The Descent of Man and Selection in Relation to Sex Volume II Part 2

It is remarkable that only small birds properly sing. The Australian genus Menura, however, must be excepted; for the _Menura Alberti_, which is about the size of a half-grown turkey, not only mocks other birds, but "its own whistle is exceedingly beautiful and varied." The males congregate and form "_corroborying_ places," where they sing, raising and spreading their tails like peac.o.c.ks and drooping their wings.[97]

It is also remarkable that the birds which sing are rarely decorated with brilliant colours or other ornaments. Of our British birds, excepting the bullfinch and goldfinch, the best songsters are plain-coloured. The kingfisher, bee-eater, roller, hoopoe, woodp.e.c.k.e.rs, &c., utter harsh cries; and the brilliant birds of the tropics are hardly ever songsters.[98] Hence bright colours and the power of song seem to replace each other. We can perceive that if the plumage did not vary in brightness, or if bright colours were dangerous to the species, other means would have to be employed to charm the females; and the voice being rendered melodious would offer one such means.

[Ill.u.s.tration: Fig. 39. Tetrao cupido; male. (From Brehm.)]

In some birds the vocal organs differ greatly in the two s.e.xes. In the _Tetrao cupido_ (fig. 39) the male has two bare, orange-coloured sacks, one on each side of the neck; and these are largely inflated when the male, during the breeding-season, makes a curious hollow sound, audible at a great distance. Audubon proved that the sound was intimately connected with this apparatus, which reminds us of the air-sacks on each side of the mouth of certain male frogs, for he found that the sound was much diminished when one of the sacks of a tame bird was p.r.i.c.ked, and when both were p.r.i.c.ked it was altogether stopped. The female has "a somewhat similar, though smaller, naked s.p.a.ce of skin on the neck; but this is not capable of inflation."[99] The male of another kind of grouse (_Tetrao urophasia.n.u.s_), whilst courting the female, has his "bare yellow sophagus inflated to a prodigious size, fully half as large as the body;" and he then utters various grating, deep hollow tones. With his neck-feathers erect, his wings lowered and buzzing on the ground, and his long pointed tail spread out like a fan, he displays a variety of grotesque att.i.tudes. The sophagus of the female is not in any way remarkable.[100]

It seems now well made out that the great throat-pouch of the European male bustard (_Otis tarda_), and of at least four other species, does not serve, as was formerly supposed, to hold water, but is connected with the utterance during the breeding-season of a peculiar sound resembling "ock." The bird whilst uttering this sound throws himself into the most extraordinary att.i.tudes. It is a singular fact that with the males of the same species the sack is not developed in all the individuals.[101] A crow-like bird inhabiting South America (_Cephalopterus ornatus_, fig. 40) is called the umbrella-bird, from its immense top-knot, formed of bare white quills surmounted by dark-blue plumes, which it can elevate into a great dome no less than five inches in diameter, covering the whole head. This bird has on its neck a long, thin, cylindrical, fleshy appendage, which is thickly clothed with scale-like blue feathers. It probably serves in part as an ornament, but likewise as a resounding apparatus, for Mr. Bates found that it is connected "with an unusual development of the trachea and vocal organs."

It is dilated when the bird utters its singularly deep, loud, and long-sustained fluty note. The head-crest and neck-appendage are rudimentary in the female.[102]

[Ill.u.s.tration: Fig. 40. The Umbrella-bird or Cephalopterus ornatus (male, from Brehm).]

The vocal organs of various web-footed and wading birds are extraordinarily complex, and differ to a certain extent in the two s.e.xes. In some cases the trachea is convoluted, like a French horn, and is deeply embedded in the sternum. In the wild swan (_Cygnus ferus_) it is more deeply embedded in the adult male than in the female or young male. In the male Merganser the enlarged portion of the trachea is furnished with an additional pair of muscles.[103] But the meaning of these differences between the s.e.xes of many Anatidae is not at all understood; for the male is not always the more vociferous; thus with the common duck, the male hisses, whilst the female utters a loud quack.[104] In both s.e.xes of one of the cranes (_Grus virgo_) the trachea penetrates the sternum, but presents "certain s.e.xual modifications." In the male of the black stork there is also a well-marked s.e.xual difference in the length and curvature of the bronchi.[105] So that highly important structures have in these cases been modified according to s.e.x.

