THE EVOLUPIONS OF WADERS. 169 
period of high-water, and that the same period is chosen by 
gunners as most generally suitable for their purpose, the method 
of election being to lie hidden in ambush near the high-water 
mark until the Waders are driven within range by the tide. 
Thus an inference is made that there is a connection between 
complexity of the evolutions and an implied uncertainty of the 
Waders regarding the position of dangerous places, or doubt as 
to the intentions of visible enemies. As the birds move in- 
shore the immediate circumstances revive their memories of 
former experiences, and lead them into exertions which other- 
wise they would not feel called upon to make. Hence the 
addition of far-reaching horizontal, vertical, and oblique move- 
ments to the simpler evolutions may be imperative, so as by the 
freedom and variety of the movements to reduce the possibility 
of death to an unlucky chance. 
I may now bring together the conditions or factors which 
are associated with or contribute towards the development of 
the more complex evolutions. When the flock is large the 
movements are often sectional, and what seems to be a succes- 
sion of waves passing through an extended flock is in many 
cases an extremely quick repetition of the simpler form of the 
evolutions by sections. The ‘‘ sheet movements” which pro- 
vide much of the spectacular display are rendered possible by 
the same circumstance, and generally grow out of the simpler 
form. They are more especially the manceuvres which take 
place prior to flight from one place to another, and before this 
happens they may be greatly prolonged. Throughout the 
‘“‘sheet movements”’ there are more or less frequent recur- 
rences of the movement in the form of the spray. If the danger 
is not pressing, greater liberty is taken in developing the move- 
ments. But the moment the position and intentions of a human 
enemy are disclosed, the evolutions generally come to an abrupt 
end in hurried and direct flight to a place of safety. Above all, 
I attach importance to two factors in the production of com- 
plexity—first, the suspected presence of human enemies in 
ambush, and, secondly, the period of occurrence when the tide 
is above mean water level, which really means a reduction of 
the available area of dry shore, with an associated implication 
of danger. In the understanding of the meaning of this faet 
