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 cerebellum 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 nther of immaturity ; the first having usually more decided 

 and contrasted colours ; the second being comparatively sombre, with 

 fiiinter 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 

 Hiinnialayan Thrush {Tardus fVhltet)^ wWich. occasionally strays into 

 Europe, retains the mottling of the dorsal plumage peculiar to the 

 unnioulted 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 {Thnmno- 

 philus) and those on certain Woodpeckers {Colaptes), respectively 

 represent the more indistinct markings of the nestling dress of the 

 ordinary Shrikes [Lunh/s) and certain other Woodpeckers {Chrysu- 

 ;B/i/Ks), 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 

 mtensity 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 \^'e ascend to the embryo) ; and the same remark 

 applies, as might be anticipated, to the shape and structure of the 

 feathers, equtlly 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 rif 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 lantkucinclce corresponds with the nestling dress of the 

 majority of other Birds of the Thrush tribe. 



It sliould 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 plumnge, when it is 

 assumed abruptly, or nearly so; in the Dwarf-bitterns, the male 

 acquires its final Hvery 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 su(--h 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 presentiug a marked sexual diversity of plu- 



mage, we have seen females of the common Redstart, Linnet, Redpole, 

 Red-backed Shrike, and Scaup Pochard, which could not be distin- 

 guished externally from males j 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 

 unfrequent occurrence. Such are accordingly the facts : but it reijuircs 

 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 Uoe, mentioned at p. 137, ^'Hh one schirrous ovary, 

 developed an autler 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 



nale 



stuu 



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 

 affurded 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 with 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 for a 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 {Circus)^ au actual change of C(dour 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 arboreits) . 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-moultiug Birds. 

 We have not space to enter more minutely into detail.— £d. 



