THESES FROM LITERATURE. 



10. Weather influences Migration. 



1 1. Contrary winds, showers or hail hinder the movement. 



12. Birds that are not yet quite strong enough for flight commence 

 the journey earlier, the stronger ones later. 



13. Land-Birds travel without distinct order; others in two con- 

 vergent lines, one of which is always longer. 



14. The leader has to work hardest, but is relieved by others. 



15. Birds wander from the North, as far as their range goes, to 

 countries where they find food 



LiNNE, Carl: „Dissertatio migratione Avium" Upsaliae 1757. 



1. Nutriment and temperature. 



2. The Migration is directed to unknown regions. 

 JENNER, E.: „Some observations on the migration of birds" 1824. 



1. Breeding and in connection therewith more need of food. 

 Faber, Friedrich : „Ober das Leben der hochnordischen Vogel" 1 826. 



1. Birds have an impulse for wandering and homing. 



2. Mild winters effect acceleration of the spring migration. 



3. Young Birds in the autumnal migration seldom travel in com- 

 pany with the old ones and, as a rule, later. 



4. The species change quarters with different winds. 



5. Originally every species was sedentary; Migration originated 

 from extension of the limits of the species. 



6. Causes: Instinct of migration, homing faculty, temperature and 

 nourishment. 



Brehm, Ch. L: „Der Zug der Vogel." Oken's „lsis" 1828 p. 912. 



1 . The gipsy habits in Birds have to be considered in judging 

 Migration critically. 



2. The previous winter has great influence on the movement. 



3. Weather during the passage has a great influence. 



4. There are Birds migrating by day, others by night. 



5. Birds travel at great heights and 



6. against the wind. 



mi M tm 



