224 
MIGRATION OP BIRDS 
of the positions of the magnetic poles. All may be contribu- 
tory factors, but any single one is not sufficient ; thus, taking the 
point of recognition of landmarks and previous knowledge 
of the route, we find that in many cases the birds are known to 
travel by night time, as evidenced by the fact that astronomers 
have reported the passage of large numbers of birds across the 
face of the moon, and that these birds are travelling at great 
heights. Again, we know that flocks of birds are attracted to 
lighthouses at night in immense numbers. Frequently, also, 
one can hear large numbers of birds flighting at night time. 
It is also a noticeable fact that when one does hear the birds 
flighting it is usually on a dark still night, especially when there 
is a mist or when rain is falling. These facts, then, rather put 
the theory of landmarks out of count to a great extent. 
Another fact which also contradicts the suggestion of land- 
marks is that already alluded to in connection with the food 
supply — namely, that in some species the young migrate first, 
unaccompanied by any adult bird over routes which they 
cannot possibly know anything about ; and in addition we 
know that the converse is the case with regard to the cuckoo : 
here the adults leave first, the young follow later ; but how ? 
Do they accompany their foster-parents ? 
There is one point which must not be overlooked when 
considering landmarks as a possible guide, and that is that 
one frequently sees certain birds which have flocked preparatory 
to migrating, ascending in large numbers towards evening, 
and, after taking short flights at a great height, returning to 
the place from which they started. Are these birds merely 
exercising, or are they getting the direction in which to move 
off later ? Major Meinertzhagen has reported that he came 
across such a flock one afternoon when he was up in an aero- 
plane, scouting. He also records the interesting fact that this 
flock consisted of birds belonging to quite distinct species — 
such as Rollers, Kestrels, and Bee-eaters. 
In support of the statement that birds travel by night, I 
should like to mention that in a certain locality not far from 
Nairobi, where I took continuous and careful observations 
during the autumn and spring movement of 1916-17, I was 
interested to find that on two occasions certain species appeared 
