THE HVOLUTIONS OF WADERS. 168 
increasingly rapid descent. The whole flock, if not too large, 
enters into the formation of the movement. It may be repeated 
any number of times, at regular intervals, unless horizontal 
movement intervenes. It is usually performed on the beat to 
windward, and with the wings outstretched. During the upcast 
the under surfaces of the birds are directed towards the sun, and 
a ‘‘reverse’”’ occurs at the summit, so that the descent is per- 
formed with the under surfaces turned away from the sun. 
Thus an observer stationed between the birds and the sun sees 
a change in the swiftly moving flock from a white and solid to 
a grey and diffuse appearance. Occasionally the upper surfaces 
face the sun during the upcast. When this is the case, the 
*‘reverse”’ seldom occurs. So that the evolutions in a relatively 
uncomplicated form consist of a succession of movements, each 
of which is made up of a swift drifting to leeward, followed by a 
beating to windward, and during the latter a rapid sequence of 
peculiar movements takes place in vertical planes. The vertical 
movements may attain to little height, or reach a considerable 
elevation. While there is no hard and fast line to be drawn 
between the lower and higher forms, the summits of a given 
series maintain a very uniform elevation. The lower form is 
that more often seen, and the preceding description applies to 
it more particularly. The higher form is mainly associated with 
landing and may want the upcast, the descent being started 
from a windward or leeward movement in a high horizontal 
plane. Very often the descent is a slow fluttering movement, 
the birds being spread out thinly like a sheet, and having their 
upper surfaces directed towards the sun. Usually, though not 
always, at a level about equal to that of the summit of the 
lower form there is a ‘‘reverse,’ causing suddenly a change 
to a conspicuously white appearance, which is maintained to 
the end of the movement, and from the time of the ‘‘ reverse”’ 
onwards the descent is hastened. To show the lower form of 
movement in perfection, the flock should be of small or moderate 
size, and there should be immediate danger from a bird of prey. 
A flock of Knots, attacked by a hawk, did not fly away as I had 
expected. They drifted swiftly to leeward over a short linear 
distance, and on the windward beat to the starting-point they 
developed a succession of the lower form of movement. This 
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