xin INSTINCT AND REASON 



339 



Sparrow's nest. Many suggestions have been made 

 to account for this fact, the best, perhaps, being that 

 there arc varieties of Cuckoo, each of which has its 

 favourite nest for laying in. When hatched, the young 

 Cuckoo has not a vestige of clown, and is perfectly 

 blind. His back from the shoulder blades downward 

 is very broad, and has a depression across the middle 

 which fills up after the twelfth day of life. This 

 remarkable form of back is very useful to the still 

 blind young bird. Using it as a shovel, he ousts the 

 other fledglings or an unhatched egg from the nest 

 sometimes before he has completed his second day, 

 when his victims may be picked up round the nest. 

 When once you have seen this blind young demon with 

 his shovel-like back to help him in his murderous 

 career, you can never forget him. The foster-mother 

 devotes all her energy to the murderer of her young. 

 Only one egg (rarely two) is laid in each nest, so 

 that the young bird may get plenty of food. 



The instinct of the Cuckoo and all the accompany- 

 ing modifications have been brought to perfection — the 

 diminutive size of the egg, only one egg (or at most 

 two) in each nest, and laid, moreover, before incubation 

 has begun, the occasional approximation of the egg in 

 point of colour to that of the foster-mother chosen, 

 the hollow back, and the self-asserting disposition of 

 the young bird. It is only in the system of migration 

 that we find imperfection. The old birds, most of 

 them, leave by August ; .the young ones sometimes 

 remain as late as October, and have to find their way 

 to Africa, even to South Africa, alone. 



The cuckoo or parasitic habit is not limited to one 



z 2 



