402 OYSTERCATCHER AND TURNSTONE 



rain, by flooding the scalps with fresh water, and causing the mussels 

 to close their valves, militates against the success of the oyster- 

 catcher. 



Although so small a proportion of shells is found to have been 

 opened through the ventral borders, owing to the fact that mussels 

 are normally attached to the beds by the byssus protruding from this 

 border, yet in a tightly closed shell the narrow round-edged fissure in 

 the middle of the ventral border is the weak point in the mussel's 

 armour. Therefore, when this surface is exposed, even if the mussel 

 is dry, it is always vulnerable to the attack of the oystercatcher. Mr. 

 Dewar says, " When oystercatchers are seen at work on dry mussel- 

 scalps, it may be taken for granted that they are searching for these 

 mussels (i.e. with the ventral surface exposed), and I have found 

 repeatedly in these cases that only those mussels were opened of 

 which the ventral borders were uppermost." 



Mussels larger than one inch and five-eighths by seven-eighths of 

 an inch are apparently not attacked. Those most frequently taken 

 vary between this and one and a quarter inch by half an inch. 

 Smaller mussels than this are taken only " when larger sizes are not 

 available." This refers to shells that are opened. Those of smaller 

 size, up to half an inch long, are swallowed entire. 



From the above it will be seen that the oystercatchers' richest 

 harvest is directly the ebb leaves the scalps exposed, and again 

 when the rising tide flows over them. At the latter time the birds 

 continue their search until they are flooded off their feet by the 

 rising tide, when they betake themselves to favourite resting-places 

 above high-water mark. They spend the time until the next ebb 

 in resting from their labours during the process of digestion of 

 their well-earned meal. They rest head to the wind, and dispose 

 themselves in long straight lines. They often stand on one leg 

 whilst at rest, as do all Waders, and Macgillivray states that he has 

 seen them crouched on the ground. 1 



1 Macgillivray, British Birds, iv. p. 156. 



