234 



GEXERAL CHARACTERS. BIRDS.- 



CHARACTERS. 



The brain in birds is proportionally smaller than in 

 the Mammalia, indicating a lower degree of general 

 intelligence. The organs of the senses, also, are for 

 the most part less perfect than in the Mammalia, and 

 the sense of touch especially must generally be possessed 

 by birds in a very low degree. The eyes, however, 

 are usually of large size and well formed, although, 

 as they have but little power of motion in the orbits, 

 these animals are under the necessity of turning their 

 heads to bring into view any objects at which they 

 desire to look. This peculiarity is induced by the form 

 of the eye, which is of a very large size, and of a more 

 or less flattened form, but bearing in front a narrowed 

 portion, the surface of which is occupied by the cornea. 

 Around this narrowed portion of the eye we find a 

 curious ring of small bony plates imbedded in the 

 sclerotic coat, and hence known as the sclerotic plates. 

 The office of these plates, which are connected with 

 delicate muscular fibres, is to increase or diminish the 

 convexity of the cornea, according as the ring which 

 they form is contracted or dilated by the action of their 

 muscles, and thus adapt the visual power of the eye to 

 the varying distances of objects. 



The organs of hearing are, next to those of sight, the 

 most highly developed in birds; but they are never fur- 

 nished with an external ear, such as we see in most 

 Mammalia. The ears open on the sides of the head, 

 behind the eyes ; they are usually surrounded by a 

 circle of feathers, which, to a certain extent, takes the 

 place of an external ear, and within these the tympanic 

 membrane may be easily seen stretched across the 

 bottom of a short passage. In the owls and other 

 nocturnal birds, the ears are of great size. 



The organs of smell are but imperfect in their con- 

 struction; the internal cavities possessing but few of 

 those convolutions, clothed with a delicate mucous 

 membrane amply supplied with nerves,, which exist in 

 most mammals. The nostrils are nearly always placed 

 on the sides of the bill, or at its base ; the only excep- 

 tions to this rule being presented by the species of 

 the singular genus Apteryx, which is peculiar to New 

 Zealand. In many birds with the nostrils placed near 

 the base of the bill, these apertures are pierced in a 

 naked skin, called the cere. The sense of taste appears 

 to be exercised by most birds in even a still lower 

 degree of perfection than that of smell, for the tongue 

 is usually of a horny texture, and it is only in the 

 parrots and a few other birds that we meet with a 

 fleshy tongue. 



The reproduction of birds is effected, as previously 

 stated, by eggs ; and they are, in fact, the only class of 

 vertebrate animals in which nothing approaching a 

 viviparous reproduction ever takes place. The Mam- 

 malia are all strictly viviparous, and therefore out of 

 the question here ; but, amongst reptiles and fishes, we 

 meet with many instances in which the eggs are hatched 

 within the body of the mother, whilst the eggs of birds 

 are invariably excluded, inclosed within a hard cal- 

 careous shell. 



The young birds are, as is well known, usually 

 hatched by the warmth of the body of their parents ; 

 the latter sitting upon the eggs during the whole time 

 that the development of the embryos is going on within 



the shells. In this occupation, which is denominated 

 incubation, both sexes frequently take part; but, in 

 many instances, the whole of this labour devolves upon 

 the female. The number of eggs laid by a hen-bird 

 varies greatly in different species ; some lay only a single 

 egg ; most of the rapacious birds lay at least two, whilst 

 the smaller birds are far more prolific, some of them 

 depositing sixteen or eighteen eggs in a single brood. 

 After the young birds are hatched, the parents attend 

 to then- wants and safety for a considerable time ; but 

 the amount of labour thus entailed upon them is very 

 different in different groups of birds, owing to variations 

 in the condition in which the young birds leave the egg. 

 In all the birds whose chief scene of activity is the air; 

 that is to say, in those groups which are most distin- 

 guished by the power of flight, and which dwell amongst 

 the branches of trees, or in other elevated situations, 

 the young are hatched in a very helpless condition; 

 and for some time after they come into the world are 

 wholly dependent upon their parents, not only for pro- 

 tection from danger, but for the nourishment necessary 

 for their further development. In the land and water 

 birds, on the contrary, the chief activity of which con- 

 sists in running or swimming, the young birds are 

 usually capable of accompanying their parents from 

 the time of their leaving the egg ; and in these, there- 

 fore, the principal duties of the latter consist in con- 

 ducting their progeny to the places in which food is 

 to be found, and in sheltering them from the perils to 

 which, in their comparatively helpless state, they are 

 exposed. Hence it has been proposed to divide all 

 birds into two primary groups the Autophagce or 

 self-feeders, in which the young can provide for them- 

 selves from the first ; and the Insessores or Perchers 

 (so called from the ordinary habits of the majority of 

 the species), in which the young require to have the 

 food brought to them by their parents. Unfortunately, 

 this rule does not strictly hold good, as regards all the 

 members of the former group ; for the young of many of 

 these are for a long time wholly dependent on their 

 parents. 



There are two other phenomena connected with the 

 general history of birds, to which we must briefly advert 

 in this place. The first of these is intimately con- 

 nected with the subjects that we have just been con- 

 sidering; this is their nest-building, or nidijication, as it 

 is usually termed. Almost all birds form a nest of some 

 kind for the reception of then- eggs during the period 

 of incubation ; and, in those species whose young are 

 hatched in a perfectly helpless condition, this also serves 

 as a cradle for the callow brood during their infancy. 

 The materials of which the nest is composed vary 

 greatly ; but the individuals of each species usually 

 exhibit a most remarkable uniformity of choice in this 

 respect. Very few, and these are all of the auto- 

 phagous section, content themselves with a hole scraped 

 in the ground in some sheltered situation ; and even 

 of these the majority take care to line the bottom of 

 the cavity with a few leaves, or other materials, to pro- 

 tect the eggs from the coldness of the ground. Some 

 birds, such as the parrots and woodpeckers, lay their 

 eggs in the holes of trees, which, however, they gene- 

 rally enlarge considerably to suit their purposes by 



