Chap. XVI. Birds — Conspicuous Coloun. 493 



with the smaller gulls, or sea-mews (Gavia),atidwilh some terns 

 t,Sterna), exactly the reverse occurs; for the heads of the young 

 birds during the first year, and of the adults during the winter, 

 are either pure white, or much paler coloured than during the 

 breeding-season. These latter cases offer another instance of the 

 capricious manner in which sexual selection appears often to 

 have acted.™ 



That aquatic birds have acquired a white plumage so much 

 oftener than terrestrial birds, probably depends on their large 

 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 these colours may possibly serve the same end as the 

 call-notes of many land-birds." A white or black bii'd when it 

 discovers and flies down to a carcase floating 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 other species, to the prey ; but 

 as this would be a disadvantage to the first fiuders, the indi- 

 viduals which were the whitest or blackest would not thus 

 procure more food than the less strongly coloured individuals. 

 Hence conspicuous colours cannot have been gradually acquired 

 for this purpose through natural selection. 



As sexual selection depends on so fluctuating an element as 

 taste, we can understand how it is that, within the same group 

 of birds having nearly the same habits, there should exist white 

 or nearly white, as well as black, or nearly black species, — for 

 instance, both white and black cockatoos, storks, ibises, swans, 

 terns, and petrels. Piebald birds likewise sometimes occur in 

 the same groups together with black and white species; for 

 instance, the black-necked swan, certain terns, and the common 

 magpie. That a strong contrast in colour is agreeable to birds, 

 we may conclude by looking through any large collection, for 

 the sexes often differ from each other in the male having the pale 



" On Larus, Gavia, and Sterna, high in the air, like marine birds 



see Macgillivray, 'Hist. Brit. Birds,' over the ocean, three or four species 



vol. V. p. 515, 584, 626. On the are almost wholly or largely white, 



Anser hyperboreus, Audubon, ' Or- and that many others are black. So 



nith. Biography,' vol. iv. p. 562. that here again conspicuous colour.'! 



On the Anastomus, Mr. Blyth, in may possibly aid the sexes in finding 



Ibis ' 1867, p. 173. ™<^1' other during the- breeding' 



»■' It may be noticed that with season, 

 'ull.ures, which roam far and wide 