It is often difficult to conjecture whether the many strange cries and notes, uttered by male birds during the breeding-season, serve as a charm or merely as a call to the female. The soft cooing of the turtle-dove and of many pigeons, it may be presumed, pleases the female.

When the female of the wild turkey utters her call in the morning, the male answers by a different note from the gobbling noise which he makes, when with erected feathers, rustling wings and distended wattles, he puffs and struts before her.[106] The _spel_ of the black-c.o.c.k certainly serves as a call to the female, for it has been known to bring four or five females from a distance to a male under confinement; but as the black-c.o.c.k continues his _spel_ for hours during successive days, and in the case of the capercailzie "with an agony of pa.s.sion," we are led to suppose that the females which are already present are thus charmed.[107] The voice of the common rook is known to alter during the breeding-season, and is therefore in some way s.e.xual.[108] But what shall we say about the harsh screams of, for instance, some kinds of macaws; have these birds as bad taste for musical sounds as they apparently have for colour, judging by the inharmonious contrast of their bright yellow and blue plumage? It is indeed possible that the loud voices of many male birds may be the result, without any advantage being thus gained, of the inherited effects of the continued use of their vocal organs, when they are excited by the strong pa.s.sions of love, jealousy, and rage; but to this point we shall recur when we treat of quadrupeds.

We have as yet spoken only of the voice, but the males of various birds practise, during their courts.h.i.+p, what may be called instrumental music.

Peac.o.c.ks and Birds of Paradise rattle their quills together, and the vibratory movement apparently serves merely to make a noise, for it can hardly add to the beauty of their plumage. Turkey-c.o.c.ks sc.r.a.pe their wings against the ground, and some kinds of grouse thus produce a buzzing sound. Another North American grouse, the _Tetrao umbellus_, when with his tail erect, his ruffs displayed, "he shows off his finery to the females, who lie hid in the neighbourhood," drums rapidly with his "lowered wings on the trunk of a fallen tree," or, according to Audubon, against his own body; the sound thus produced is compared by some to distant thunder, and by others to the quick roll of a drum. The female never drums, "but flies directly to the place where the male is thus engaged." In the Himalayas the male of the Kalij pheasant "often makes a singular drumming noise with his wings, not unlike the sound produced by shaking a stiff piece of cloth." On the west coast of Africa the little black-weavers (Ploceus?) congregate in a small party on the bushes round a small open s.p.a.ce, and sing and glide through the air with quivering wings, "which make a rapid whirring sound like a child's rattle." One bird after another thus performs for hours together, but only during the courting-season. At this same season the males of certain nightjars (Caprimulgus) make a most strange noise with their wings. The various species of woodp.e.c.k.e.rs strike a sonorous branch with their beaks, with so rapid a vibratory movement that "the head appears to be in two places at once." The sound thus produced is audible at a considerable distance, but cannot be described; and I feel sure that its cause would never be conjectured by any one who heard it for the first time. As this jarring sound is made chiefly during the breeding-season, it has been considered as a love-song; but it is perhaps more strictly a love-call. The female, when driven from her nest, has been observed thus to call her mate, who answered in the same manner and soon appeared.

Lastly the male Hoopoe (_Upupa epops_) combines vocal and instrumental music; for during the breeding-season this bird, as Mr. Swinhoe saw, first draws in air and then taps the end of its beak perpendicularly down against a stone or the trunk of a tree, "when the breath being forced down the tubular bill produces the correct sound." When the male utters its cry without striking his beak the sound is quite different.[109]

[Ill.u.s.tration: Fig. 41. Outer tail-feather of Scolopax gallinago (from Proc. Zool. Soc. 1858).]

In the foregoing cases sounds are made by the aid of structures already present and otherwise necessary; but in the following cases certain feathers have been specially modified for the express purpose of producing the sounds. The drumming, or bleating, or neighing, or thundering noise, as expressed by different observers, which is made by the common snipe (_Scolopax gallinago_) must have surprised every one who has ever heard it. This bird, during the pairing-season, flies to "perhaps a thousand feet in height," and after zig-zagging about for a time descends in a curved line, with outspread tail and quivering pinions, with surprising velocity to the earth. The sound is emitted only during this rapid descent. No one was able to explain the cause, until M. Meves observed that on each side of the tail the outer feathers are peculiarly formed (fig. 41), having a stiff sabre-shaped shaft, with the oblique barbs of unusual length, the outer webs being strongly bound together.

