ON INCUBATION. 143 



It may be worth noting en passant that the Common 

 and Little Terns remove the shells of hatched eggs from 

 the vicinity of the nest, whereas the Sandwich Tern 

 apparently never does so. 



It is certain that the habit of the Black-headed Gull, 

 Common and Sandwich Terns is to commence incubation 

 as soon as the first egg has been laid. Gulls are notorious 

 egg-stealers, and there can be no question that if the 

 Black-headed Gull has the opportunity it will rob the 

 nests of its own species. In several cases single eggs 

 disappeared from clutches of three ; they had evidently 

 been sucked. On one occasion I saw a Gull carrying a 

 large object in its beak, being pursued by another bird of 

 the same species ; the first dropped its booty. On 

 reaching the place I found it to be an egg of the Black- 

 headed GuU, almost on the point of hatching. It is when 

 the birds have been disturbed from their nests, I think, that 

 the opportunity is seized and the eggs taken. 



Likewise, in the case of the Terns, several instances 

 were noted of eggs being sucked just after the birds had 

 been disturbed. Though the Black-headed GuUs were 

 never detected in the act, there is no doubt that they were 

 the culprits, for no other species of egg-sucking bird 

 ventured on to the Terns' nesting-ground. Frequently 

 the GuUs were chased off, but at other times little notice 

 appeared to be taken of them. As Terns usually nest 

 on Gull-haunted ground, there can be no doubt that the 

 covering of the eggs by the birds from the time the first 

 has been deposited is of great value to the species, 

 especially as the eggs are not laid on consecutive days, 

 for the risk of losing them is reduced to a minimum, 

 being in fact practically nil where the birds are left 

 undisturbed. This is most markedly the case with the 

 Sandwich Tern, for the eggs are very conspicuous, and 

 three days frequently elapse between laying. 



Now, each of these three species is weU known to exhibit 

 a great range of variation in the colouring of its eggs. 

 I think we have here the explanation. It is evident that 



