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The colours, patterns, and textures of passerine feathers are considered important taxonomic characters, especially in determining genera, species, and subspecies. Plumage is also occasionally used in a very broad way to indicate evolutionary levels. Spots, streaks, and dull colours are generally considered more primitive than bold or complicated patterns and bright colours, but there are many exceptions to this rule.

Passerines often are sexually dimorphic in their plumage, with adult males wearing brighter colours and more striking patterns than do females. In some families, notably tanagers (Thraupidae), wood warblers (Parulidae), and New World orioles (Icteridae), the temperate zone species show more sexual dimorphism than do tropical members of the same families. In addition, many species (especially those in temperate climates) are seasonally dimorphic, with a bright plumage during the breeding season and a dull one in winter. Juvenile plumages of both sexes tend to be cryptically coloured (that is, adapted for concealment), as is that of the adult female.

Virtually any colour may be found in one passerine or another, and the order offers a wide array of specialized feather types, such as the waxlike tips on the flight feathers of waxwings (Bombycillidae); the tufts of stiff feathers in some honeyeaters (Meliphagidae); iridescent “spangles” in some manakins, sunbirds, and tanagers; and the almost unbelievable array of “wires,” iridescent gorgets, velvety ruffs, racquet tails, and filamentous plumes of the birds-of-paradise.

Another taxonomically important character is the number and distribution of feathers (pterylosis) on the bodies of passerines. From external appearance all birds seem to be more or less evenly covered by feathers; in actual fact, however, most birds have their feathers growing from relatively narrow tracts (pterylae) in the skin. From the pterylae the feathers fan out and cover the remainder of the bird’s body. In passerines, the feathers are arranged in eight distinguishable tracts, with apteria (relatively bare skin) between them. Variations in tract width and length and especially differences in feather number and distribution are often useful in determining relationships. Of particular interest are the occurrence of apteria within tracts and the configuration of the ventral tract. Also used in classification are the numbers of flight feathers. The remiges (flight feathers on the wings) of most passerines consist of 10 primaries on the “hand” (manus) and 9 secondaries on the forearm (ulna). In all perching birds the 10th (outermost) primary is reduced to some degree, and in many families only 9 may be found. The number of secondaries is more variable, with some species having as many as 14 (the satin bowerbird, Ptilonorhynchus violaceus). Tail feathers (rectrices) also vary; most passerines have 12, but the number ranges from 6 to 16.

Of importance in some species is the relative length of the primaries. This “wing formula” is often useful to differentiate between species of such difficult groups as the New World flycatchers and the Old World warblers (Sylviidae).

Syrinx

In a group of birds as vocal as the passerines, it is natural that the structure of the vocal apparatus should have evolutionary significance. Differing from the mammalian larynx in both location and structure, the syrinx consists of a resonating chamber at the lower end of the windpipe (trachea), with associated membranes, cartilages, and muscles. These modifications involve elements of the bronchi (the two tubes connecting the trachea with the lungs) as well as those of the trachea. Since the mid-19th century the basic subdivisions of the order Passeriformes have been based primarily on the structure of the syrinx. Syrinx morphology has also provided characters useful for modern taxonomic revisions of such groups as the tyrant flycatchers (Tyrannidae).

Syringeal muscles are classified into two groups: extrinsic muscles, which connect the syrinx with other parts of the anatomy, and intrinsic muscles, which extend from one part of the syrinx to another. The number, shape, and attachments of the intrinsic muscles are likely to remain important in passerine classification. Those birds in which the muscles are inserted on the middle of the bronchial semi-rings (C-shaped cartilages that strengthen the bronchi) are sometimes called mesomyodian (most members of the suborder Tyranni), and those with the insertion on the ends of the semi-rings are acromyodian (Menuridae, Passeri). The broadbills (Eurylaimidae) and a few others have no intrinsic muscles. Further distinction is made in the number of pairs of intrinsic muscles, most importantly in the Passeri, which have four.

The passerine syrinx exists in four basic types:

1. Unspecialized: relatively little modification of the tracheobronchial region; few, if any, cartilaginous specializations, and no intrinsic muscles; found in broadbills (Eurylaimidae), pittas (Pittidae), New Zealand wrens, asities (Philepittidae), plantcutters, most cotingas, and a few manakins and tyrant flycatchers.
2. Tracheophone: most of the specializations limited to the tracheal region; intrinsic muscles number zero to two pairs; pessulus (a bony bar lying at the junction of the bronchi) absent; found in all members of the Furnarioidea (South American ovenbirds, woodcreepers, antbirds, and tapaculos).
3. Intermediate tracheobronchial: various modifications of cartilages and membranes; one or two pairs of intrinsic muscles; pessulus present or absent; found in the sharpbill (Oxyruncus) and most manakins and tyrant flycatchers.
4. Oscine (acromyodean): complex musculature involving four pairs of intrinsic muscles (but three pairs in lyrebirds and scrub-birds); some cartilaginous specializations; pessulus present (except in larks).