ANNUAL MIGRATIONS 129 



after heavy snow although strong, well fed and per- 

 fectly equal to taking an immediate southward 

 flight of a few hundred miles. Physically they are 

 evidently fit to migrate; physiologically they are 

 incapable. 



Analogous, though differing in detail, is the case 

 of birds whose gonads are diseased and fail to func- 

 tion normally. These individuals will probably 

 also fail to migrate. In the Province-wide hunt for 

 crows after liberation of our experimental birds, 

 half a dozen wild crows that had failed to go south 

 were shot and sent in for examination. All were 

 males and every one showed a pathological condi- 

 tion of the testes. 



The behaviour of the experimentals was entirely 

 different. No control was ever heard to sing during 

 the winter months. The experimentals, on the 

 other hand, were in full song at Christmas even at 

 zero. In all, 92 experimentals were liberated (in 

 batches under varying weather conditions) during 

 the four winters. Of these, 2>^ individuals (over 40 

 per cent) were never again seen or retaken. The 

 percentage of departures varied appreciably with 

 circumstances. Summarising the results it may be 

 stated that there appeared to be no discernible cor- 

 relation with barometric pressure, but that ex- 

 tremely low temperatures and conditions that pre- 



