PASSERIFORM

also called passerine, or perching bird any member of the order Passeriformes, the largest order of birds and the dominant avian group on Earth today. The passeriform birds are true perching birds, with four toes, three directed forward and one backward. Considered the most highly evolved of all birds, passerines have undergone an explosive evolutionary radiation in relatively recent geological time and now occur in abundance on all continents except Antarctica and on most oceanic islands. Their rapid evolution and adaptation to virtually all terrestrial environments have resulted in a large number of species, some 5,100, compared to only about 3,500 species for all other birds. The order Passeriformes is divided by most taxonomists into four suborders: Eurylaimi, Tyranni, Menurae, and Passeres. The first three, containing about 1,100 species, are considered more primitive and are often grouped informally as the suboscines for convenient comparison with the very large fourth suborder, the oscines, or songbirds (about 4,000 species). Passerines, like other small land birds with fragile bones and arboreal habits, are rarely fossilized; hence the fossil record gives few clues to their evolution. Their origins, both in time and ancestral type, can only be guessed. Some authorities believe that passerines arose during the Early Cretaceous Era (about 120,000,000 years ago); others believe that it was much later, not until the Late Cretaceous or even Paleocene Epoch (perhaps 65,000,000 years ago). The earliest known passerine fossils are from the upper Eocene Epoch (about 40,000,000 years ago). One of these is an extinct genus assigned to the modern family of tapaculos (Rhinocryptidae), another is thought to be related to the titmice (Paridae), and still another to the starlings (Sturnidae). Palaeospiza, a primitive oscine classified as a separate family (Palaeospizidae) between the larks and the swallows, is known only from the Colorado Oligocene Epoch (about 35,000,000 years ago). As drying conditions during the Miocene Epoch (26,000,000 to 7,000,000 years ago) reduced the forests and encouraged the spread of grasslands, bird families exploited the newly appearing drier and less forested habitats and radiated into them. There for the first time appear remains of crows (Corvidae), thrushes (Turdidae), wagtails (Motacillidae), shrikes (Laniidae), and wood warblers (Parulidae). During the Pliocene Epoch (from 7,000,000 to 2,500,000 years ago) the warm, dry conditions continued, and most paleornithologists now believe that all of the living passerine families were in existence by its close. They also believe that most modern species of birds arose during the Pleistocene Epoch (about 2,500,000 to 10,000 years ago), a period of almost constant change during which there were four major advances and retreats of the glaciers. Most of the passerines in the fossil record are from the Pleistocene or Recent epochs and represent either living species or close relatives. Evolution since the retreat of the last ice sheet (about 11,000 years ago) has been mainly at the subspecies level. With the lack of a significant fossil record, nothing is known of the type or types of birds from which passerines arose. Studies of the anatomy of modern forms have led to a general agreement that the perching birds developed from more than one ancestral type (polyphyletic origin). But until more evidence is unearthed, nothing further can be said. Classification Distinguishing taxonomic features The classification of passerines is probably the single most vexing problem in avian taxonomy today. Few of the subgroupings (especially oscine families) have strikingly different characteristics, the fossil record is inadequate to elucidate the complete evolutionary history of the order, and the order has undergone an explosively radiative evolution that has resulted in a large number of anatomically similar species. Classification is further complicated by the persistence of intermediate and relict forms and a high degree of convergence (similarity due to different lines' being subject to the same types of natural selection) among distantly related groups. It is therefore necessary to examine each passerine species in careful detail (including its anatomy) to be certain of its affinities. With some 5,100 species in the order, this is a monumental task, one that is by no means complete. Traditionally, passerine families have been defined on the basis of a careful (usually anatomical) study of only a few individuals; the great majority of the birds have been assigned to established families solely because they externally resemble the studied species in plumage, bill, leg, and foot characters, as well as in what is known of their life history, behaviour, geographical distribution, etc. It is unfortunate that in the early years of avian taxonomy there was a strong tendency to try to include each newly discovered species, regardless of its geographical range, in some previously defined palearctic family. In the mid-20th century, taxonomists began re-examining the generally accepted 19th-century groupings of birds into families. In these investigations some of the features on which earlier classifications were based have been found to be convergent or too variable to be useful in certain groups (e.g., bill shape, tarsal scutellation). Consequently, passerine taxonomists have been left with a rather sparse body of morphological information upon which to base a classification. In recent years ornithologists have made a concerted effort both to augment some of the century-old work on passerine anatomy and to explore new avenues of morphology, behaviour, reproductive patterns, biochemistry, and zoogeography to help define and relate the many families of perching birds. Much of this work is still in progress and has not yet been incorporated into classification systems. Among traditional and newly studied taxonomic features are external characterssuch as rictal bristles and other specialized feathers, colours and patterns of the fleshy parts of the mouth, morphology of the bill and nostrils, colour patterns of adults and young; internal anatomical characterssuch as the number of cervical (neck) vertebrae, the condition of the deep plantar tendons, anterior and posterior spines and processes of the