222 BRITAIN'S BIRDS AND THEIR NESTS. 



enough to be of any importance as foster-parents for 

 British Cuckoos. 



The Cuckoo'^s egg usually bears a fairly close resemblance 

 to the eggs of the rightful owner of the nest in which 

 it is laid, and this is at the root of most of the problems 

 connected with this aspect of the Cuckoo's life. Some- 

 times the degree of similarity is very great, but usually 

 the Cuckoo's egg is slightly superior in size ; it would be 

 very much superior were it not that the egg happens to 

 be exceptionally small in proportion to the dimensions 

 of the parent. In some cases the resemblance in colour 

 is not striking, and this is especially so in the case of 

 the Hedgesparrow, the Redstart, and other birds laying 

 unspotted light-blue eggs. Sometimes the Cuckoo's egg 

 is also of this colour, but very often it bears no sort of 

 resemblance to those of the foster -parent. Why this 

 should be so it is difficult to say ; but there may be 

 some truth in the suggestion that this is a mere excep- 

 tion, recorded with a frequency out of all proportion to 

 its importance, owing to its conspicuousness. Whether 

 these rather obviously imposed eggs ever suffer ejection 

 by the foster-parents does not seem to be on record ; but 

 if not, it is difficult to see what is the raison d'etre of all 

 this adaptive colouring unless some disadvantage of this 

 nature is thereby avoided. 



Assuming that this is the case, we have still to 

 account for the great individual variation within the 

 species. Were the Cuckoo's eggs all of one type, and 

 its choice of foster-parents confined to one or a few 

 similar species, it would be quite intelligible. Such, 

 indeed, is the case with certain foreign kinds. But in 

 our species we find a variety of egg-types corresponding 

 to a number of foster-parents. The most obvious ex- 

 planation would be that the egg was laid first, and then 



