BIRDS. 561 



summer; the young in their immature state and the adults 

 in their winter dress heing gray and black. With many 

 kinds of gulls (Larus) the head and neck become pure 

 white during the summer, being gray or mottled during 

 the winter and in the young state. On the other hand, 

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

 terns (Steraa) 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 colored 

 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. Conse- 

 quently 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 con- 

 spicuous either by being perfectly white or intensely black; 

 so that these colors may possibly serve the same end as the 

 call-notes of many land-birds, f A white or black bird 

 when it discovers and flies down to a carcass floating on the 

 sea or cast upon the beach, will be seen from a great dis- 

 tance, and will guide other birds of the same and other 

 species to the prey; but as this would be a disadvantage to 

 the first finders, the individuals which were the whitest or 

 blackest would not thus procure more food than the less 

 strongly colored individuals. Hence conspicuous colors 

 cannot have been gradually acquired for this purpose 

 through natural selection. 



As sexual selection depends on so fluctuating an element 



*0n Larus, Gavia and Sterna, see Macgillivray, "Hist. Brit. 

 Birds," vol. v, pp. 515, 584, 626. On the Anser liyperboreus, Audu- 

 bon, " Ornith. Biography," vol. iv, p. 562. On the Anastomus, Mr. 

 Blyth, in " Ibis," 1867, p. 173. 



f It may be noticed that with vultures, which roam far and wide 

 high in the air, like marine birds over the ocean, three or four species 

 are almost wholly or largely white, and that many others are black. 

 So that here again conspicuous colors may possibly aid the sexes in 

 finding each other during the breeding season. 



