284 THE ZOOLOGIST. 



and early spring, when the available feeding-grounds are greatly 

 enlarged at low water. The vertical range of the low-water line 

 is much greater than that of the high-water line, so that the 

 advantage gained in the lower zones of the beach during spring 

 tides is more than lost during neaps. For the same reason, the 

 diminution of tidal range in passing from spring to neap tides is 

 not directly proportional to the loss in height of the tide, but 

 diminishes at a greater rate. The tidal range during neap tides 

 is, therefore, a factor of importance in controlling the numbers 

 of a settlement. The area under observation was too small to 

 allow of a comparative study of the effects of locally different 

 tidal ranges. The occurrence of the fortnightly succession of 

 highest tides between the hours of one and five has, in the day- 

 time, an important bearing on the welfare of the Oystercatcher 

 .in relation to human activities. 



All three divisions of a sea-beach recognized by Pearse in 

 Massachusetts * occur in irregular sequence on the south side of 

 the Forth. They are the rock-beach, sand-beach, and the mud- 

 flat. The first and last are habitats of Mytilus ; the second is 

 of minor importance to the economy of the Oystercatcher. As 

 the division was found to be a natural one, the rock-beach and 

 the mud-flat will be taken, for comparison, as the two locally 

 typical habitats of the Oystercatcher. Along the sixty odd 

 kilometres of coastline from the Forth Bridge to Belhaven, 

 Dunbar, there were found seven permanent winter settlements 

 of the Oystercatcher. Of these, one is a purely mud-flat station, 

 two are mud-flat, rock-beach stations, while the remaining four 

 are confined to rock-beaches only. In addition, two large Mytilus 

 stations, not permanently occupied by Oystercatchers, were 

 under observation. In the next section an account is given of 

 the general features of the stations^examined, in order to afford 

 some idea of the nature of the ground in this locality. 



2. Mud-flat Habitat. — Estuary of the East Lothian Tyne. 

 The river flows into a small estuary locked from the North Sea 

 by a long sandbar. The substratum consists of sand in the 

 outer and marginal parts of the estuary, and of a fairly firm mud 

 in the centre of the area, through which the channel flows. 

 Weed is not common. There is a plentiful supply of Mussels of 



* ' Rev. Knowledge,' 1915, p. 59 (no reference given). 



