288 THE ZOOLOGIST: 



Kedhouse and at Seafield. Neither has a resident stock of 

 Oystercatchers, and visits are irregular, occurring mostly in 

 the early morning. At the former station the rocks descend 

 abruptly to a sand-beach, and have little breadth. The larger 

 sizes of Mussels preponderate, but lie well within 150 metres of 

 the grass. In the latter, the eastward rocks have a similar 

 character. Towards the west the rock-beach is prolonged out to 

 sea by a succession of more or less isolated reefs carrying plenty 

 of Mussels. The foreshore here is much disturbed. Neither of 

 these areas has a refuge within ordinary range. 



III. — Discussion of the Winter Environment. 



Summing up the conditions common to all the occupied 

 stations of the rock-beach and mud-flat habitats on the south 

 shore of the Firth of Forth, we have the following : — A body of 

 seawater ; Mytilus in sufficient quantity, of a suitable size, in 

 accessible situations, and at or more than 150 metres from the 

 nearest danger point ; a place of refuge which can be resorted 

 to for safety during the period of high water, and at other times 

 when the feeding-grounds are disturbed. 



Eesort to a place of refuge at certain times is a local adjust- 

 ment to human interference. In the early part of the winter, 

 when disturbance is infrequent, the birds rarely leave Aberlady 

 Bay during diurnal high water, and if they do, it is with mani- 

 fest reluctance. At high water they collect at one part of the 

 shore, which is a constant for the purpose, and takes the place 

 of the refuge as the headquarters within the territory. As the 

 winter advances, the birds are more liable to be driven from the 

 high-water mark. They then proceed to the refuge. Later still, 

 in the daytime they " anticipate " disturbance by not coming in 

 to the high-water mark at all during spring tides, and proceed 

 to the refuge one or two hours before the time of high water. 

 But throughout the whole winter, during the diurnal high water 

 of neap tides, and the nocturnal high water of all tides, the birds 

 remain at the headquarters in the bay. In the former instance, 

 the low gradient of the beach enables the birds to remain on 

 the high-water line beyond the range of gunfire. In little dis- 

 turbed localities, permanently occupied winter stations occur 

 which have no special refuges. The Oystercatchers which feed 



