42 THE RIDDLE OF MIGRATION 



change of outlook, of interests, of habits. Attain- 

 ment of maturity witnesses a metamorphosis of 

 mind and body. In animals changes may be even 

 more striking. In many species of those birds in 

 which sexual maturity is an annually recurring 

 event, a switching from one plumage to another or 

 from one mode of behaviour to another may be 

 observed every year. Thus the bobolink, in his 

 bright spring clothes is a combative individual for- 

 ever singing, ready to fight any other male, inter- 

 ested in the opposite sex, willing to build a nest, to 

 collect food for his offspring and so on, but when the 

 summer draws to a close and he dons his sober 

 winter apparel, all these activities slip from his 

 memory. Fighting and singing appeal to him no 

 more than does the opposite sex. He is quite con- 

 tent to be a silent and inconspicuous member of a 

 flock of males without individuality or personal 

 interests. For the time being he is sexless. When 

 at the end of April, he returns from the south to the 

 States, he arrives in the company of other males, 

 without the pugnacity which will develop later, but 

 already in song. His gonads are enlarging. They 

 have, however, not yet attained the maximum but 

 neither has he reached the zenith of his sexual 

 behaviour. Such behaviour depends directly on the 

 internal secretions of the testes. This assertion can 



