igij.'j Breeding-habits of tJie Cuckoo. 199 



tlie more heavily niaikcd. The markings lying deep in the 

 shell, and the gronnd-culonr, are due to some earlier dyeing 

 process, probably due to bile-matter. 



This, howqyer, docs not solve the difficulty of the various 

 colours which so ofccn occur in eggs of the same species, 

 such as the Cuckoo, Guillemot, Tree-Pipit, Gulls, and 

 Terns, nor does it explain the occurrence of a rufous or 

 pink type in eggs of the Blackcap, Dartford \Yarbler, and 

 other species. 



Dr. Key and others, dealing with the Cuckoo, have 

 adduced the theory that the food supplied to the young, 

 which differs with the foster-parents selected, has an 

 influence on the future egg-coloration ; but this seems 

 an untenable, though ingenious, solution. For if young 

 birds be removed from the nest at an early age and fed 

 on more or less artificial food, no alteration in the colour 

 of their eggs occurs, and the Canary, after generations of 

 rearing on foods which must differ from those eaten in a 

 natural state, and, more remarkable still, after the type of 

 plumage has been greatly changed, still continues to lay 

 eggs of the original type. Nor is it difficult to recall other 

 instances to show that this idea, though attractive, only 

 illustrates the tendency to fit facts to suit theories, for, 

 if true, it would elucidate the mystery of the varying 

 colourings of Cuckoos' eggs. 



In some instances it seems possible to trace a connection 

 between egg-coloration and locality. It is noticeable, for 

 instance, that eggs of the Common Guillemot from many 

 colonies show a distinct predominance of a certain colora- 

 tion such as a white, yellowish, or deep blue ground-colour, 

 and in a certain locality in the Midlands I observed that 

 the majority of eggs of the Garden-Warbler were of an 

 unusually pale type. On the other hand, a visit to a colony 

 of Gulls or Terns at once disposes of any attempt to make 

 this a general rule, and the only possible deduction may be 

 that certain families laying eggs of a given type may have 

 established themselves and flourished in such localities. 



