222 
MIGRATION OF BIRDS 
birds which breed only in the far north, yet are commonly 
found in this country in winter. 
Now with regard to the causes of migration ; nothing 
definite is known, so we can only put forward suggestions. 
Take first of all the autumnal or southward movement. 
In some cases the increasing scarcity of food may be all-powerful 
and prove a sufficient reason ; for it is well known that birds 
will travel to great distances in order to obtain food. Thus 
we find it recorded by a recent observer in Northern Siberia 
that certain waders disappeared when food — particularly insect 
life — became scarce, as a result of a fall in temperature. 
The limited amount of food in a certain area may account 
for the fact that in certain species of birds, which are double- 
brooded, the young of the first brood are driven away by their 
parents, thus assuring nourishment for the second brood ; 
and in the case of Partial Migrants some birds may move south 
in the autumn, knowing that if they remained there would not 
be sufficient food for them and for the large number of 
immigrants from other countries. 
That climatic conditions — such as fall of temperature, in- 
creasing rains and wind — play an important part is well demon- 
strated when we consider the movement of birds in the Arctic 
zone ; yet, on the other hand, we have instances where certain 
sea-birds, such as Puffins, return to their breeding-grounds on 
exactly the same date in each year, no matter what the weather 
conditions are like. 
These two causative agents may appear to be sufficient 
when applied to the southward movement, but what governs 
the return in spring ? Here we fail almost entirely and have 
to fall back on that much abused term Instinct, which, in 
other words, may be taken to mean ‘ love of the country of their 
birth ’ ! This cannot be proved or disproved. 
When considering the return movement of migrants from 
this country, it is interesting to note that when this movement 
takes place our local birds are breeding and all conditions ap- 
pear most favourable. Why don’t the migrants remain here 
to breed ? That some individuals, belonging to species well 
recognised as migrants, do breed in this country is now recog- 
nised, but whether these individuals or their young ever do 
