SEXUAL BE IT AVTOUR 9I 



4. THE SPECIAL PROBLEM OF MIGRATION 



The subject of animal migration has always captured 

 the public interest, and particular attention has been 

 given to the regular and spectacular movements of birds. 

 A consideration of the problem is relevant here because 

 migratory movements are a form of sexual behaviour, 

 and the flow and ebb of the tide of anim.als for the 

 purposes of breeding forms one of the most remarkable 

 cycles of sexual activity known. 



However, the analysis of this problem, particularly 

 from an experimental point of view, is hampered by 

 extraordinary difficulties. Observations and experi- 

 ments must be made in the field, and in this case the field 

 is some hundreds or even thousands of miles long. The 

 simplest starting point is from the observation that the 

 spring movements of birds towards their breeding areas 

 m.ay be related to the growth of their gonads. Sometimes 

 these movements are only short as when the curlew 

 travels some 40 or 50 miles from the seashore to the 

 high moors, but sometimes they are extraordinarily 

 long as when the arctic tern travels more than 10,000 

 miles from the Antarctic to the Arctic. In those species 

 which have been examined it has been invariably found 

 that the gonads begin their seasonal growth just before 

 the birds begin to move.^^^ So it seems that there may be 

 a general correlation between spring migration and 

 increasing gonad activity, and possibly the reverse 

 movement in the autumn may be related to decreasing, 

 or minimum, gonad activity. 



Clearly the experimental examination of this thesis 

 presents alm.ost overwhelming difficulties and the efforts 

 of those who have tried can only be considered as valiant. 

 The pioneer in this work was again Rowan ^'^^ ^yho started 

 with the Canadian juncos. After he had modified their 

 reproductive cycle by means of artificial light in autumn, 

 he released the birds together with the untreated 

 controls. Each day food was put out for them because, of 



