298 FACULTIES OF BIRDS. 
northerly regions, return to their own country when' 
that rigour has abated. kag i 
M. Brehm, who has given much attention to the 
subject of the migration of birds, considers the 
following facts as established. Every bird has its 
native country, where it freely reproduces, and re- 
mains part of the year, travelling in the remain- 
der. Most birds spend half the year at their home, 
and pass the other half in travelling. Some, par- 
ticularly birds of prey, travel by day, but by far 
the greater part travel by night; and some perform 
their migrations indifferently either by day or night, 
They seem to pass the whole of their migration 
without sleez, for they employ the day in seeking 
their food, stopping in the places where they are 
most likely to find it. They commonly keep very 
high in the air, and always at nearly the same dis- 
tance from the earth, so that they rise very high 
over mountains, and fly lower along valleys. They 
require a wind that blows against them, as a con- 
trary wind assists and raises them. ‘This last state- 
ment must, however, we imagine, be subject to 
some very large exceptions. 
The same writer thus answers the rather difficult 
question, ‘‘ What decides birds to emigrate?” Itis 
not want of nourishment, for most of them com- 
mience their migration while there is still abundance 
im the country they are leaving. Atmospherical 
currents are not the cause, nor do the changes of 
season explain it, as the greatest number set off 
while the weather is yet fine; and others, as the 
larks and starlings, arrive while the season is bad, 
Atmospherical influences can only hasten the mi- 
gration in autumn, but must rather retard or de- 
range it in spring. It is the presentiment of what is 
to happen which determines birds to begin their 
journey. It is an instinct which urges them, and 
which initiates them into the meteoric alterations 
that are preparing. ‘They have a particular faculty 
