SEXUAL BEHAVIOUR 93 



released them together with the untreated controls. He 

 too found that these controls could be repeatedly trapped 

 in the immediate neighbourhood of the laboratory, and 

 that the experimental birds vanished. Only one of the 

 latter was subsequently recovered, but this, significantly 

 enough, had travelled 200 miles north in 10 days. 



Thus there is at least a prima facie case that the basis 

 of the spring migratory movements of birds lies in the 

 activation of the gonads, and, from what has been said 

 earlier in this chapter, that the effect on the nervous 

 system is achieved through the intermediary of the sex 

 hormones. Clearly much work remains to be done before 

 this can be firmly established, but it is the simplest and 

 most probable explanation of the facts as they are known 

 to-day. 



As regards the autunm movement in the opposite 

 direction there is relatively little information. In some 

 species this movement takes place when the gonads are 

 shrinking, but perhaps in most it commences only after 

 they are fully regressed with sex hormone secretion at a 

 minimum. Some experimental observations made in 

 autumn are based on the fact that, towards migration 

 time, those birds which migrate at night become more 

 and more restless, particularly during the hours of 

 darkness. This restlessness has been called the migration 

 urge, and when it reaches a maximum, it interferes with 

 sleep and ends in the movement to the winter quarters. 

 Using female redstarts, Schildmacher^'^^' ^''^ found that 

 this autumn restlessness can be eliminated by injections 

 of oestrogens. Later he also eliminated it by inducing 

 gonad growth in the Continental robin by means of extra 

 light, i'3 and in winter he found it possible to induce a 

 precocious spring migration urge by the same means, i^* 



Attempts to influence migration have also been made 

 by means of castration experiments, but it has been 

 found that both the spring and autumn movements may 

 be completed normally by birds in this condition. This 

 would appear to contradict conclusively the present 



