Passerines: Passeriformes

These birds make up the biggest group of birds - more than half of all birds. They are to be found everywhere. None is very large, and most are fairly small. Their feet have three toes in front and a long one behind, which enables them to perch easily - although, of course, other birds can perch too. Many, but by no means all, can sing well and, in a few cases, the song must be heard to be sure of the bird's identity.

Passerines are by far the largest order of birds and comprise about two thirds of all avian species. They include small to at most moderately large birds; among their European representatives, the smallest, the Goldcrest, weighs about 6 g and the largest, the Raven, about 1,200 g. Most of them have a short to medium-length beak, the shape of which varies with the type of diet. Seed-eaters have a hard, conical beak (Fig. 1), which is sometimes notched (e.g. in buntings, Fig. 2) and is thus well suited for gripping and crushing seeds. Conversely, insectivorous birds mostly have a thin, pointed beak, which is sometimes mod­ified to 'forceps' for extracting insects from cracks in bark (e.g. treecreepers, Fig. 3), and is sometimes flat, with a wide base and with bristles growing from the upper mandible, as an aid to the bird for catching insects on the wing (e.g. flycatchers, Fig. 4). Crossbills have a uniquely formed beak, whose crossed mandibles are used for opening the scales of fir and pine cones to reach the seeds. Passerines have free feet with moderately long toes and sharp claws. Three of the toes are directed forwards and the first toe points backwards (Fig. 5), as adaptation to an arboreal mode of life, since most passerines live in trees. Many inhabit open, unwooded country, however, and live on the ground, in which case the surface of their feet is increased by long, straight claws (e.g. larks, Fig. 6).

Passerines have moderately thick plumage, but poorly developed down. The Dipper is an excep­tion, since it has a thick coat of down, obviously an adaptation to its underwater habits. Most passerines are very good fliers. The length and timing of the moult vary with the species. The length of the complete moult (after nesting, when much of the plumage has been worn away) is very closely associated with migration. This associa­tion is of vital importance, since no bird can undertake the long journey to its winter quarters with tatty wings. The flight feathers may either be changed before the birds leave (e.g. the Pied Flycatcher or the Lesser Whitethroat), in which case the moult is very fast and the birds are hardly able to fly at all during it, or they may be shed after the birds have arrived in their winter quarters (e.g. the Spotted Flycatcher and the Garden Warbler). Sexual differences in size and colour are found in a whole range of passerine species.

The vocal manifestations of passerines are exceedingly varied and have a range of seven to eight octaves. Their diversity is associated with a vocal organ (the syrinx) whose structure is very similar in all passerines and is the factor ranking them in one order. The syrinx (Fig. 7) lies at the bifurcation of the trachea (a) into two bronchi (b); it is a small, drum-like structure formed by the fusion of a few tracheal and bronchial cartilages (c) with several pairs of small song muscles (d) inserted on its outer surface. At the spot where the inner walls of the bronchi con­verge, there is a bar of cartilage (e) which tapers off to form a membranous vocal chord (f). The passage of air through the syrinx is regulated by two glottal spaces, which are controlled by outer (g) and inner (h) glottal labia and are bordered by the tympanic membranes (i,j). The voice is produced by vibration of the membranous vocal chord and stretching of the tympanic membranes by means of the song muscles and is amplified by the surrounding lung sacs, which act as reso­nators. Passerines include the best songbirds and it may therefore come as a surprise to learn that they also include birds like the Raven, the Carrion Crow and the Jay, whose croaking and screeching can hardly be described as song. Nevertheless, they also are the proud possessors of a syrinx, formed in just the same way as the syrinx of the most celebrated songbirds.

Passerine nests can be found in trees and bushes, in holes and burrows and on the ground. Sometimes they are very simple structures and sometimes works of art. The size of the clutches is very variable. The young are typically nidicolous; they are dependent on parental care until they are fledged and in some cases for some time afterwards. They beg for food, screaming with their beaks held wide open, and in this respect they differ from the young of most other birds. Young reared in holes remain there somewhat longer (15- 20 days) than those reared in open nests (12-14 days). Premature departure from the nest (in nine days or even sooner) is typical of passerines nesting on or just above the ground. Since enemies have easy access to such nests, dispersal of the young into the surrounding area reduces the danger of the whole family being destroyed; in the worst event only one of the young is captured, and not all of them as would be the case if they remained in the nest. It is thus a kind of defence measure.