F. B. KIRKMAN : VARIATION IN NESTS OF TERNS. 103 



grass," the other with " fine pebbles." While no doubt 

 the species generally dispenses with a lining (Fig. 7), 

 further observation may show that variations are not 

 infrequent. One may note in passing that the preference 

 of the Little Tern for unlined nests may possibty account 

 for the comparative scarcity of this species. 



Let us turn now to consider the bearing of the above 

 facts, beginning with the variation in sites. It seems 

 reasonable to assume that if there is any truth in the theory 

 of protective coloration the normal (i.e., the safest) 

 nesting site of the Arctic and Common Tern, and, indeed, 

 of all the grey and white Terns and Gulls, is the closely 

 packed shingle such as one finds on the beach or the vast 

 stretches of Romney Marsh. Sitting amid the vague 

 outlines of black, white, and grey stones, a Tern is prac- 

 tically invisible. Something of this correspondence of 

 colour is visible in Plate III. There is no reason to think 

 that the invisibility is necessary to the safety of the 

 Tern itself, for it is the last bird to be caught napping 

 on its nest. But its advantage as a means of protecting 

 eggs and young from discovery is obvious. 



By placing its eggs among herbage, on the bare sand 

 patches, on rocks, or even in the shingle beds among the 

 sandhills, where the stones are seldom closely packed 

 (Fig. 6), the Tern sacrifices all the advantages to be 

 derived from its coloration. In such sites it is a con- 

 spicuous object. That it should be able to effect the 

 change with comparative impunity seems to argue that 

 the species has no longer many egg-stealing foes to fear. 

 Under any circumstances it would require a bird of no 

 mean courage or strength to pillage the nests of Terns, 

 for they have an unpleasant habit of descending almost 

 vertically, with the velocity of a bolt, upon unwelcome 

 intruders, and striking with the beak. At Walney 

 the young Black-headed Gulls, whose mottled brown 

 plumage evidently caused their identity to be mistaken, 

 suffered severely from this practice, often being struck 

 down in mid-flight, and the more easily as they were 



