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Show 172 SEXUAL SELECTION : BIRDS. PART H. rrhis way of viewing the relation, as far as it holds good, between the bright colours of female birds and their manner of nesting, receives some support from certain ana}oaous cases occurring in the Sahara Desert. Here, as in ;ost other deserts, various birds, and many .other animals, have had their colours adapted in a wonderful manner to the tints of the surrounding surface. Nevertheless there are, as I am informed by the Rev. 1\ir. Tristram, some curious exceptions to the rule; thus the male of the Monticola cyanea is conspicuous from his bright blue colour, and the female almost equally .conspicuous from her mottled brown and white plumage; both sexes of two species of Dromolrea are of a lustrous black; so that these three birds are far from receiving T>rotection from their colours, yet they are able to survive, for they have acquired the habit, when in danger, ,of taking refuge in holes or crevices in the rocks. "With respect to the above-specified groups of birds, in which the females are conspicuously coloured and build concealed nests, it is not necessary to suppose that each separate species bad its nidifying instinct specially modified; but only that the early progenitors of each group were gradually led to build domed or concealed nests; and aftervvarcls transmitted this instinct, together with their bright colours, to their modified descendants. This conclusion, as far as it ean be trusted, is interesting, namely, that sexual selection, together with equal or nearly equal inheritance by both sexes, have indirectly determined the manner of nidification of whole groups of birds. EYen in the groups in which, according to 1\ir. Wallace, the females from being protected during 'nidification, have not had their Lright colours eliminated through natural selection, the males often differ in a slight, and occasionally in a considera.Lle degree, from CHAP. XV. COLOUR AND NIDIFICATION. 173' the females. This is a significant fact, for such differences in colour must be accounted for on the principle of some of the variations in the males having been from the first limited in their transmission to the same sex· as it can hardly be maintained that these difference; especially when very slight, serve as a protection t; the female. Thus all the species in the splendid groupof the Trogons build in holes ; and Mr. Gould gives figures 20 of both sexes of twenty-five species, in all of which, with one partial exception, the sexes differ sometimes slightly, sometimes conspicuously, in colour,the males being always more beautiful than the females,.. though the latter are likewise beautiful. All the species of kingfisher build in holes, and with most of the species the sexes are equally brilliant, and thus far Mr. Wallace's rule holds good ; but in some of the Australian species the colours of the females are rather l~ss vivid than tho~e of the male; and iu one splendidly- coloured spemes, the sexes differ so much that they were at first thought to be specifically distinct.21 Mr. R. B. Sharpe, who has especially studied this ?roup,. has shewn me some American species (Ceryle} m whiCh the breast of the male is belted with black. Again, in Carcineutes, the difference between the s~xes is conspicuous: in the male the upper surface IS dull-blue banded with black, the lowet· surface being partly fawn-coloured, and there is much red abou.t the head; in the female the upper surface is. reddish-brown banded with black, and the lower surface white with black markings. It is an interestino· fact, as shewing bow the same peculiar style of ~exual 20 See his' Monograph of the Trogonidm,' first edition. ~ 1 Namely Cyanalcyon. Gould's • Hnndbook of 1he Birds of Austmlin,' vol. i. p. 133; soc, also, p. 130, 136. |