He found that by blowing on these feathers, or by fastening them to a long thin stick and waving them rapidly through the air, he could exactly reproduce the drumming noise made by the living bird. Both s.e.xes are furnished with these feathers, but they are generally larger in the male than in the female, and emit a deeper note. In some species, as in _S. frenata_ (fig. 42), four feathers, and in _S. javensis_ (fig. 43), no less than eight on each side of the tail are greatly modified.

Different tones are emitted by the feathers of the different species when waved through the air; and the _Scolopax Wilsonii_ of the United States makes a switching noise whilst descending rapidly to the earth.[110]

[Ill.u.s.tration: Fig. 42. Outer tail-feather of Scolopax frenata.]

[Ill.u.s.tration: Fig. 43. Outer tail-feather of Scolopax javensis.]

In the male of the _Chamaepetes unicolor_ (a large gallinaceous bird of America) the first primary wing-feather is arched towards the tip and is much more attenuated than in the female. In an allied bird, the _Penelope nigra_, Mr. Salvin observed a male, which, whilst it flew downwards "with outstretched wings, gave forth a kind of cras.h.i.+ng, rus.h.i.+ng noise," like the falling of a tree.[111] The male alone of one of the Indian bustards _(Sypheotides auritus_) has its primary wing-feathers greatly ac.u.minated; and the male of an allied species is known to make a humming noise whilst courting the female.[112] In a widely different group of birds, namely the Humming-birds, the males alone of certain kinds have either the shafts of their primary wing-feathers broadly dilated, or the webs abruptly excised towards the extremity. The male, for instance, of _Selasphorus platycercus_, when adult, has the first primary wing-feather (fig. 44), excised in this manner. Whilst flying from flower to flower he makes "a shrill, almost whistling, noise;"[113] but it did not appear to Mr. Salvin that the noise was intentionally made.

[Ill.u.s.tration: Fig. 44. Primary wing-feather of a Humming-bird, the _Selasphorus platycercus_ (from a sketch by Mr. Salvin). Upper figure, that of male; lower figure, corresponding feather of female.]

Lastly, in several species of a sub-genus of Pipra or Manakin, the males have their _secondary_ wing-feathers modified, as described by Mr.

Sclater, in a still more remarkable manner. In the brilliantly-coloured _P. deliciosa_ the first three secondaries are thick-stemmed and curved towards the body; in the fourth and fifth (fig. 45, _a_) the change is greater; and in the sixth and seventh (_b_, _c_) the shaft "is thickened to an extraordinary degree, forming a solid h.o.r.n.y lump." The barbs also are greatly changed in shape, in comparison with the corresponding feathers (_d_, _e_, _f_) in the female. Even the bones of the wing which support these singular feathers in the male are said by Mr. Fraser to be much thickened. These little birds make an extraordinary noise, the first "sharp note being not unlike the crack of a whip."[114]

[Ill.u.s.tration: Fig. 45. Secondary wing-feathers of _Pipra deliciosa_(from Mr. Sclater, in Proc, Zool. Soc. 1860). The three upper feathers, _a_, _b_, _c_, from the male; the three lower corresponding feathers, _d_, _e_, _f_, from the female.

_a._ and _d._ Fifth secondary wing-feather of male and female, upper surface. _b_ and _e_. Sixth secondary, upper surface. _c_ and _f_.

Seventh secondary, lower surface.]

