OCR Text |
Show 228 SEXUAL SELECTION: BIRDS. PART u. pheasant and some oth r b1.1 " d s s bo ul. d b.e encumbered with I)]umes so long as to impede theu flight. f . In the same manner, as t 11 e ma1 o s a lone o vanous 1 d species are black, the ~le ma1 e s b · g dull-co oure ; Cl.ll • h 11 so in a few cases the males alone are eiltheBr lwl b~ ·ely or partially whi· te, as W"l th the sovera e - u . s of South America (Ohasmorhyncl~us), the Antarctic goose (Bernicla antarctica), the silver-pheasant, ~~' whilst the females are brown or obscurely. m_ott e . Therefore, on the same principle ~s before, It IS p~·obable that both sexes of many bn·ds, su~h as white cockatoos, several egrets "th th · · beautiful plumes WI eu . . ' eertam. 1'b 1' ses, gul ls , terns ' &c ., have acqmred then1 more or less completely white _plum~ge through ~exua selection. The species which mhabit snowy r.egwns of course come under a different head. The white plun:aO'e of some of the above-named birds apprea1:s 1~ b~th sexes only when they are mature. !hl~ IS likewise the case with certain gannets, troplc-bn·ds, &c., and with the snow-goose (Anser hyper~~reus). As the latter breeds on the "barren grounds, when ~ot covered with snow, and as it migrates southwar.d durmg the winter, there is no reason to suppose t~at 1ts snowwhite adult plumage serves as a p~otect10n. In the case of the Anastomus oscitans prevwusly. alluded to, we have stiH better evidence that the white plum~ge is a nuptial character, fo~ it is .de:veloped only durmg the summer; the young m theu 1:nmature state, an:l the adults in their winter dress, bemg grey and black. With many kinds of gulls (Larus), the head ~ncl neck become pure white during the summer, bemg grey or mottled during the winter and in the young state. On tho other hand, with the smaller gulls, or sea-mews (Gavia), and with some terns (Sterna), exac.tly the .reverse occurs; for the heads of the young bn·ds durmg CHAt'. XVI. CONSPICUOUS COLOURS. 229 tho first year, and of the adults during the winter, are either pure white, or much paler-coloured than during tho breeding-season. These latter cases offer another instance of the capricious manner in which sexual selection appears often to have acted. 54 The cause of aquatic birds having acquired a white plumage so much more frequently than terrestrial birds, probably depends on their largo size and strong powers of flight, so that they can easily defend themselves or escape from birds of prey, to which moreover they are not much exposed. Consequently sexual selection has not here been interfered with or guided for the sake of protection. No doubt, with birds which roam over the open ocean, the males and females could :find each other much more easily when made conspicuous either by being perfectly white, or intensely black; so that those colours may possibly serve the same end as the ca11-notos of many ]and-birds. A white or black bird, when it discovers and flies clown to a carcase floatinO' b on the sea or cast up on the beach, will be seen from a great distance, and will guide other birds of the same and of distinct species, to the prey; but as this would Le a disadvantage to the first :finders, the individuals which were the whitest or blackest would not thus have ?ro?~red more food than the less strongly coloured mdrVIcluals. Hence conspicuous colours cannot have Leen gradually acquired for this purpose through natural selection.55 64 On Lo.rus, Gavin, and Sterna, see Macgillivray, 'Hist. Brit. Birds,' v~l. v. p. 515, 584, G2G. On tho Anser hyperb01·eus, Audubon, ' Ornith. Bwgro.phy,' vol. iv. p. 562. On tho Ano.stumus, Mr. Blyth, in 'Ibis,' 18G7, p. 173. "" It may bo noticed that with vultures, which roam far and wide through the higher regions of tho atmosphere, like marino birds ovor the ocean, three or four specjes arc almost wholly or largely white, and |