IX. THE CAUSE OF THE MIGRATORY 

 MOVEMENT 



WE have now considered some of the principal phenomena which 

 the migratory movement of birds presents to us, and it is, in con- 

 clusion, left for us to inquire into the immediate cause which 

 prompts these creatures to undertake their migration journeys. 



It would be interesting to discover what induces birds wintering 

 in the more southern parts of Africa, where they are subjected to 

 hardly any changes of climate, suddenly to leave these stations for 

 their breeding homes in the north ; or, again, what it is that urges 

 the individuals of a species, the nesting stations of which are 

 situated, say, in central Germany, to start on their journey a 

 month earlier than other members of the same species the breed- 

 ing homes of which are in northern Scandinavia, and which have 

 been passing their winter in North Africa. The latter allow the 

 migrant stream of their more southern kinsmen to pass over them 

 unmoved, as though they were fully conscious that their own time 

 for departure is not yet ripe, and that their breeding quarters are 

 still held bound in the depths of winter. Phenomena similar to 

 these are also presented by different species of the same genus, as, 

 for instance, in the case of the two species of Bluethroat, Sylvia 

 suecica and S. leucocyana, of which the first has its nesting 

 stations within the confines of the Arctic Circle of the Old World, 

 while those of the latter hardly ever extend beyond northern 

 Germany. Both species pass their winter in Upper and Central 

 Africa; nevertheless the southern white-spotted species arrives 

 here as early as the end of March and up to the middle of April, 

 even though the weather be still inclement ; whereas the northern 

 red-spotted form does not migrate through this island until May, 

 when warmer weather has set in. 



Now what could possibly induce the birds of the southern 

 species to start on their journey as early as the end of March, and 

 what could cause their northern relatives to tarry until May in the 

 common winter quarters ? for we can hardly suppose either that 



