THE EVOLUTIONS OF WADERS. 165 
bearing on the psychology of the evolutions than any mentioned 
hitherto. When a column of spray is thrown up, one sees the 
whole curve complete for an instant before it collapses. That is 
to say, some particles do not pass over the summit, and only 
rise in the upcast column to heights which correspond relatively 
to their heights at the beginning of the impulse. If the flock of 
Waders is small, the phenomenon is absent, because there is not 
sufficient material for its production. When the flocks are of 
moderate or large size, the details are reproduced with wonderful 
exactness. Those Waders which enter later into the curve 
ascend the upcast of the column to a greater or lesser height 
according to their earlier or later entry into the movement, and 
their ascent comes to a sudden end, wherever they may be, with 
the general collapse of the form of movement. 
It may have been gathered that there are reasons for believing 
in the defensive character of the evolutions. I propose now to 
give some of these reasons, and will revert first to the case of 
the hawk and the Knots. Before the hawk arrived the Knots 
were resting on an isolatea and exposed reef. They rose when 
the hawk approached, and performed the evolutions I have 
already described. They continued to do so until the hawk 
went away. Then they resettled on the rock. During several 
minutes the hawk made most persistent efforts to break into or 
detach a portion of the closely packed, swiftly moving flock. 
The attempts not only failed, but the entire performance ap- 
pealed to me as being unusually inept for so adroit a bird. 
Since the date of that event I have seen several encounters of 
hawks with Knots, and also with Dunlins, and in all cases the 
results were similar. The Carrion and Hooded Crows attack 
Waders at every opportunity. If the flock is resting on a reef, 
the crow approaches cautiously on foot, apparently in the hope 
of reaching the edge of the flock and seizing one of the smaller 
Waders. The Waders forestall this move by rising and perform- 
ing evolutions similar to those shown in the hawk’s case. But 
the crow never follows. It retires a little way, and waits till the 
birds settle, when the process begins again. ‘The observer is 
walking along the shore, and sees ahead a flock of Dunlins or 
Knots. If the birds make their escape by passing him instead 
of going on ahead, they will probably perform. They rise, and 