The diversity of the sounds, both vocal and instrumental, made by the males of many species during the breeding-season, and the diversity of the means for producing such sounds, are highly remarkable. We thus gain a high idea of their importance for s.e.xual purposes, and are reminded of the same conclusion with respect to insects. It is not difficult to imagine the steps by which the notes of a bird, primarily used as a mere call or for some other purpose, might have been improved into a melodious love-song. This is somewhat more difficult in the case of the modified feathers, by which the drumming, whistling, or roaring noises are produced. But we have seen that some birds during their courts.h.i.+p flutter, shake, or rattle their unmodified feathers together; and if the females were led to select the best performers, the males which possessed the strongest or thickest, or most attenuated feathers, situated on any part of the body, would be the most successful; and thus by slow degrees the feathers might be modified to almost any extent. The females, of course, would not notice each slight successive alteration in shape, but only the sounds thus produced. It is a curious fact that in the same cla.s.s of animals, sounds so different as the drumming of the snipe's tail, the tapping of the woodp.e.c.k.e.r's beak, the harsh trumpet-like cry of certain water-fowl, the cooing of the turtle-dove, and the song of the nightingale, should all be pleasing to the females of the several species. But we must not judge the tastes of distinct species by a uniform standard; nor must we judge by the standard of man's taste. Even with man, we should remember what discordant noises, the beating of tom-toms and the shrill notes of reeds, please the ears of savages. Sir S. Baker remarks,[115] that "as the stomach of the Arab prefers the raw meat and reeking liver taken hot from the animal, so does his ear prefer his equally coa.r.s.e and discordant music to all other."

_Love-Antics and Dances._-The curious love-gestures of various birds, especially of the Gallinaceae, have already been incidentally noticed; so that little need here be added. In Northern America, large numbers of a grouse, the _Tetrao phasianellus_, meet every morning during the breeding-season on a selected level spot, and here they run round and round in a circle of about fifteen or twenty feet in diameter, so that the ground is worn quite bare, like a fairy-ring. In these Partridge-dances, as they are called by the hunters, the birds a.s.sume the strangest att.i.tudes, and run round, some to the left and some to the right. Audubon describes the males of a heron (_Ardea herodias_) as walking about on their long legs with great dignity before the females, bidding defiance to their rivals. With one of the disgusting carrion-vultures (_Cathartes jota_) the same naturalist states that "the gesticulations and parade of the males at the beginning of the love-season are extremely ludicrous." Certain birds perform their love-antics on the wing, as we have seen with the black African weaver, instead of on the ground. During the spring our little white-throat (_Sylvia cinerea_) often rises a few feet or yards in the air above some bush, and "flutters with a fitful and fantastic motion, singing all the while, and then drops to its perch." The great English bustard throws himself into indescribably odd att.i.tudes whilst courting the female, as has been figured by Wolf. An allied Indian bustard (_Otis bengalensis_) at such times "rises perpendicularly into the air with a hurried flapping of his wings, raising his crest and puffing out the feathers of his neck and breast, and then drops to the ground;" he repeats this manuvre several times successively, at the same time humming in a peculiar tone. Such females as happen to be near "obey this saltatory summons," and when they approach he trails his wings and spreads his tail like a turkey-c.o.c.k.[116]

But the most curious case is afforded by three allied genera of Australian birds, the famous Bower-birds,-no doubt the co-descendants of some ancient species which first acquired the strange instinct of constructing bowers for performing their love-antics. The bowers (fig.

46), which, as we shall hereafter see, are highly decorated with feathers, sh.e.l.ls, bones and leaves, are built on the ground for the sole purpose of courts.h.i.+p, for their nests are formed in trees. Both s.e.xes a.s.sist in the erection of the bowers, but the male is the princ.i.p.al workman. So strong is this instinct that it is practised under confinement, and Mr. Strange has described[117] the habits of some Satin Bower-birds, which he kept in his aviary in New South Wales. "At times the male will chase the female all over the aviary, then go to the bower, pick up a gay feather or a large leaf, utter a curious kind of note, set all his feathers erect, run round the bower and become so excited that his eyes appear ready to start from his head; he continues opening first one wing, and then the other, uttering a low, whistling note, and, like the domestic c.o.c.k, seems to be picking up something from the ground, until at last the female goes gently towards him."

Captain Stokes has described the habits and "play-houses" of another species, the Great Bower-bird, which was seen "amusing itself by flying backwards and forwards, taking a sh.e.l.l alternately from each side, and carrying it through the archway in its mouth." These curious structures, formed solely as halls of a.s.semblages, where both s.e.xes amuse themselves and pay their court, must cost the birds much labour. The bower, for instance, of the fawn-breasted species, is nearly four feet in length, eighteen inches in height, and is raised on a thick platform of sticks.

[Ill.u.s.tration: Fig. 46. Bower-bird, Chlamydera maculata, with bower (from Brehm).]

