Form and function

Feet and legs

The single feature that distinguishes passerines from all similar birds is their “perching” foot. In this foot type, all four toes are well developed and free from one another; in some families (wrens and most suboscines), the front toes may be partially fused at the base, but the distal portions (extremities) are functionally free. The hind toe (hallux) is joined on the same level with the front toes and opposes them, so that the foot can grip a perch. The only exception to this passerine foot type is found in the well-named Paradoxornis paradoxus, or three-toed parrotbill (Panuridae), in which the outer toe is reduced to a short clawless stump, fused to the middle toe; other species of Paradoxornis have normal feet.

Although all passerines can perch, not all do so habitually. A number of species (some tapaculos, Rhinocryptidae; larks; pipits, Motacillidae) are largely terrestrial and have feet modified for walking and running; the terrestrial foot is differently proportioned from the typical perching one, often with longer toes and longer, straighter claws (particularly on the hallux), probably as an aid in maintaining balance when running. The dippers, or water ouzels (Cinclus), are semiaquatic, but, although they successfully swim on the water surface and walk underwater searching for food on stream bottoms, they have retained the typical passerine foot. The single slight difference in the Cinclus foot is that the claw of the middle toe sometimes has a thin horny flap (of unknown function) on its inner border. Some other passerines, notably swallows, live a largely aerial life and have small and weak feet. The typical arboreal songbird has a well-developed foot, with the middle front toe longer than the others. Birds such as woodcreepers and nuthatches that often cling to vertical surfaces have strong, curved, sharp claws. Those that spend much of their time walking and scratching on the ground (although not limited to terrestrial activity) tend to have heavy, straighter, and rather blunt claws. Most passerines, however, have moderately curved sharp claws that are suited to grip a variety of rounded or rough surfaces.

The lower leg of passerines, the tarsometatarsus (usually called simply the tarsus), is normally covered by a horny sheath (podotheca). Exceptions include some swallows, which have feathered tarsi. Although the various different patterns of scale size and distribution of the normal unfeathered podotheca have been used by some taxonomists to differentiate families or groups of families, study has revealed so much variability in the tarsal patterns of certain families that it is no longer considered a reliable family character; it may still be useful as a generic or specific character. In most oscines the posterior (plantar) surface of the tarsus is bilaminate—that is, covered by two long plates, or laminae.

Bill

The bills of passerines are extraordinarily diverse in size, shape, and proportions. This diversity was long thought to be indicative of the birds’ relationships and so was used as a prime taxonomic character. It is now believed, however, that bills are evolutionarily plastic, reacting with relative ease to selective pressures, particularly to changes in feeding habits. Thus, on a broad scale, a passerine’s bill shape reveals less about its family affinities than it does about its food preferences, and, although bill shape may be an aid to determining a bird’s relationships, it must be considered in the light of other features and of the degree of variation found in the family. Two frequently cited examples of the adaptiveness of bills are the Darwin’s finches of the Galapagos Islands and the Hawaiian honeycreepers, Drepanididae (see evolution: Adaptive radiation). Each is a closely interrelated group of birds with different kinds of bills in the several species and genera. Bills of the drepanidids range from heavy, seed-cracking, grosbeaklike bills through thin, pointed, insectivorous types to the long, decurved (curved downward) bills of nectar feeders. These Hawaiian birds are now thought to be members of a single family of nine genera. On the basis largely of bill shape, they were once classified into four different families and 18 genera.

Most birds, including passerines, show little sexual dimorphism (difference between sexes) in bills except for minor differences in size (reflecting general body size differences) and sometimes in colour. The most outstanding exception is the extinct huia (Heteralocha acutirostris, Callaeidae), originally classified as two different species. The male of this New Zealand bird had a strong chiselling bill, whereas the female had a long, decurved, pliable bill. Reportedly, the two sexes fed cooperatively, the male digging in decaying wood and the female probing in crevices to extract grubs. The species unfortunately was prized by the Maoris, who used the white-tipped tail feathers in ceremonial headdresses, as well as by Europeans, and, after most of its habitat had been destroyed, the huia was hunted to extinction about the end of the 19th century.

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Passerine bills may be broadly classified into eight morphological and functional types:

1. Insectivorous: a generalized type found in many passerines, ranging from relatively straight and pointed (as in the wood warblers, Parulidae), through bills with a slight or pronounced hook (some New World flycatchers), to those that are short, with a wide gape and usually surrounded by rictal bristles (stiff hairlike feathers)—as in aerial feeders, such as swallows. Most insectivorous bills are relatively light in build, but this depends on the type of insect usually taken by the species and also on how generalized a feeder it is.
2. Omnivorous: unspecialized in shape and function but usually strongly built, as in crows and jays (Corvidae).
3. Toothed: strongly hooked at the tip and with a “tooth” (notch) on either tomium (cutting edge) of the upper mandible; adapted to tearing up large, relatively soft prey. This is the typical bill of shrikes (Laniidae) but is also found in some unrelated birds, such as the Australian bell-magpies (Cracticidae) and some tanagers.
4. Tearing: a relatively light bill with a strong hook at the tip, for tearing open objects, such as flowers, to obtain the insects and nectar within. Found in flower piercers (Diglossa, Thraupidae).
5. Probing: relatively narrow and often downcurved; slender in species that probe flowers for tiny insects and nectar (sunbirds; some Hawaiian honeycreepers) but more heavily constructed in those that probe in wood or under tree bark (creepers, Certhia; some woodcreepers).
6. Frugivorous: variable but usually rather wide; ranges from lightly built with a wide gape for swallowing whole fruits (found in some cotingas, and in the swallow-tanager, Tersina) to more heavily built for tearing apart tougher fruits (some tanagers).
7. Serrated: conical, with a finely serrated edge, adapted for feeding on leaves, buds, shoots, and fruit. Found only in the plantcutters (Phytotoma, Cotingidae).
8. Conical: adapted for seed eating. Ranges from exceedingly stout and blunt (such as the hawfinch, Coccothraustes, which can crack remarkably hard objects, such as cherry pits) to relatively small and pointed (siskins, Carduelis). Some forms specialized for particular kinds of seed extraction (such as crossbills, Loxia, which feed on pine seeds).

This classification indicates morphological and functional types of bills, but it does not imply that a species with a particular type of bill will feed only on the food for which it is best adapted. Although some birds are extremely specialized in their feeding habits, most are opportunistic feeders, seizing upon whatever food is readily available and can be “handled” with the bill. Hence, many basically granivorous or frugivorous birds catch insects, especially when feeding nestlings, and many insectivorous species exploit seasonally available plant food. Yellow-rumped warblers (Dendroica coronata) and tree swallows (Iridoprocne bicolor), for example, feed on bayberries in fall and winter, and eastern kingbirds (Tyrannus tyrannus) and other New World flycatchers eat a variety of fruits and berries in season.

The mandibles of passerines, like those of all other birds, are composed of bone covered with a horny sheath, the ramphotheca. The ramphotheca is worn down by normal use and, in most birds, is capable of growing to replace the lost material. In individuals with damaged bills or those (such as cage birds) that do not have the opportunity to wear down the constantly growing ramphotheca, the bills overgrow at the tip.