78 THE BIOLOGY OF THE SEASONS 



be represented by the perpendicular and base of a right- 



w | v ^-^ 

 angled triangle + _^*^^ , the vernal flight may be 



S ^ E 



represented by the hypotenuse. When the autumnal 

 flight is altogether from north to south, the vernal flight 

 cannot be more direct ; but, as we have seen, there are many 

 birds which show in Autumn an east to west and then a 

 southerly movement. As facts accumulate, it will be 

 possible to test Gatke's interesting suggestion as to the 

 greater directness of the Spring flight, which, as yet, some- 

 what outstrips the evidence. If there is this marked 

 change of flight in Spring, it will increase the difficulty of 

 understanding the path-finding. 



We are still very much in the dark in regard to the 

 termini of the autumnal southward flight, and it is a great 

 satisfaction to be able to point to definite facts secured by 

 ringing individual birds. Thus Dr. Thienemann has 

 proved up to the hilt that many North European storks 

 winter in North Africa and go as far south as Natal. That 

 a large proportion of our Summer visitors go to Africa is 

 probably true, but definite information is scanty. What 

 is known in most cases is simply that no birds of a particular 

 kind are left in Autumn in a northern country, and that 

 there is a practically simultaneous arrival of a large repre- 

 sentation of this particular kind of bird in a southern country. 

 But it is a big step from this general fact to the particular 

 statement that night- jars from the English Lake District 

 go to East Africa by the Great Lakes. Yet it is this kind 

 of statement which one desires to make. 



A second question in regard to which we lack adequate 

 information is as to the altitude of the migratory flight. 

 There is much reason to fear that many of the statements 

 made in this connection are greatly exaggerated. Observers 

 with telescopes have seen birds crossing the moon's disc 



