MIGRATORY SPRING FLIGHT. 113 
the air, as the weather permits, and no calling or de- 
coys will allure them. Fresh birds soon take the 
places of those who first came, and the weather deter- 
mines their advance or retreat. Often they are driven 
back by steady freezes or snowstorms nearly to the 
frost line, for the weather in the early spring is al- 
ways variable. 
A scouting party of pintails or mallards consists of 
several flocks of old birds, evidently appointed and 
sent out by generals from the outposts of the front to 
prospect and ascertain the area of the water question. 
They appear suddenly after a change of weather, the 
result of a warm current, a thaw, a rainstorm, or 
whenever open water appears derived from air holes, 
either made by fishes, muskrats or hunters. The lat- 
ter cut channels in the ice, and by tipping up the edges 
of the cut pieces slide them under the solid parts, 
taking care to leave no fragments upon the surface 
or upon the edges of the ice; an open channel is thus 
formed which soon becomes increased by the agita- 
tion of the water, being rippled by the wind or by the 
jumping of the fishes who soon flock to these im- 
provised air spaces. These air holes are, indeed, a 
vital necessity for fishes in shallow lakes; lacking 
which millions die from non-oxygenation. A beau- 
tiful illustration of instinctive preservation is  fre- 
quently seen near the frost line, where muskrats are 
scarce, and shallow lakes are frozen over; there fishes 
will jump all night, making a ripple which agitates the 
water and prevents deep holes from freezing over; 
should these holes freeze the fishes having collected 
to these deeper places would soon die from want of 
oxygen, did they not obtain a fresh supply in this 
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