_Decoration._-I will first discuss the cases in which the males are ornamented either exclusively or in a much higher degree than the females; and in a succeeding chapter those in which both s.e.xes are equally ornamented, and finally the rare cases in which the female is somewhat more brightly-coloured than the male. As with the artificial ornaments used by savage and civilised men, so with the natural ornaments of birds, the head is the chief seat of decoration.[118] The ornaments, as mentioned at the commencement of this chapter, are wonderfully diversified. The plumes on the front or back of the head consist of variously-shaped feathers, sometimes capable of erection or expansion, by which their beautiful colours are fully displayed. Elegant ear-tufts (see fig. 39 ante) are occasionally present. The head is sometimes covered with velvety down like that of the pheasant; or is naked and vividly coloured; or supports fleshy appendages, filaments, and solid protuberances. The throat, also, is sometimes ornamented with a beard, or with wattles or caruncles. Such appendages are generally brightly coloured, and no doubt serve as ornaments, though not always ornamental in our eyes; for whilst the male is in the act of courting the female, they often swell and a.s.sume more vivid tints, as in the case of the male turkey. At such times the fleshy appendages about the head of the male Tragopan pheasant (_Ceriornis temminckii_) swell into a large lappet on the throat and into two horns, one on each side of the splendid top-knot; and these are then coloured of the most intense blue which I have ever beheld. The African hornbill (_Bucorax abyssinicus_) inflates the scarlet bladder-like wattle on its neck, and with its wings drooping and tail expanded "makes quite a grand appearance."[119] Even the iris of the eye is sometimes more brightly coloured in the male than in the female; and this is frequently the case with the beak, for instance, in our common blackbird. In _Buceros corrugatus_, the whole beak and immense casque are coloured more conspicuously in the male than in the female; and "the oblique grooves upon the sides of the lower mandible are peculiar to the male s.e.x."[120]

The males are often ornamented with elongated feathers or plumes springing from almost every part of the body. The feathers on the throat and breast are sometimes developed into beautiful ruffs and collars. The tail-feathers are frequently increased in length; as we see in the tail-coverts of the peac.o.c.k, and in the tail of the Argus pheasant. The body of this latter bird is not larger than that of a fowl; yet the length from the end of the beak to the extremity of the tail is no less than five feet three inches.[121] The wing-feathers are not elongated nearly so often as the tail-feathers; for their elongation would impede the act of flight. Yet the beautifully ocellated secondary wing-feathers of the male Argus pheasant are nearly three feet in length; and in a small African nightjar (_Cosmetornis vexillarius_) one of the primary wing-feathers, during the breeding-season, attains a length of twenty-six inches, whilst the bird itself is only ten inches in length.

In another closely-allied genus of nightjars, the shafts of the elongated wing-feathers are naked, except at the extremity, where there is a disc.[122] Again, in another genus of nightjars, the tail-feathers are even still more prodigiously developed; so that we see the same kind of ornament gained by the males of closely-allied birds, through the development of widely different feathers.

It is a curious fact that the feathers of birds belonging to distinct groups have been modified in almost exactly the same peculiar manner.

Thus the wing-feathers in one of the above-mentioned nightjars are bare along the shaft and terminate in a disc; or are, as they are sometimes called, spoon or racket-shaped. Feathers of this kind occur in the tail of a motmot (_Eumomota superciliaris_), of a kingfisher, finch, humming-bird, parrot, several Indian drongos (_Dicrurus_ and _Edolius_, in one of which the disc stands vertically), and in the tail of certain Birds of Paradise. In these latter birds, similar feathers, beautifully ocellated, ornament the head, as is likewise the case with some gallinaceous birds. In an Indian bustard (_Sypheotides auritus_) the feathers forming the ear-tufts, which are about four inches in length, also terminate in discs.[123] The barbs of the feathers in various widely-distinct birds are filamentous or plumose, as with some Herons, Ibises, Birds of Paradise and Gallinaceae. In other cases the barbs disappear, leaving the shafts bare; and these in the tail of the _Paradisea apoda_ attain a length of thirty-four inches.[124] Smaller feathers when thus denuded appear like bristles, as on the breast of the turkey-c.o.c.k. As any fleeting fas.h.i.+on in dress comes to be admired by man, so with birds a change of almost any kind in the structure or colouring of the feathers in the male appears to have been admired by the female. The fact of the feathers in widely distinct groups, having been modified in an a.n.a.logous manner, no doubt depends primarily on all the feathers having nearly the same structure and manner of development, and consequently tending to vary in the same manner. We often see a tendency to a.n.a.logous variability in the plumage of our domestic breeds belonging to distinct species. Thus top-knots have appeared in several species. In an extinct variety of the turkey, the top-knot consisted of bare quills surmounted with plumes of down, so that they resembled, to a certain extent, the racket-shaped feathers above described. In certain breeds of the pigeon and fowl the feathers are plumose, with some tendency in the shafts to be naked. In the Sebastopol goose the scapular feathers are greatly elongated, curled, or even spirally twisted, with the margins plumose.[125]

