CONCERNING THE CUCKOO 117 



a change merely to obtain so small an advantage. 

 The alternative conclusion is that the cuckoo must 

 have developed this peculiarity under conditions 

 different from those now existing. 



We may be able to realize to some extent what 

 those conditions must have been, if we try to 

 imagine for a moment what would happen if the 

 cuckoo of the present day were to return to its abori- 

 ginal habits and endeavour to rear its own young. 

 Two considerations immediately present themselves. 

 A single pair would in the first place be quite 

 unable to feed and rear an ordinary brood. In 

 the second place, the young birds would not tolerate 

 each other in the nest. There have been rare cases 

 known where the cuckoo has deposited two eggs in 

 the same nest, and one of the young birds has been 

 known to eject the other after a prolonged struggle. 



The cuckoo, whether from some change in environ- 

 ment, such as the disappearance of its natural food 

 or through some other cause, is evidently at the 

 present time a bird which finds great difficulty in 

 feeding itself. Each bird is said to have its own 

 feeding grounds, which it defends against all comers, 

 and the early migration, and the significant fact 

 that the caterpillars which other birds reject form 

 the staple food, all point to the conclusion that the 

 cuckoo obtains sufficient food only with difficulty. 

 Now it is not difficult to conceive what the effect 

 upon the young was when these conditions first 

 arose and the cuckoo was stiU a normally nesting 

 bird. The nesting period is the time when the 

 demand for food is greatest, and the rivalry must 

 , immediately and in the first place have made itself 

 felt among the young birds. The advantage must 



