22 BIRDS' NESTS 



these slight variations which, when beneficial, are 

 preserved by natural selection, that birds adapt 

 themselves to any changed conditions of life. I 

 have, for instance, several times remarked a com- 

 plete absence of feathers from the lining or interior 

 of the nest of the Common Wren ; also considerable 

 variation in the lining of the nest of the Magpie ; whilst 

 every observer must have remarked the great amount 

 of difference in the nests in a large rookery ; whilst, 

 lastly, as a case in point there are many Puffins that 

 never collect any material at all, whilst others make 

 quite a warm nest of dry grass and feathers. I noted 

 similar variation in the nest arrangements of the 

 Fulmar Petrel, some individuals making quite an 

 elaborate nest, others contenting themselves with 

 little or no artificial resting-place for the egg. 



As Dr Wallace most forcibly says, with birds as 

 with man, " when once a particular mode of building 

 has been adopted and has been confirmed by habit 

 and by hereditary custom, it will be long retained, 

 even when its utility has been lost through changed 

 conditions." Now, we know that although many 

 habits have long since ceased to be of any service, 

 they are retained. We have, for instance, the case 

 of hole-building Ducks covering their eggs like their 

 congeners nesting in open situations ; Jackdaws 

 making a most elaborate nest in a position where one 

 even of the slightest description is of small necessity ; 

 Swans adding to their nest (undoubtedly a habit origin- 



