Evolution and systematics

With more than 5,700 species—approximately 59% of the total number of bird species—passerines make up the single largest order of birds in the world. Indeed, many researchers roughly divide all birds into two major categories: passerines, and everything else.

Other than the large size of the order, however, there is very little that can be generally said about passerines. Members of this species-rich order are usually small, morphologically uniform, terrestrial birds that eat mainly seeds, fruit, nectar, and/or insects. Australian lyrebirds (Menura) and ravens (Corvus corax; weight approximately 60 oz [1,700 g]) are the largest members of the order, and bushtits (Psaltri-parus) and pygmy tits (Psaltria exilis) are the smallest.

Nearly all taxonomists agree that Passeriformes are mono-phyletic, meaning they share a common ancestor, but beyond that there is little agreement about the evolutionary history and genetic relationships of this order. Robert J. Raikow defines the group with five derived characters: a wing tendon architecture that is unique; except in one genus (Conopophaga), a distinctive palate called "aegithognathous"; unique, bundled sperm structure; and a highly specialized foot and leg that facilitate perching, with a large hallux (rear toe) that is specially arranged, deep plantar tendons, and simplified foot muscles. Researchers agree that these characters are unique to the order, but some also cite other, traditional morpho logical features as defining the group. These features include, among others, the arrangement of the toes (anisodactyl, or three toes forward, one toe pointing rearward), an incumbent (non-elevated), independently acting hallux, and distinctive syringeal architecture. Raikow, however, claims that all these features are more general within birds as a whole, and therefore not useful for establishing passerine mono-phyly.

DNA analysis may prove to be a critical tool for defining the phylogeny, or genealogical relationships, among birds, especially in the case of subdivision of the order. Except for the architecture of the syrinx and feet, passerines are remarkably similar in morphology. Differences in the syrinx have allowed for two generally recognized suborders, Tyranni (suboscines) and Passeri (oscines). However, beyond these large suborders, classification has been extremely problematic. Convergent and parallel evolution has produced structural features and behaviors that are remarkably similar in birds that are, in fact, not closely related. However, divergent features have evolved in some species that are closely related. As a consequence, many species and genera have been traditionally, but possibly incorrectly, grouped together in families on the basis of similar morphologic features. Charles Sibley, Jon Ahlquist, and Burt Monroe have proposed a reorganization of passerines, particularly oscines, using DNA hybridization techniques. However, though these techniques promise to provide additional insight into these relationships,

An Altamira oriole (Icterus gularis) at its nest in south Texas. (Photo by Larry Ditto. Bruce Coleman Inc. Reproduced by permission.)

it may still take some time before passerine systematics are fully understood.

Fossil record

Although Passeriformes fossils are well represented in the Miocene, there is little in the way of a fossil record for the first passerine species. The oldest known fossils of passerine origin were two bones found in Upper and Uppermost Oligocene strata in France.

There are two competing hypotheses for the time and place of origin of the passerines. The paucity of fossil evidence for passerines prior to the Oligocene and the presence of numerous early fossils in the Northern Hemisphere are used to support the hypothesis that passerines arose in Laurasia sometime after the Cretaceous extinction. However, techniques using DNA molecular clock interpretations suggest that passerines may have had a southern, Gondwanan origin in the Early Cretaceous. According to some researchers, the primitive suboscines originated from early passerines in Gondwana when South America separated from Africa. This is a hypothesis that is strongly supported by the overwhelming num ber of passerines found in South America today, where there are more than 3,000 species.

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