AVES. 
159 
The brain, in Birds, offers the same general characters as in the rest of the Ovipara ; 
but is distinguished by its very considerable proportionate size, which often even sur- 
passes that of this organ in the Mammalia. It is principally on the tubercles analo- 
gous to the corpora striata that this volume is dependent, and not upon the 
hemispheres, which are very small and without convolutions. The cerehellum is 
tolerably large, and almost without lateral lobes, being principally formed by the 
vermiform process. | 
The trachea of Birds has its rings entire ; at its bifurcation is a glottis, most usually 
furnished with peculiar muscles, and named the lower larynx ; it is there that the voice 
rity, — the other of immaturity ; the first having usually more decided 
and contrasted colours ; the second being comparatively sombre, with 
fainter or more blended colours, which however are commonly broken 
into various streaks or spots, and other different mottlings : where the 
latter condition, however, becomes permanent, the variegations of the 
adult bird are in general more distinctly defined ; thus a beautiful 
Himmalayan Thrush {Turdus fVhitei) , occasionally strays into 
Europe, retains the mottling of the dorsal plumage peculiar to the 
unmoulted young of other Thrushes, but the colours of those mottled 
feathers are much more finely brought out ; in like manner the distinct 
transverse bars on the adult plumage of the Bush-shrikes {Thamno- 
philus) and those on certain Woodpeckers {Colaptes), respectively 
represent the more indistinct markings of the nestling dress of the 
ordinary Shrikes (Lanius) and certain other Woodpeckers {Chryso- 
pfi/as), which barred plumage is succeeded in the latter by an adult 
garb devoid of those markings : this increased distinctness is however 
less apparent in some cases, as in that of the Bittern of North Ame- 
rica, the adult markings of which correspond, feather by feather, (their 
intensity being but inconsiderably enhanced,) with those of the im- 
mature Dwarf-bitterns already referred to. 
Accordingly, then, it is in the first plumage of Birds that the affinity 
of allied groups is ordinarily most apparent, as is analogously the case 
with the young of animals in general (the distinctions of all essen- 
tially allied groups of which continue to decrease till they disappear 
successively, as we ascend to the embryo) ; and the same remark 
applies, as might be anticipated, to the shape and structure of the 
feathers, equally with their colouring. Thus, the nestling garb is 
always much less firm than that subsequently attained ; and those 
feathers which are acuminate in the adult are rounded, or but slightly 
narrowed, in the young, and in general become gradually more 
elongated and pointed at each successive moult, till they have ac- 
quired their final shape and developement : the dorsal feathers of the 
common Heron, and clothing plumage of the Starling, may be cited 
in exemplification. In this respect, also, as with their colouring, the 
feathers of some species, compared with those of others proximately 
allied, are specifically arrested at various stages of developement : the 
adult plumage of the Bitterns represents in this particular the imma- 
ture garb of the Herons generally ; and in the weakness of texture of 
the dorsal feathers, equally with their mottled markings, the mature 
livery of the lantKocinclm corresponds with the nestling dress of the 
majority of other Birds of the Thrush tribe. 
It should be remarked that in some cases where the typical plumage 
is finally attained, this is only after a series of moultings more or less 
numerous, each successive stage of which may or may not present a 
nearer approximation to it in different species ; it being thus assumed 
gradually, or abruptly ; and, in such cases, it is generally acquired by 
the male sex sooner than by the female, where both ultimately arrive 
at it. In the European Oriole, the male alone attains the typical garb, 
but not before its third or fourth change of plumage, when it is 
assumed abruptly, or nearly so; in the Dwarf-bitterns, the male 
acquires its final livery at the first moult, the female not before the 
third or fourth moult, presenting an intermediate garb in the mean 
while, which is ultimately exchanged for the same livery as that of its 
mate. The amount of constitutional vigour tends to determine the 
period at which the more advanced condition of plumage is obtained, 
in the ratio of the average period required for its assumption : thus, 
we perceive little or no irregularity in those instances where the 
typical dress is gained at the first renewal, but considerable irregu- 
larity where the period of its assumption is ordinarily protracted ; and 
it would seem that in the latter case the females are more apt to 
acquire ultimately the most advanced livery, than in those instances 
where the male alone regularly obtains it at the first moult ; though, 
as there is always a tendency on the part of vigorous females to throw 
out the masculine attire, it may be that this apparent difference arises 
simply from the fact of such females being liable to escape notice, 
from their consequent similarity to the other sex inducing a belief 
that they belong to it, and so precluding further examination. Of 
species thus usually presenting a marked sexual diversity of plu- 
mage, we have seen females of the common Redstart, Linnet, Redpole, 
Red-backed Shrike, and Scaup Pochard, w'hich could not be distin- | 
guished externally from males ; and all of them contained eggs in the 
ovarium. 
