Description
The order is divided into three suborders, Tyranni (suboscines), Passeri (oscines), and the basal Acanthisitti. Oscines have the best control of their syrinx muscles among birds, producing a wide range of songs and other vocalizations (though some of them, such as the crows, do not sound musical to human beings); some such as the lyrebird are accomplished imitators. The Acanthisittids or New Zealand wrens are tiny birds restricted to New Zealand, at least in modern times; they were long placed in Passeri; their taxonomic position is uncertain, though they seem to be a distinct and very ancient group.
Most passerines are smaller than typical members of other avian orders. The heaviest and altogether largest passerines are the Thick-billed Raven and the larger races of Common Raven, each exceeding 1.5 kg (3.3 lb) and 70 cm (28 in). The Superb Lyrebird and some birds-of-paradise, due to very long tails or tail coverts, are longer overall. The smallest passerine is the Short-tailed Pygmy-Tyrant, at 6.5 cm (2.6 in) and 4.2 g (0.15 oz).
The foot of a passerine has three toes directed forward and one toe directed backwards, called anisodactyl arrangement. This arrangement enables the passerine birds to perch upon vertical surfaces, such as trees and cliffs. The toes have no webbing or joining, but in some cotingas the second and third toes are united at their basal third. The hind toe joins the leg at the same level as the front toes. In other orders of birds the toe arrangement is different. The leg muscle of passerine birds contains a special adaption for perching. It will automatically tighten and become stiff if the bird starts to lose hold of the branch on which it is perching. This enables passerines to sleep while perching without falling off. This is especially useful for passerine birds that develop nocturnal lifestyles.
Most passerine birds develop twelve tail feathers, though the Superb Lyrebird has sixteen. Certain species of passerines have stiff tail feathers, which help the birds balance themselves when perching upon vertical surfaces.
The chicks of passerines are altricial: blind, featherless, and helpless when hatched from their eggs. This requires that the chicks receive a lot of parental care. Most passerines lay coloured eggs, in contrast with non-passerines, most of whose eggs are white except in some ground-nesting groups such as Charadriiformes and nightjars, where camouflage is necessary, and some parasitic cuckoos, which match the passerine host's egg.
Clutches vary considerably in size: some larger passerines of Australia such as lyrebirds and scrub-robins lay only a single egg, most smaller passerines in warmer climates lay between two and five, whilst in the higher latitudes of the northern hemisphere hole-nesting species like tits can lay up to a dozen and other species around five or six.
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