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Show 224 SEXUAL 'ELEO'l'ION: BIRDS. PAlU IT. nearly the same dangers. It is therefore prob~ble that strongly-pronounced colours have been ~cqmrod by tree-haunting birds through sexual select10n, but that green tints have had an advantage through natural selection over other colours for the sake of protection. In regard to birds which live on the gro~n~, everyone admits that they arc coloured so as to Imitate the surrounding surface. How difficult it is to see a par-tl ·idrre snipe woodcock certain plovers, larks, and b' ' ' . • night-jars when crouched on the .g~oun.d. Amma1s m-habitinO' deserts offer the most str1kmg mstances, for tho b bare surface affords no concealment, and all the smaller quadrupeds reptiles and birds depend for safety on ' ' . 1 51 • their colours. As Mr. Tnstram has remarkec , m rccrard to the inhabitants of the Sahara, all are prote~ ted by their "isabellinc or sand-colour." Calling ~o my recollection tho desert-birds which I had see~ m South America, as well as most of tho ground-buds in Great Britain, it appeared to me that both sexes in such cases arc generally coloured nearly alike. Accorclingly I applied to Mr. Tristram, with respect to the birds of tho Sahara, and he has kindly given me the following information. There arc twenty-six species, belonging to fifteen genera, which manifestly have had their plumaO'C coloured in a protective manner; and this colouri~cr is all the more striking, as with most of these birds it is different from that of their congeners. Both sexes of thirteen out of the twenty-six species are coloured in tho same manner; but these belonO' to genera in which this rule commonly pre-b . vails, so that they tell us nothing about the protective colours being the same in both sexes of desert-birds. Of "' 'Ibis,' 1859, vol. i. p. 429, et seq. COLOUR AND PROTECTION. 225 tho. other thirteen species, three belong to genera in whiCh tho sexes us.ually differ from each other, yet they have the sexes alike. Iu the remaininO' ten species the male differs from the female; but tl~e difference i; cohn'f i1n ed· chiefly to the under surface of the pl umage, w te 1 Is concealed when the bird crouches on the grouud ; the head and back being of the same sandcolo~ red hue in both sexes. So that in these ten spec10s the upper surfaces of both sexes have been ~ctod on and rendered alike, through natural selection for the sake of protection; whilst the lower surfaces of tho ~ales alone have been diversified through sexual seloctwn, for the sake of or·nament. Here, as both ~exes are equally well protected, we clearly see that the f~males h~ve n~t. been prevented through natural selectiOn from mhentmg the colours of their male parents: '~e must look to the law of sexually limited transmis ·swn, as before explained. . In al~ par·ts of ~he world both sexes of many softbilled birds, especially those which frequent reeds or sedges, arc obscurely coloured. No doubt if their colours had been brilliant, they would have been mu?h more ?onspicuous to thei1· enemies; but whether their dull tmt~ have been specially gained for the sake of protectiOn seems, as far as I can judge rather doubt~ul. It is still more doubtful whethe~· such dull tmts can have been gained for the sake of ornal~ ont. We must, however, bear in mind that male .bu~s, though dull-coloured, often differ much from their females, as with the common sparrow and this leads to the belief that such colours have b~en gained t~rough sexual selection, from being attractive. Many ?f the soft-billed birds are songsters; and a discussion ~n a former chapter should not be forgotten, in which It was shewn that the best songsters are rarely orna- VOL. II. Q |
