THESES FROM LITERATURE. 



7. Birds are afraid of crossing the open sea ; tiiey either take a 

 ronndabout wayorfoliow directions offering resting-places — 

 Helgoland, Capri. 

 Kessler: „Einige Beitrage zur Wandergeschichte derZugvogel" Mosc. 

 Bull. 1853. 



1 . There-are summer-birds, winter-birds and periodically wandering 

 birds. 



2. Migrants which arrive earliest, remain longest, those arriving 

 latest generally leave earliest. 



3. Summer- and winter guests often meet in the same place. 



4. Weather has great influence upon Migration. 



5. The arrival of Birds of passage and of winter guests seems 

 to depend upon the weather in their northern home. 



6. The order in which the species arrive in various localities is not 

 always the same. 



7. The advance is very unequal, in some species quicker, in others 

 slower. 



8. The oscillations are stronger in the South, than in the North. 



9. Some species seem to migrate from S. W. to N. E.. others 

 from S. to N. The Migration-direction is not always from S. to N. 

 but when crossing rivers also from S. W. to N. E. 



10. Migrating Birds rarely follow distinct routes, but advance, pretty 

 uniformly along the whole extent of the front (broad front). 



1 1. Birds have no presentiment of weather. 



12. Oscillation in early comers is more marked. 



On Method. 



Only observations continued for years in as close a system as 

 possible, can give positive results. 



MiDDENDORFF, A. von : „Die Isepiptesen Russlands etc." 1885. Extra- 

 copy from Mem. de I'Acad. des Sciences, St. Petersbourg. 



iszi 36 oas 



