368 THE STRUCTURE AND LIFE OF BIRDS chap. 



the north follow their example. In the Transvaal 

 are many resident Reedwarblers, and mingled with 

 them during the southern summer are some of our 

 Reedwarblers that nest in the north. How wonderful 

 it is that these near relations should have formed 

 habits so different and that those who have departed 

 from the ancient traditions of the Reedwarblers 

 (supposing it to be the migrants who are the inno- 

 vators) should be present to watch the others main- 

 taining the primitive customs of the race ! 



The Cause of Migration. 



Two of the problems of migration rank in import- 

 ance above the many others that meet us. These are 

 (i) why birds migrate, (2) how they are able to find 

 their way. The former of these two problems, though 

 the easier of the two, is a difficult one. The Swallow 

 would seem to be well enough off in South Africa. 

 Why, then, start on a voyage which must be perilous 

 even for an old and experienced traveller ? Why 

 adopt a plan of life which must mean frightful mor- 

 tality among the young ? For of the many Swallows 

 hatched only a small proportion return, and the 

 numbers washed up by the sea after a storm show 

 what has become of them. These arguments sound 

 forcible, but they will not stand investigation. Swal- 

 lows and other migratory species maintain their 

 numbers quite as well as those that are resident. 

 Among all birds, whatever their habits, the death-rate 

 is very high. It is true that migration has to answer 

 for the deaths of a very large percentage, but there is 



