The sixth order of Birds is that of the Insessores, or Perchers - often spoken of as the Passeres, or "Passerine" Birds. They are defined by Owen as follows: "Legs slender, short, with three toes before and one behind, the two external toes united by a very short membrane" (fig. 345, E, F).

"The Perchers form the largest and by far the most numerous order of birds, but are the least easily recognisable by distinctive characters common to the whole group. Their feet, being more especially adapted to the delicate labours of nidi-fication, have neither the webbed structure of those of the Swimmers, nor the robust strength and destructive talons which characterise the feet of the Birds of Rapine, nor yet the extended toes which enable the Wader to walk safely over marshy soils and tread lightly on the floating leaves of aquatic plants; but the toes are slender, flexible, and moderately elongated, with long, pointed, and slightly curved claws.

Fig. 345.   A, Head of Hoopoe (Upupa epops), showing the Tenuirostral type of beak. B, Head of Red backed Shrike (Lanius collurio), showing the Dentirostral type of beak, C, Head of White bellied Swift (Cypselus melba), showing the Fissirostral type of beak. D, Head of Corn bunting (Emberiza miliaria), showing the Coni rostral type of beak. E, Foot of the Yellow Wagtail (Motacilla sulphurea). F, Foot of a Finch (Fringilla).

Fig. 345. - A, Head of Hoopoe (Upupa epops), showing the Tenuirostral type of beak. B, Head of Red-backed Shrike (Lanius collurio), showing the Dentirostral type of beak, C, Head of White-bellied Swift (Cypselus melba), showing the Fissirostral type of beak. D, Head of Corn-bunting (Emberiza miliaria), showing the Coni-rostral type of beak. E, Foot of the Yellow Wagtail (Motacilla sulphurea). F, Foot of a Finch (Fringilla).

"The Perchers in general have the females smaller and less brilliantly coloured than the males; they always live in pairs, build in trees, and display the greatest art in the construction of their nests. The young are excluded in a blind and naked state, and wholly dependent for subsistence during a certain period on parental care. The brain arrives in this order at its greatest proportionate size; the organ of voice here attains its greatest complexity, and all the characteristics of the bird, as power of flight, melody of voice, and beauty of plumage, are enjoyed in the highest perfection by one or other of the groups of this extensive and varied order."

The structure of the feet, then, gives the definition of the order, but the minor subdivisions are founded on the nature of the beak; this organ varying in form according to the nature of the food, "which may be small or young birds, carrion, insects, fruit, seeds, vegetable juices, or of a mixed kind" (Owen).

In accordance with the form of the beak, the Insessores have been divided into four great sections or sub-orders, known as the Conirostres, Dentirostres, Tenuirostres, and Fissirostres.