THE CUCKOO 87 



natural selection hold. There is a certain margin of freedom, 

 and a disadvantageous habit may be outbalanced by other 

 beneficial ones. Birds will usually brood an inserted egg of 

 about the same size as their own ; it is quite easy to get a 

 nestful of young song-thrushes hatched and reared in a 

 blackbird's nest, though the eggs are very distinct in colour. 

 Sometimes the owner of the nest does resent the arrival of 

 the cuckoo's egg, and covers it up with a little more lining. 

 Though the parasitic habit is clearly a disadvantage to the 

 foster-parents, it is 

 harder to decide 

 whether it is an ad- 

 vantage to the cuc- 

 koo. Obviously it 

 saves parents an im- 

 mensity of trouble ; 

 but the avoidance of 

 trouble is usually not 

 a good principle in the 

 long-run either for 

 men or birds. The 

 highest birds — judged 

 by standards of intelligence — are not only monogamous 

 for the nesting season, but for life. By all the moral 

 rules, cuckoos ought to be a declining species ; but it 

 must honestly be said that they are not. They are numer- 

 ous in every type of landscape, only avoiding towns. We 

 are left to presume that they exist in spite of their peculiar 

 habits rather than because of them, as is certainly true of 

 the foster-parents' share in the story. Not all cuckoos 

 are parasitic, and we find certain American members of the 

 tribe building nests of their own, though very poor ones. It 

 would throw much light on the question of the help or harm- 