[Ill.u.s.tration: Fig. 47. Paradisea rubra, male (from Brehm)]

[Ill.u.s.tration: Fig. 48. Lophornis ornatus, male and female (from Brehm).]

In regard to colour hardly anything need here be said; for every one knows how splendid are the tints of birds, and how harmoniously they are combined. The colours are often metallic and iridescent. Circular spots are sometimes surrounded by one or more differently shaded zones, and are thus converted into ocelli. Nor need much be said on the wonderful differences between the s.e.xes, or of the extreme beauty of the males of many birds. The common peac.o.c.k offers a striking instance.

Female Birds of Paradise are obscurely coloured and dest.i.tute of all ornaments, whilst the males are probably the most highly decorated of all birds, and in so many ways, that they must be seen to be appreciated. The elongated and golden-orange plumes which spring from beneath the wings of the _Paradisea apoda_ (see fig. 47 of _P. rubra_, a much less beautiful species), when vertically erected and made to vibrate, are described as forming a sort of halo, in the centre of which the head "looks like a little emerald sun with its rays formed by the two plumes."[126] In another most beautiful species the head is bald, "and of a rich cobalt blue, crossed by several lines of black velvety feathers."[127]

[Ill.u.s.tration: Fig. 49. Spathura underwoodi, male and female (from Brehm).]

Male humming-birds (figs. 48 and 49) almost vie with Birds of Paradise in their beauty, as every one will admit who has seen Mr. Gould's splendid volumes or his rich collection. It is very remarkable in how many different ways these birds are ornamented. Almost every part of the plumage has been taken advantage of and modified; and the modifications have been carried, as Mr. Gould shewed me, to a wonderful extreme in some species belonging to nearly every sub-group. Such cases are curiously like those which we see in our fancy breeds, reared by man for the sake of ornament: certain individuals originally varied in one character, and other individuals belonging to the same species in other characters; and these have been seized on by man and augmented to an extreme point-as the tail of the fantail-pigeon, the hood of the jacobin, the beak and wattle of the carrier, and so forth. The sole difference between these cases is that in the one the result is due to man's selection, whilst in the other, as with Humming-birds, Birds of Paradise, &c., it is due to s.e.xual selection,-that is to the selection by the females of the more beautiful males.

I will mention only one other bird, remarkable from the extreme contrast in colour between the s.e.xes, namely the famous Bell-bird (_Chasmorhynchus niveus_) of S. America, the note of which can be distinguished at the distance of nearly three miles, and astonishes every one who first hears it. The male is pure white, whilst the female is dusky-green; and the former colour with terrestrial species of moderate size and inoffensive habits is very rare. The male, also, as described by Waterton, has a spiral tube, nearly three inches in length, which rises from the base of the beak. It is jet-black, dotted over with minute downy feathers. This tube can be inflated with air, through a communication with the palate; and when not inflated hangs down on one side. The genus consists of four species, the males of which are very distinct, whilst the females, as described by Mr. Sclater in a most interesting paper, closely resemble each other, thus offering an excellent instance of the common rule that within the same group the males differ much more from each other than do the females. In a second species (_C. nudicollis_) the male is likewise snow-white, with the exception of a large s.p.a.ce of naked skin on the throat and round the eyes, which during the breeding-season is of a fine green colour. In a third species (_C. tricarunculatus_) the head and neck alone of the male are white, the rest of the body being chesnut-brown, and the male of this species is provided with three filamentous projections half as long as the body-one rising from the base of the beak and the two others from the corners of the mouth.[128]