As the assumption of the typical plumage, then, in species wherein 
it is tardily acquired, is especially dependent on the amount of con- 
stitutional vigour, it follows that captive Birds should generally arrive 
more slowly at their final livery, than those individuals which are 
unconfined ; and it might be predicated, also, that instances of captive 
females assuming the male plumage, in those species wherein the 
females ordinarily differ from the males, would be of comparatively 
uufrequent occurrence. Such are accordingly the facts : but itre()uires 
to be noticed, that any effectual injury to the ovarium, or other cause 
of sterility, also occasions female Birds to throw out the masculine 
livery (just as the Doe, mentioned at p. 137, with one schirrous ovary, 
developed an antler on the same side), this fact being very commonly 
noticed in Pheasants and domestic Poultry. On the other hand, how- 
ever, it is still more remarkable that a male bird, analogously injured, 
will sometimes even moult back from the typical plumage to that pro- 
per to the female and young ; though caponized fowls retain their 
male costume. 
We have thus far treated on the subject only under its most simple 
phase, as observed in those species which renew their plumage in 
autumn only ; and have entered somewhat into detail, from experience 
of the great assistance rendered by a knowledge of the characters thus 
afforded in tracing the affinities of groups, by simple inspection of the 
plumage: being enabled thus to perceive the systematic relationship 
of various genera at a glance, which is not obvious in the rest of their 
external characters, nor even in this one to persons unacquainted 
with the normal progressive changes characteristic of the particular 
group. In illustration, let it be supposed that a species of Sparrow 
existed (which is quite probable), the males of which, like the 
females of the House Sparrow, retained permanently the colouring of 
the nestling garb of the latter, (or, in other words, that its plumage 
presented the same analogy avith that of the House Sparrow which 
the common Bunting’s plumage does to that of its congeners): the 
affinity of such a species to the Tree Sparrow, both sexes of which 
exhibit at all ages a style of colouring corresponding to that peculiar 
to the adult male of the House Sparrow, would be rendered intelli- 
gible by the mutation incidental to the latter, even though no actual 
similitude were traceable between the plumage of the Tree Sparrow 
and that of the imagined species. There are numerous groups, then, 
the relationship of which may be at once recognized on the principle 
here indicated. 
Among those species which retain their first plumage till the second 
autumn, its aspect undergoes considerable variation in some, from 
different causes. Thus, in the Osprey, Gannet, and some others, the 
upper parts are fora while conspicuously speckled with terminal white 
spots, on a dark ground-colour ; which spots gradually disappearing, 
as the terminal edges of the feathers are naturally shed, leave the 
back uniformly dark-coloured and plain. In certain other groups, as 
in some Harriers (Circt/s), an actual change of colour takes place in 
the feathers, to a variable extent. 
In those species of Birds which undergo a double moult, the sexes 
are generally similar, or nearly so, in both states of plumage, and 
always in the winter dress ; and even the summer and winter liveries 
do not in all cases differ, as may be observed in the Tree Pipit 
{Anthus arboreus) . Where the contrary prevails in both sexes, the 
young, in their first down, are subject to possess the colouring of the 
adult summer garb, as noticeable in the common Guillemot and 
Razorbill ; and, in the plumage which succeeds the down, to resemble 
the mature winter dress, or to present a combination of the two, 
which is not uncommon — particularly among the small waders, which 
subsequently attain their proper winter clothing plumage by a moult 
towards the close of autumn. When the breeding livery of the male 
and female differs, the same law prevails as in single-moulting Birds. 
We have not space to enter more minutely into detail.— Ed. 
