forward on a rapidly descending curve in the interval between 
the wing-beats. This gives rise to what is known as an 
“undulating”’ flight. But the large passerines, like the 
crows, differ conspicuously in their method of progress. With 
them the wing beats relatively slowly, so that its shape can 
be readily seen ; and their course is direct-—hence the familiar 
saying ‘‘straight as the crow flies.” Further, the imner 
webs of the outer primary quills are, what is called “ emar- 
ginate,”’ that is to say, the width of the web is suddenly 
reduced towards the tip of the feather, so that the out- 
stretched wing has a conspicuously fringed appearance, as 
may be seen at a glance at the beautiful pen-and-ink sketches 
on another page. The eagles and falcons have similar 
emarginations. 
But to return for a moment to the smaller passerines. 
There are very few of our native species which could be dis- 
tinguished in the field by their flight alone. For the most 
part one has to rely on this and clues afforded by character- 
istic markings: while a further aid is afforded by at least a 
slight knowledge of the haunts of birds. One would not 
expect to find a wheatear in a wood, or a wren in a reed-bed. 
The wagtails are among the easiest of the “ undulating ”’ 
fliers to distinguish, if only because of the great length of the 
tail. The pied-wagtail, with its black and white plumage—or 
black, grey, and white in the winter—can be identified at a 
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