AERIAL JOURNEYS. 
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of greater or less length, guided through the trackless fields of air by 
an instinct which we find it difficult to understand. We must not 
confound, however, those which migrate with those which only travel, 
or enjoy, as it were, an aerial ramble. The former set forth every year 
at or about the same date, and follow always the same direction ; tlie 
latter do not remove until urged by some iinperious necessity. Neither 
the date nor the direction of their flight is determined befoi'ehand, and 
it comes to an end Avith the cause which impelled it. Then there are 
others who traverse a very limited area, simply abandoning one locality 
for another situated at no great distance apart. It is in this last sense 
that the bird may be called a migratory individual. Some of our 
favourite English species, for instance, keep always near a particular 
spot ; but every year, or even oftener, they move their nest, under what 
we must suppose to be in their opinion a sufficient motive. 
But we wish to speak rather of those general migrations, which 
every autumn deprive us of so many of our feathered favourites, to 
bring them back in the spring ; or which carry off" our aquatic birds 
before the winter has fettered the streams with ice. The return of the 