The coloured plumage and certain other ornaments of the males when adult are either retained for life or are periodically renewed during the summer and breeding-season. At this season the beak and naked skin about the head frequently change colour, as with some herons, ibises, gulls, one of the bell-birds just noticed, &c. In the white ibis, the cheeks, the inflatable skin of the throat, and the basal portion of the beak, then become crimson.[129] In one of the rails, _Gallicrex cristatus_ a large red caruncle is developed during this same period on the head of the male. So it is with a thin h.o.r.n.y crest on the beak of one of the pelicans, _P. erythrorhynchus_; for after the breeding-season, these h.o.r.n.y crests are shed, like horns from the heads of stags, and the sh.o.r.e of an island in a lake in Nevada was found covered with these curious exuviae.[130]

Changes of colour in the plumage according to the season depend firstly on a double annual moult, secondly on an actual change of colour in the feathers themselves, and thirdly on their dull-coloured margins being periodically shed, or on these three processes more or less combined.

The shedding of the deciduary margins may be compared with the shedding by very young birds of their down; for the down in most cases arises from the summits of the first true feathers.[131]

With respect to the birds which annually undergo a double moult, there are, firstly, some kinds, for instance snipes, swallow-plovers (Glareolae), and curlews, in which the two s.e.xes resemble each other and do not change colour at any season. I do not know whether the winter-plumage is thicker and warmer than the summer-plumage, which seems, when there is no change of colour, the most probable cause of a double moult. Secondly, there are birds, for instance certain species of Tota.n.u.s and other grallatores, the s.e.xes of which resemble each other, but have a slightly different summer and winter plumage. The difference, however, in colour in these cases is so slight that it can hardly be an advantage to them; and it may, perhaps, be attributed to the direct action of the different conditions to which the birds are exposed during the two seasons. Thirdly, there are many other birds the s.e.xes of which are alike, but which are widely different in their summer and winter plumage. Fourthly, there are birds, the s.e.xes of which differ from each other in colour; but the females, though moulting twice, retain the same colours throughout the year, whilst the males undergo a change, sometimes, as with certain bustards, a great change of colour. Fifthly and lastly, there are birds the s.e.xes of which differ from each other in both their summer and winter plumage, but the male undergoes a greater amount of change at each recurrent season than the female-of which the Ruff (_Machetes pugnax_) offers a good instance.

With respect to the cause or purpose of the differences in colour between the summer and winter plumage, this may in some instances, as with the ptarmigan,[132] serve during both seasons as a protection. When the difference between the two plumages is slight it may perhaps be attributed, as already remarked, to the direct action of the conditions of life. But with many birds there can hardly be a doubt that the summer plumage is ornamental, even when both s.e.xes are alike. We may conclude that this is the case with many herons, egrets, &c., for they acquire their beautiful plumes only during the breeding-season. Moreover, such plumes, top-knots, &c., though possessed by both s.e.xes, are occasionally a little more highly developed in the male than in the female; and they resemble the plumes and ornaments possessed by the males alone of other birds. It is also known that confinement, by affecting the reproductive system of male birds, frequently checks the development of their secondary s.e.xual characters, but has no immediate influence on any other characters; and I am informed by Mr. Bartlett that eight or nine specimens of the Knot (_Tringa canutus_) retained their unadorned winter plumage in the Zoological Gardens throughout the year, from which fact we may infer that the summer plumage though common to both s.e.xes partakes of the nature of the exclusively masculine plumage of many other birds.[133]

From the foregoing facts, more especially from neither s.e.x of certain birds changing colour during either annual moult, or changing so slightly that the change can hardly be of any service to them, and from the females of other species moulting twice yet retaining the same colours throughout the year, we may conclude that the habit of moulting twice in the year has not been acquired in order that the male should a.s.sume during the breeding-season an ornamental character; but that the double moult, having been originally acquired for some distinct purpose, has subsequently been taken advantage of in certain cases for gaining a nuptial plumage.

