten (<1.8 million years ago) yield numerous to near-exclusively extant species, or their chronospecies and paleosubspecies.
In the Americas, the fossil record is more scant. Apart from the indeterminable MACN-SC-1411 (Pinturas Early/Middle Miocene of Santa Cruz Province, Argentina), an entirely extinct lineage of perching birds has been described from the United States:
This list is one proposed taxonomic order. It places related species/groups next to each other. The subdivisions follow Lovette and Bermingham's 2000 study of the c-mos proto-oncogene nDNA sequence.
Initially 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 they contained the superfamilies Corvoidea and minor lineages, and the superfamilies Sylvioidea, Muscicapoidea, and Passeroidea, respectively.
Several taxa turned out to be isolated species-poor lineages and consequently new families had to be established; 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 ten species, 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. Likewise, major "wastebin" families such as the Old World warblers and Old World babblers have turned out to be paraphyletic.
This process is still continuing. Therefore, the arrangement as presented here is not definite and may be subject to change.
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. It was attempted to preserve as much of the former arrangement, while giving priority to adequately address the relationships between the families.
Songbirds or oscines
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