MIGRATION OF BIRDS. 393 



removal from one place to another, it may be asserted that the 

 greater part of the animals of Africa are forced to make snch 

 changes. The difference in the commencement of the rainy 

 season throughout the grassy or wooded uplands, both north 

 and south of the equator, naturally influences the ripening of 

 fruits and the development of insect life, upon which the higher 

 classes of animals depend for subsistence. Hence arises the 

 necessity for the latter to leave tracts which are beginning to 

 get dry, and to go in search of others which yield the necessary 

 means of subsistence in greater abundance. A good example 

 of this wandering caused by the necessity of finding food is 

 afforded in the Chrysospizza hctca, Licht., which frequents the 

 steppe in flocks at the first commencement of the summer 

 rain, and builds its nests in the acacia brushwood, but, as soon 

 as the steppe becomes withered and dry, collects in small com- 

 panies and removes to the banks of the larger rivers and brooks, 

 visiting even towns and villages. These birds may thus be 

 seen in the winter months by hundreds in Khartum, together 

 with Passer domesticus, L. The same is true of some weaver- 

 birds. It is, then, quite natural that, when the steppe yields 

 no more food, its feathered inhabitants should retire, in part at 

 least, to the south, where an abundant feast awaits them. I 

 mention as a well-known example Uypliantica cethiopica, Sund., 

 which in summer frequents the tablelands of Kordofan, Sen- 

 nar, and Takale, bat in the winter chooses for its abode the 

 region between Sobat and the 4th parallel of N. lat., be- 

 cause the beds of rushes in this tract supply suitable dwell- 

 ings and food. But besides the need of food, which causes 

 birds as well as all other animals to change their residence, 

 it must be remembered that limited districts, which are able in 

 ordinary times to harbour a certain number of birds, become 

 too small for them, particularly in the breeding season, because, 

 on the one hand, each pair demands a larger space, and on the 

 other, the consumption, of insects especially, is considerably 

 increased. Hence results a movement which in our territorv 

 is chiefly from south to north, though migrations to the south 

 are not unknown. 



It would be superfluous to give examples of this movement 

 from south to north ; all that have hitherto been called African 



