296 BIRD MIGRANTS ON THE YENESAY. 



country show that these migrations are, as we have 

 said, in progress over almost the whole country generally, 

 and are as general (if we may venture to use the 

 simile) as the snow-flakes which fall over vast areas, 

 during the occurrence of extensive atmospheric disturb- 

 ances. 



We have already noticed certain facts which prove 

 that these migrations occur in both the arctic and 

 antarctic regions, so far as the evidence of facts upon 

 the American continent is concerned, and it would be 

 possible to collect evidence to show that in all proba- 

 bility they take place periodically almost everywhere 

 upon the whole terrestrial surface of the globe. * Exact 

 observations upon these and similar matters are how- 

 ever as yet comparatively scanty ; as it is only of late 

 years that these interesting phenomena have been made 

 the subject of systematic investigation. 



In the first place, the flight of migratory birds takes 

 place for the most part by night, when their move- 

 ments are not easily observed. It frequently occurs 

 that the only evidence of their passing above is the 

 sound of their different call notes; and even in the 

 daytime the vast altitude at which they fly frequently 

 renders anything like exact observations impossible, 

 for it seems probable and indeed certain that many of the 

 smaller species pass at such great heights above the earth 

 that under the most favourable circumstances they 

 are entirely beyond the range of human vision, f 



It is probable that a number of causes combine to 

 induce birds to ascend to these great heights during 



* In Australia and other southern lands for instance, enormous 

 flights of wild-fowl are constantly seen during their autumns and springs. 



f The Migration of Birds, an Attempt to reduce Avian Season Flight 

 to Law, by Charles Dixon, 1892, p. 71. 



