368 THE STRUCTURE AND LIFE OF BIRDS cuHapP. 
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 
(1) 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 
