OYSTERCATCHEB AND NATURAL ENVIRONMENT. 289 



on the Mussel-bank at the mouth of the River Awe, or the 

 Mussel-scalps on lonely parts of the coast of Northumberland, 

 have no distinctive refuge. They pass the time of diurnal high 

 water on a constant part of the high-water mark. When dis- 

 turbed, they proceed to little-frequented fields in the vicinity. 



Oystercatchers show a decided preference for Mussels about 

 3'5 cm. in length. Why, has not been determined. Probably 

 Mussels ranging closely about this size provide food in due 

 proportion to the amount of labour required to get it. When 

 the Mussels are larger, too much energy is needed to open them; 

 when they are smaller, too many have to be opened, and the 

 more fragile shell is apt to be crushed into the body of the 

 mollusc. In this condition they are commonly refused. 



The situation of the feeding-ground at or more than 150 

 metres from the nearest danger point is also an adjustment, 

 probably local, to human intervention. In the Firth of Forth 

 large areas of Mussel-scalp, providing an abundance of food, lie 

 too near the shore line owing to steepness of the gradient. 

 These areas are seldom visited. When they occur near to or 

 within a permanent winter station, they are visited only at 

 night or in the early morning. 



To most of the permanent stations in the Firth there are 

 attached areas of wet sandy beach, generally lying close to the 

 feeding-grounds. These areas are much used, especially after a 

 spell of feeding, for the purpose of crowding together, preening 

 and resting, and, in fine weather, for the sunning reaction. The 

 presence of such an area is, however, not essential, for the con- 

 dition is absent from the Lamb territory. The habit is not 

 protective during the resting periods. The birds are very 

 conspicuous while on the sand, whereas on the rocks or on 

 mud, crowded with Mussels, they are often hard to distinguish. 

 Crowding together appears to form an essential part of the habit. 

 Owing to the irregularities of rocky areas and of mud-banks, 

 dotted with clumps of Mussels, crowding can rarely be possible 

 in these localities. A smooth expanse of sand is better adapted 

 to the purpose. It is also possible the soft wet sand is a 

 necessary condition of the response. 



Along the. Forth the Oystercatcher has few enemies, and 

 even the depredations of man — the most important destructive 