sternum, syringeal muscles, palatal and other bones of the skull, feather tracts, jaw and tongue musculature, hyoid (tongue) apparatus, aortic-arch system, pneumatic fossa (cavity) of the humerus, and types of spermatozoa; biochemical analysis of substancessuch as egg white, eye lens, plasma proteins, and hemoglobins; and an array of behavioral traitssuch as reproductive behaviour, nest building, methods of scratching, etc. Thus, although the current system is by no means satisfactory, improvement is in the offing, and it is hoped that in the near future modern studies will lead to a more firmly drawn passerine classification. also called Passerine, or Perching Bird, any member of the largest order of birds (Passeriformes), containing about 5,100 species, as compared with 3,500 species for all other birds. The passeriform birds are true perching birds, with four toes, three directed forward and one backward. This order is generally divided into the suboscines, containing some 1,100 species, and the oscines, or songbirds, with about 4,000 species. Passerines form the dominant avian group on Earth today. They are regarded as the most highly evolved of all birds and occur in abundance on all continents except Antarctica and on most oceanic islands. Passerines are small to medium-sized land birds. Most are insectivorous, and different groups have evolved various ways of obtaining their food. Humans have long enjoyed passerines for their songs and their almost infinite variety of colours, patterns, and behavioral traits. This admiration can be hazardous to the existence of some bird species. They are widely kept as cage birds, and until the mid-20th century a number of species of passeriforms were commonly slaughtered for their colourful feathers. Many countries now prohibit the capture and sale of nearly all native songbirds. The birds are still, however, an important food source in certain countries and are considered delicacies in others. Passerines are a major factor in maintaining balance in nature. They consume large amounts and varieties of foodinsects, grains, fruits, small amphibiansand in turn serve as food for other animals. They also act as hosts for parasites, and they pollinate plants and flowers and carry seeds to new locations. They can be agricultural pests by their depredations of grain crops. Most passerines are solitary nesters; a single monogamous pair maintains a territory large enough to support all their mating-season activitiescourtship, mating, nesting, and food gathering. Nests may be located on the ground, on riverbanks, in rock crevices, on ledges, or in a wide variety of trees, grasses, and shrubs. Most species build cup-shaped open nests. Others, such as the ovenbird, form domed or ball-shaped nests of mud with small entrances on the side or top. Passerines lay clutches of anywhere from 1 to 14 eggs. As a rule, the female alone incubates the eggs, although occasionally incubation is shared with the male. Incubation lasts from 11 to 21 days. In most species the male helps feed the young, which remain in the nest for 8 to 30 or 35 days but most commonly from 10 to 15 days. After the young fledge, they stay near the nest for some days or weeks before becoming fully independent. An outstanding characteristic of most perching birds is their ability to sing. The 4,000 songbird (or oscine) species are most notable for their vocalizations, but other members of the order also can produce a variety of sounds. The songbirds have a highly complex vocal organ called a syrinx. Only the male of most passerine species, however, sings a true song. A characteristic behaviour pattern peculiar to passerines is the practice of anting. Approximately 30 passerine families actively or passively manipulate ants exuding formic acid to crawl through their feathers. Some authorities believe this is a form of stimulation, but others argue that the birds do this to help with feather maintenance. They avoid the selection of stinging ants. Some passerines also follow columns of army ants in order to devour other insects and small animals fleeing from their path. Although all passerines can perch, not all do so habitually. A number of ground-dwelling species (some tapaculos, larks, pipits) have modified the characteristic four-toed perching foot into a flatter, longer version to aid in walking and running. Other passerines, such as swallows, which spend much time airborne, have small, weak feet. And still others, such as woodcreepers and nuthatches, which often cling and climb vines and trees, have strong, curved, sharp claws. Most passerines have moderately curved sharp claws. Passerine bills are broadly classified into eight types that indicate the bill's function or the bird's major food source. The bill types are insectivorous, omnivorous, toothed, tearing, probing, frugivorous (heavily built for tearing apart tough fruits), serrated (found in birds needing plant cutters), and conical (for seed eating). Male passerines tend to have brighter colours and more striking patterns on their feathers than do females and juvenile birds, which are cryptically coloured to aid in concealment. Many species, especially those in temperate climates, have a bright plumage during breeding season and a dull one in winter. Studies of modern birds have led to a general agreement that perching birds developed from more than one ancestral type; however, the fossil record gives little definite evidence of direct ancestors. Although some authorities believe that passerines arose about 120 million years ago, the earliest fossils found date from about 40 million years ago. Additional reading Specialized works and technical review articles include Peter L. Ames, The Morphology of the Syrinx in Passerine Birds (1971); Charles G. Sibley, A Comparative Study of the Egg-White Proteins of Passerine Birds (1970); K.E.L. Simmons, Anting and the Problem of Self-Stimulation, Journal of Zoology, 149:145162 (1966); and E.F. Potter, Anting in Wild Birds: Its Frequency and Probable Purpose, Auk, 87:692713 (1970). Taxonomic works include Jean Delacour and Charles Vaurie, A Classification of the Oscines (Aves) (1957); and Ernst Mayr and J.C. Greenway, Jr., Sequence of Passerine Families, Breviora, vol. 58 (1956). Mary Heimerdinger Clench Oliver L. Austin, Jr. The Editors of the Encyclopdia Britannica