GENERAL CHARACTERS. BIRDS. GENERAL CHARACTERS. 



233 



pulsiori. The bases of the tail feathers are concealed, 

 like those of the wings, by softer feathers, forming coverts; 

 these are usually of moderate or small size, but some- 

 times attain an extraordinary degree of development. 



It seems almost unnecessary to indicate the beautiful 

 a daptation of the covering of birds to their habits and 

 mode of life. Organized as they are for the most rapid 

 passage through the air, their numerous feathers, lying 

 one over the other and all directed backwards, offer no 

 impediments to their flight ; whilst, at the same time, 

 the very pressure of the atmosphere, as the bird pursues 

 its swift course, will only cause the feathers to lie more 

 closely, and thus present an increased obstacle to the 

 penetration of the cold air to the skin of the bird a 

 circumstance of no small importance when we consider 

 the high temperature of the bodies of these creatures. 

 The downy coat which everywhere intervenes between 

 the external feathers and the skin, is an additional pro- 

 tection against the influence of cold, especially by 

 giving room for a certain quantity of warm air, the 

 escape of which is prevented by the outer feathers. 

 The structure of all the feathers, even those of the 

 wings and tail, renders them very light, a necessary 

 condition for animals whose principal activity is in the 

 air. In the aquatic birds a further provision is necessary ; 

 the feathers of these must resist the passage, not only 

 of air, but of water. For this purpose they are found 

 to be constantly lubricated with a peculiar oily secre- 

 tion, which renders them perfectly waterproof; it is 

 obtained from a peculiar gland situated on the tail, 

 which the birds press with their bills when cleaning and 

 arranging their plumage. 



In most birds the whole surface, with the exception 

 of the bill and feet, is clothed with feathers, but a few 

 have the head, or even the head and neck, quite bare. 

 In most of these the skin is folded so as to form wrinkles 

 or wattles, which are often adorned with the most bril- 

 liant colours. 



The horny bill or beak which incloses the jaws of 

 all birds, is usually of a more or less conical form ; but 

 this is modified almost infinitely to suit the requirements 

 of the different species. Thus, in the hawks or other 

 predaceous birds, the upper mandible of the bill is 

 strongly hooked at the tip, and many of the species 

 possess a tooth on each side at some little distance 

 from the apex ; the parrots possess a strongly hooked 

 bill, which assists them in climbing ; the wading birds 

 are often endowed with long bills, adapted for the cap- 

 ture of their food in mud and water ; and the ducks 

 have a more or less flattened bill, fringed along the 

 margins, and admirably fitted for straining their food 

 from the soft mud in which it is often found. All 

 these and many other variations in the form and struc- 

 ture of the bill, will, however, come under our con- 

 sideration hereafter, in describing the characters of 

 the different species of birds, so that it is unnecessary 

 to dwell upon them any longer here. The cere is a 

 naked skin, clothing the base of the bill in many birds, 

 and inclosing the nostrils. 



In the structure and clothing of the feet, we find 

 characters of perhaps equal importance with those pre- 

 sented by the bill and wings. The number of toes is 

 usually four, and of these three are commonly turned 

 VOL. i. 



forwards, and one the great toe backwards. In some 

 of the waders and aquatic birds, the hind toe is entirely 

 wanting, as it is also in the ostriches ; the true, or 

 African ostrich, is also destitute of one of the front 

 toes, so that it has but two in each foot. In the parrots 

 and woodpeckers, and some other climbing birds, the 

 outer toe is directed backwards as well as the great toe, 

 so that there are two each way (see fig. 134) and 

 the cuckoo has the power of turning the outer toe 

 either backwards or forwards at its pleasure. The 

 swifts have all the four toes turned in one direction. 

 In most birds the toes are united at the base by a small 

 fold of skin or web, which is generally insignificant, but 

 often shows itself very distinctly. In the true water 

 birds, these webs attain a much larger development, 

 generally uniting the anterior toes quite down to their 

 tips, and in some cases even extending back along the 

 inner margin of the foot to the hinder toe. 



In a few birds, of which the eagle is one, the clothing 

 of feathers extends down to the very toes, but in the 

 majority the whole foot, from the heel downwards, is 

 quite naked, and in some of the waders and water birds 

 this naked part even extends for some distance up the 

 shank. The skin of the foot is of a homy texture, 

 sometimes scaly or granulated, sometimes divided into 

 distinct horny plates of larger or smaller size. The 

 toes are always terminated by claws, the form of which, 

 like that of the bill, usually furnishes a good clue to the 

 habits of the animal. Thus, in the predaceous species 

 we find the claws long, strong, sharp, and curved, 

 forming most formidable weapons ; the smaller grani- 

 vorous and insectivorous birds are also usually provided 

 with long, curved claws, but these are slender and 

 weak, and only of service to them in clinging to the 

 surfaces of objects. Those species which, like the 

 pheasant and our common fowl, find much of their 

 food by scratching in the earth, are furnished with 

 stout nail-like claws ; and these organs become still 

 more nail-like in many of the aquatic birds. In the 

 pheasant family, the male birds are frequently furnished 

 with one or more spurs or accessory claws, attached to 

 the back of the tarsus, at some little distance above the 

 hinder toe. 



In their internal structure, birds present much resem- 

 blance to the Mammalia; they possess the same organs, 

 arranged, with one or two exceptions, nearly in the 

 same manner, and the vital operations are carried on 

 by precisely the same means in both these great classes. 

 The principal difference in the structure of the diges- 

 tive organs consists in the dilatation of the resophagus, 

 or gullet, into a spacious bag called the crop, in which 

 the food is retained for some time after being swallowed. 

 The walls of this bag contain a great number of glands, 

 the fluid secreted from which soaks the food, and thus fits 

 it for the action of the true digestive organs. The true 

 stomach is separated from the crop by a continuation 

 of the oasophagus, furnished with very thick, glandular 

 walls. The office of these glands is to secrete the gas- 

 tric juice ; and the stomach itself, in those birds which 

 feed on hard substances, such as seeds, is very muscu- 

 lar, and lined internally with a hard coating, which 

 materially assists in the trituration of the food. This 

 stomach is well known as the gizzard. 



2 G 



