54 THE ZOOLOGIST. 



least two matters which demand further explanation. One is 

 the peculiar attitude of the bird in the moment of the attack, 

 especially as regards the bill, and the other the nature of the 

 pushing or chipping stroke. Having repeatedly verified the 

 original observation that the bill is held with the sides vertical 

 in the operation of overturning a Limpet, I have no doubt that 

 it is actually so, and the problem comes to be, Why does the 

 Oystercatcher instinctively ignore the expediency of pushing the 

 bill flatly under the Limpet ? The answer is bound up with the 

 nature of the sharp push or chipping stroke. In the first place, 

 a Limpet, when bone dry externally, adheres to the rock so 

 tenaciously that no pressure likely to be' within the capacity of 

 the 0} 7 stercatcher is able to move it. In the second, a Limpet, 

 when it is moist as in a pool, in the tidewash, or soon after the 

 ebb has left it, is generally relaxed and slightly raised from the 

 rock. The Limpets which fall into the second category are 

 those sought after by the Oystercatcher, and it is an additional 

 advantage if there be a slight irregularity, either in the shell or 

 the surface of the rock, at one point on the margin of the shell. 

 When, however, one of these Limpets is experimentally warned 

 by tapping it gently, it draws itself tightly on to the rock, and 

 passes into the first category. Thus the main essential is appa- 

 rently a taking of the Limpet unawares, and a secondary ad- 

 vantage the existence of a slight local increase of the normal 

 gap between a relaxed shell and the rock. Limpets, of course, 

 are to be found in the tidewash with the shell separated from 

 the rock more than enough to admit the depth of a bill, and 

 others are attached to such hopelessly irregular pieces of rock 

 that the insertion of the bill is an easy matter. On the other 

 hand, a Limpet, whether it is relaxed or holding firmly, is abso- 

 lutely safe when it is sunk in a depression of its own making in 

 the rock. For such as these the peculiar method of the Oyster- 

 catcher is not called into play. After the tide has receded, and 

 even before the rocks have dried, the average Limpet presents so 

 small a crack between the shell and the rock that it would be 

 impossible to push the bill under the shell without the aid of 

 some special mechanism. This is supplied by the sharp push 

 or chipping stroke. When it is made experimentally in the case 

 of the smaller shells, the result is exactly as I have described it 



