2-^4 General Characters. BIRDS. General 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 Avhich 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. 
Tlie 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 witliin 
the shells. In this occupation, which is denominated 
incubation, both sexes frequently take part ; but, in 
many instances, the whole of this labour devolves iq)on 
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, wliilst 
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 their wants and safety for a considerable time ; but 
the amount of labour thus entailed upon them is very 
difterent 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 ad\-ert 
in this place. Tire first of these is intimately con- 
nected wth the subjects that we have just been con- 
sidering; this is tlieir nest-building, ornidification, as it 
is usually termed. Almost all birds form a nest of some 
kind for the reception of their 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 
