144 BRITISH BIRDS. 



in all three species there has been a strong tendency 

 towards individual variation in the colouring of the eggs. 

 As they are covered from the time they are laid, the 

 action of natural selection (in the form of egg-sucking 

 birds, which would otherwise check variation, by destroy- 

 ing all those eggs which departed from the type which 

 agreed most closely in colouring with the environment) 

 has been in abeyance, and therefore, as variation has 

 continued practically unchecked, we now have the 

 exceptional range of colouring shown in the eggs of the 

 Black-headed Gull, Common and Sandwich Terns. 



Two other species noted for the wide range of variation 

 in the colouring of their eggs are the Razorbill {A. tarda) 

 and the Guillemot [U . troile). As these birds only deposit 

 a single egg, the case appears to be analogous with those 

 previously mentioned. 



Only three nests of the Little Tern {S. minuta) were 

 noted hatching off, but these were of great interest. 

 In one clutch of two eggs the first hatched on June 29th, 

 the second was hatching on July 1st. In the two other 

 cases, each a clutch of two, the eggs hatched at the same 

 time. Further observation is needed, but it may be 

 suggested that as the species has not the physical force 

 of the Common and Sandwich Terns, it has suffered more 

 from the ravages of egg-sucking birds ; therefore a type 

 of egg has been evolved, the protective colouring of which 

 saves them from the keen vision of the birds that seek 

 them ; and so, in some cases at all events, the birds do 

 not incubate until the full clutch has been laid, for natural 

 selection working on other Hnes has caused it to be 

 immaterial that they should do so. 



The eggs of the Curlew {N. arquata) are protected to 

 some extent by their coloration, as is the case with 

 other Limicoline birds. A nest found on April 26th, which 

 then contained two eggs, was visited daily ; it was not 

 until April 30th that the third and last egg was laid. 

 In the only other nest of this species that I have watched 

 there was also a considerable interval between the laying 



