OBSERVATIONS ON THE OYSTERCATCHER. 55 



from direct observation of the Oystercatcher at work. With the 

 larger shells, while it is easy to reproduce the natural occur- 

 rence, it is not so easy to find out what actually occurs during 

 the rapid succession of events. The blow must be delivered at 

 a low angle ; otherwise it serves merely to warn the Limpet to 

 settle down more firmly on the rock. And the instrument must 

 be held with its sides, or rather the plane of its depth, at right 

 angles to the plane of the ground. I find that at the moment of 

 the stroke, made under these conditions, the instrument repre- 

 senting the bill does not pass under the shell, but gives a sharp 

 blow to the edge of the shell. The blow shifts the Limpet 

 bodily a fraction of an inch in the direction of the pressure. The 

 animal then holds on, and the upper part of its body undergoes 

 distortion, pressing the advancing edge of the shell firmly on to 

 the rock, and raising the near edge against which the instrument 

 is pushed. In some cases this proves sufficient. If the shell is 

 out of the water, and the stroke has been made quickly, a 

 rushing sound is heard, as of air passing in under the foot, and 

 the shell topples over. In others the instrument must be passed 

 under the shell. To do this, hard pushing is required to distort 

 the body of the Limpet, and to raise the near edge of the shell 

 still more. Probably the obliquely truncated tip of the bill, as 

 it occurs in at least some Oystercatchers, assists in raising the 

 edge of the shell under pressure. After getting the tip past the 

 edge of the shell, the instrument must then be pushed under the 

 edge of the foot, a proceeding which is made easier by swaying 

 the instrument from side to side. This manoeuvre produces an 

 abrasion on the edge of the shell similar to that proceeding from 

 a similar cause in nature. As soon as the instrument passes 

 to a varying distance under the foot the rushing sound is heard, 

 and the shell topples over. If, however, the movement is carried 

 out slowly the rushing sound is not heard. Leverage by raising 

 the base of the instrument is a disadvantage at an early stage, 

 as in the likely event of failure the Limpet gets a better grip of 

 the rock during the return movement. It, however, hastens the 

 overturning when the hold of the animal is weakening. Instead 

 of this form of leverage, rotation of the instrument through 

 barely a quarter of a circle is equally helpful at this later stage. 

 Experimentally, the instrument passes more easily under the 



