XIII INSTINCT AND REASON 339 
Sparrow’s nest. Many suggestions have been made 
to account for this fact, the best, perhaps, being that 
there are varietics of Cuckoo, each of which has its 
favourite nest for laying in. When hatched, the young 
Cuckoo has not a vestige of down, 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 cach 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 
Z2 
