56 MANNER OF MIGRATION. 
highways of migration. Through telescopic observations 
it has been learned that migrating birds travel at a great 
height. The exact height remains to be determined, but 
it is known that many migrants are at least a mile above 
the earth. From this elevation they command an ex- - 
tended view, and in clear weather prominent features of 
the landscape are doubtless distinguishable to their pow- 
erful vision at a great distance. 
It is when fogs and storms obscure the view that birds 
lose their way. Then they fly much lower, perhaps seek- 
ing some landmark, and, should a lighthouse lie in their 
path, they are often attracted to it in countless numbers. 
Thousands of birds perish annually by striking these 
lights during stormy fall weather. In the spring the 
weather is more settled and fewer birds are killed. | 
Although birds are guided mainly by sight, hearing is 
also of assistance to them on their migrations. Indeed, at’ 
night, young birds, who have never made the journey be- 
fore, must rely largely upon this sense to direct them. It 
is difficult for us to realize that on favorable nights during 
the migratory season myriads of birds are passing through 
the dark and apparently deserted air above us. Often 
they are so numerous as to form a continuous stream, and 
if we listen we may hear their voices as they call to one 
another while flying rapidly onward. 
Some idea may be formed, of the multitude of birds 
which throng the upper air on favorable nights during 
their migration by using a telescope. One having a two- 
inch object glass will answer the purpose. It should be 
focused on the moon, when the birds in passing are sil- 
houetted against the glowing background. At the proper 
focal distance they appear with startling distinctness. In 
some cases each wing-beat can be detected, and with a 
large glass it is even possible to occasionally recognize 
the kind of bird. 
