57 



Warblers, Thrushes and the like, this limit of visible elevation 

 may perhaps not amount to much." Of course a Thrush, or any- 

 smaller bird, becomes invisible at only a very moderate height, but 

 one may perhaps be excused a feeling of surprise to read on page 

 76 that — " Under normal conditions the migrations of most 

 species proceed at a height of at least 20,000 feet," or, roughly 

 speaking, a trifle less than four miles. When we recollect that, 

 the highest mountain peak of the Himalayas does not exceed 

 29,000 feet, we get some idea of what the elevation must be at 

 which, according to the theories of the author, migration is 

 performed by the more robust species. 



Having learned so much respecting the character of normal 

 migration, it becomes necessary to further inquire what is the 

 author's idea of normal and abnormal weather, for if the state of 

 the atmosphere governs the manner and height at which the 

 migratory flight is performed, then the latter can only be de- 

 scribed as normal or abnormal in a secondary sense, as it is the 

 direct outcome of the former. 



On page 76 we find the following : — " Birds naturally choose 

 for their migrations those strata of the atmosphere which oifer 

 the most favourable conditions to their progress. It is, however, 

 a fact of peculiar interest, that during both migration-periods of 

 the year, all species, without exception, approach in largest num- 

 bers to the earth's surface when very light south-easterly winds, 

 accompanied by clear warm weather, happen to prevail for any 

 length of time in the lower regions of the atmosphere. If autumn 

 brings a long spell of weather of this kind, we may not only 

 reckon on the appearance of large numbers of all our common 

 visitors during September and October, but may also look forward 

 with certainty to the frequent occurrence of species very rare in 

 Europe, and originating from the far East. . . . from all 

 these facts it appears that the meteorological conditions discussed 

 above are those best adapted to the migrations of birds, and that 

 the latter betake themselves to strata of the atmosphere in which 

 such conditions prevail " (page 77.) Herr Gatke's remarks have 

 been previously quoted to the effect that visible migration is the 

 abnormal phase of the movement, but as the height of the flight 

 is governed by the prevailing weather — in the light of his theories 



b 



