MIGRATION. 
407 
What may be the causes which combine to drive a bird 
so far from its home it is impossible for us to say. Storms 
at sea, doubtless, drive aquatic birds inland; but how a 
small Thrush manages to get from America to Northern 
Asia, and travel half through Europe, still remains to us 
a mystery. There is so much in bird-life, however, which 
is dark and mysterious ; how much more so, indeed, must 
be these apparent errata of Nature. 
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