It appears at first sight a surprising circ.u.mstance that with closely-allied birds, some species should regularly undergo a double annual moult, and others only a single one. The ptarmigan, for instance, moults twice or even thrice in the year, and the black-c.o.c.k only once: some of the splendidly-coloured honey-suckers (Nectariniae) of India and some sub-genera of obscurely-coloured pipits (Anthus) have a double, whilst others have only a single annual moult.[134] But the gradations in the manner of moulting, which are known to occur with various birds, shew us how species, or whole groups of species, might have originally acquired their double annual moult, or having once gained the habit, have again lost it. With certain bustards and plovers the vernal moult is far from complete, some feathers being renewed, and some changed in colour. There is also reason to believe that with certain bustards and rail-like birds, which properly undergo a double moult, some of the older males retain their nuptial plumage throughout the year. A few highly modified feathers may alone be added during the spring to the plumage, as occurs with the disc-formed tail-feathers of certain drongos (_Bhringa_) in India, and with the elongated feathers on the back, neck, and crest of certain herons. By such steps as these, the vernal moult might be rendered more and more complete, until a perfect double moult was acquired. A gradation can also be shewn to exist in the length of time during which either annual plumage is retained; so that the one might come to be retained for the whole year, the other being completely lost. Thus the _Machetes pugnax_ retains his ruff in the spring for barely two months. The male widow-bird (_Chera progne_) acquires in Natal his fine plumage and long tail-feathers in December or January and loses them in March; so that they are retained during only about three months. Most species which undergo a double moult keep their ornamental feathers for about six months. The male, however, of the wild _Gallus bankiva_ retains his neck-hackles for nine or ten months; and when these are cast off, the underlying black feathers on the neck are fully exposed to view. But with the domesticated descendant of this species, the neck-hackles of the male are immediately replaced by new ones; so that we here see, with respect to part of the plumage, a double moult changed under domestication into a single moult.[135]

The common drake (_Anas boschas_) is well known after the breeding-season to lose his male plumage for a period of three months, during which time he a.s.sumes that of the female. The male pintail-duck (_Anas acuta_) loses his plumage for the shorter period of six weeks or two months; and Montagu remarks that "this double moult within so short a time is a most extraordinary circ.u.mstance, that seems to bid defiance to all human reasoning." But he who believes in the gradual modification of species will be far from feeling surprise at finding gradations of all kinds. If the male pintail were to acquire his new plumage within a still shorter period, the new male feathers would almost necessarily be mingled with the old, and both with some proper to the female; and this apparently is the case with the male of a not distantly-allied bird, namely the _Merganser serrator_, for the males are said to "undergo a change of plumage, which a.s.similates them in some measure to the female." By a little further acceleration in the process, the double moult would be completely lost.[136]

Some male birds, as before stated, become more brightly coloured in the spring, not by a vernal moult, but either by an actual change of colour in the feathers, or by their obscurely-coloured deciduary margins being shed. Changes of colour thus caused may last for a longer or shorter time. With the _Peleca.n.u.s onocrotalus_ a beautiful rosy tint, with lemon-coloured marks on the breast, overspreads the whole plumage in the spring; but these tints, as Mr. Sclater states, "do not last long, disappearing generally in about six weeks or two months after they have been attained." Certain finches shed the margins of their feathers in the spring, and then become brighter-coloured, while other finches undergo no such change. Thus the _Fringilla tristis_ of the United States (as well as many other American species), exhibits its bright colours only when the winter is past, whilst our goldfinch, which exactly represents this bird in habits, and our siskin, which represents it still more closely in structure, undergo no such annual change. But a difference of this kind in the plumage of allied species is not surprising, for with the common linnet, which belongs to the same family, the crimson forehead and breast are displayed only during the summer in England, whilst in Madeira these colours are retained throughout the year.[137]

_Display by Male Birds of their Plumage._-Ornaments of all kinds, whether permanently or temporarily gained, are sedulously displayed by the males, and apparently serve to excite, or attract, or charm the females. But the males will sometimes display their ornaments, when not in the presence of the females, as occasionally occurs with grouse at their balz-places, and as may be noticed with the peac.o.c.k; this latter bird, however, evidently wishes for a spectator of some kind, and will shew off his finery, as I have often seen, before poultry or even pigs.[138] All naturalists who have closely attended to the habits of birds, whether in a state of nature or under confinement, are unanimously of opinion that the males delight to display their beauty.

Audubon frequently speaks of the male as endeavouring in various ways to charm the female. Mr. Gould, after describing some peculiarities in a male humming-bird, says he has no doubt that it has the power of displaying them to the greatest advantage before the female. Dr.

Jerdon[139] insists that the beautiful plumage of the male serves "to fascinate and attract the female." Mr. Bartlett, at the Zoological Gardens, expressed himself to me in the strongest terms to the same effect.