THE UNDERLYING IDEA 83 
while these hatched out, and the young were on the 
nest until they were old enough to fly; but before they 
had left she had slipped a fresh egg among them, 
ready to start a new batch. Whenever I saw the nest 
throughout the entire summer, I found in it either 
eggs, or young, or both.” Such reproductive energy 
as this is hard to beat; compared with this rate of in- 
crease, the ordinary bird is the exponent of race sui- 
cide. How can a robin hope to compete with this 
family industry? What can a bluebird offer that will 
approach such chances of a worthy successor when his 
work shall be finished? 
These, then, are the most important points in which 
the English sparrow has varied from his sparrow 
cousins and made of himself the most successful town 
dweller in the bird world. He has become clannish 
and gained the advantages of cooperation. He has 
used man’s highways and cars by means of which to 
expand his area. He has cultivated the presence of 
man and thus gained protection from his enemies, 
food from man’s waste, and nesting sites on man’s 
house. He has assumed a varied diet. The male has 
become handsome. He has given up migrating, and 
thus secured the best nesting sites. He has learned 
to produce many offspring. With all his versatility, 
why should he not succeed? 
Thrown into competition with our native birds, he 
