evidently more than compensated by the sky-matching power which uniform, 

 pure white gives to this most vital and most dangerous portion of the gull, 

 either when he is resting on the water, with head held erect, or — and perhaps 

 more particularly — when, as he flies or swims, his head is stooped toward, 

 to, or even beneath, the surface, in search of food. 



White or largely white-marked heads are common to a good many other 

 birds, not counting the habitual swimmers, which get their living from the 

 water; witness the Bald Eagle, the Osprey, the Great Blue Heron, etc. In 

 all these cases they perform the same service of 'obliteration' against the 

 sky. Some gulls, on the other hand, such as the Black-head {Larus ridi- 

 bundus), of Europe, and the Laughing Gull (L. atricilla), of America, have 

 dusky hoods enveloping the entire head. All or nearly all the kinds thus 

 marked are inhabitants of bays and lakes and marshes rather than the open 

 sea. Furthermore, the dark hood is worn only by the adults in the breeding 

 season, when, amid the blackish mud and dusky shadows of the salt marsh 

 or inland swamp, they well serve as 'ruptive'* masks. So do the jetty 

 crown-caps of nesting terns — except that these belong to obliterative shading 

 as well as to 'ruptive' pattern. (See Fig. 55.) Like the gulls' hoods, they 

 are as a rule features of the breeding season only — in the autumn largely 

 giving place to white, the regulation sky-matching color. But the black 

 markings on the quill feathers both of gulls and terns are worn throughout 

 the year, and probably serve both as ' distractive ' marks f when the birds 

 are fishing, and as combined 'distractive' marks and 'picture patterns' when 

 they are brooding on shore amid shadows and other dark landscape- 

 details. 



For the most part, however, the coloration of these gulls and terns in adult 

 plumage is suited to the sky and sea rather than to the land, and they are apt 

 to be conspicuous on their breeding grounds, by virtue of their paleness. 

 Their downy young, on the contrary, are almost always well 'obliterated' 



* See Chapter XIII, p. 78. 

 t See Chapter XXII, p. 151. 



74 



