OCR Text |
Show 206 SEXUAL SELECTION: BIRDS. PART If. those with duller plumage and grey legs were the T~ales or the young. In an Australian tree-creeper ( Ol~ma:ieris eryih'rops) the female differs from the male m "being adorned with beautiful, radiated, rufous mark" ings on the throat, the mal~ ha:ing .thi~' part quite ,, plain." Lastly in an Australian mght:Jar the .fe:nale "always exceeds the male in size and m the bnlhance "of her tints; the males, on the other hand, have two "white spots on the primaries more conspicuous than "in the female." 25 We thus see that the cases in which female birds are more conspicuously coloured than the males, with the young in their immature plumage rese~bling the a~ult males instead of the adult females, as m the prevwus class, are not numerous, though they are distributed in various Orders. The amount of difference, also, between the sexes is incomparably less than that which frequently occurs in the last class ; so that the cause of the difference, whatever it may have been, has acted on the females in the present class either less energetically or less persistently than on the males in the 'last class. Mr. Wall ace believes that the males have had their colours 25 For tho Milvago, sec' Zoology of the Voyage of the "Beagle,"' llirds 1841, p. 16. For tho ClimRcteris and night-jar (Eurostopodus), sec G~uld's 'Handbook of tho Birds of Australia,' vol. i. p. 602 and fJ7. The Now Zealand shieldruko (Tadorna variegctta) offers a quito anomalous case: tho head of the female is pure white, and her back is redder thnn that of the mo.le; tho head of the mal() is of a rich dark bronzed colour, and his back is clothed with finely pencilled slate-coloured feathers, so that he may altogether bo considered as the more beautiful of tho two. He is lurgor and more pugnacious than the female, and does not sit on the eggs. So that in ail these respects this species comes unucr our first claes of cases; but Mr. Sclater (' Proc. Zool. Soc.' 1866, p. 150) was much surprised to observe that the young of both sexes, when about three months old, resembled in their dark heads and necks tho adult males, instead of tho adult females; so that it would appear in this case that the females ;have been modified, wLilst the male~ unu the young have retained a former state of plumage. CHAP. XVI. THE YOUNG Lll(E THE ADULT MALES. 207 rendered less conspicuous for the sake of protection during the period of incubation ; but the difference between the sexes in hardly any of the foregoing cases appears sufficiently great for this view to be safely accepted. In some of the cases the brighter tints of tho female are almost confined to the lower surface, and the males, if thus coloured, would not have been exposed to danger whilst sitting on the eggs. It should also be borne in mind that the males are not only in a slight degree less conspicuously coloured than the females, but are ofless size, and have less strength. They have, moreover, not only acquired the maternal instinct of incubation, but are less pugnacious and vociferous than the females, and in one instance have simpler vocal organs. Thus an almost complete transposition of the instincts, habits, disposition, colour, size, and of some points of structure, has been effected between the two sexes. Now if we might assume that the males in the present class have lost some of that ardour which is usual to their sex, so that they no longer search eagerly for the females; or, if we might assume that the females have become much more numerous than the males-and in the case of one Indian Turnix the females are said to be "much more commonly met with than the males" 26_ then it is not improbable that the females would have been led to court the males, instead of being courted by them. This indeed is the case to a certain extent with some bird~, as :ve have seen with the peahen, wild t~rkey, and. certam kmds of grouse. Taking as our guide the habits of most male birds, the greater size and strength and the extraordinary pugnacity of the females of the T~rnix and Emu, must mean that they endeavour to dnve away rival females, in order to gain possession of 26 J erdon,.' Birds of India,' vol. iii. p. 5!J8. |