INTERNAL MIGRATIONS. 137 



deal in individual species. The same remarks 

 apply to North Africa. Many birds there are 

 regular migrants, coming up from the desert in 

 spring, where they have been spencUng the winter 

 in the various oases, and breeding on the slopes of 

 the Atlas. Their fly-lines may be traced down 

 certain valleys, through gorges, and from oasis to 

 oasis, followed just as unerringly as those of birds 

 whose migrations extend far across the sea. 



I had the pleasure of observing two very inter- 

 esting instances of this vertical migration whilst 

 travelling in Algeria, one of which had hitherto 

 escaped the notice of naturalists. This was the 

 vertical movement of Tristram's Warbler [Sylvia 

 algeriensis), a species that was originally discovered 

 by Canon Tristram in the remote oases of the Sahara, 

 and whose habitat was stated by that naturalist to 

 be ".only in the southern desert." This region, 

 however, is but its winter quarters, for in summer 

 I found it distributed throughout the Djebel Aures, 

 from the plateau of Batna, 3500 feet above sea- 

 level, up to 6000 feet near Oued Taga. The 

 second species was the gay and lively Bush Chat 

 (Pratincola mousseri), which Canon Tristram found 

 in increasing numbers as he went south into the 

 desert during winter ; but in summer exactly the 

 reverse conditions prevail, and I found it equally 

 common from Batna up to 6000 feet, becoming 

 less common on the lower and southern slopes of 

 the Atlas to the oasis of Biskra, which is only 360 

 feet above sea-level. Again, the Chat {Saxicola 



