4G0 THE ZOOLOGIST. 



trusion. The rock-beach, even when it takes the form of skerries, 

 has less depth of foreshore, and a variety of surface features, 

 which, in some places, are a direct assistance to human approach. 



On these grounds the mud-flat is regarded as an optimum 

 habitat of the Oystercatcher, on the south shore of the Firth of 

 Forth, in so far as food activities are concerned. It is obvious, 

 however, that a mud-flat will not form an optimum habitat 

 in a district having a large human population, unless it has 

 attached to it a place of refuge, available during high water and 

 at other times, when the feeding-grounds are disturbed. There 

 are other non-essential conditions of which one — the wet sand- 

 beach — has been definitely recognized. 



In more general terms, the conditions of an optimum habitat 

 in winter are: (1) a body of sea-water; (2) edible Mussels in 

 sufficient quantity and of a suitable size (mostly not under 

 2'5 cm. in length), stationed on an area which has shallow 

 gradients and sinuous contour lines, and which is not exposed to 

 severe wave-action ; (3) areas of soft wet sand near to the 

 feeding-grounds, attractive in connection with the crowding and 

 other reactions. Wherever the presence of man makes itself 

 felt, there fall to be added : (4) separation of the feeding-grounds 

 from the nearest potential source of danger by a distance of, at 

 least, 150 metres ; and (5) a place of refuge which is not too far 

 away, and affords a considerable amount of security during the 

 period of high water, and in the event of human disturbance of 

 the feeding-grounds. 



In the Firth of Forth, the large human population of its 

 banks is, on the whole, adverse to the Oystercatcher as a species. 

 Extensive feeding-grounds exist on both shores, which are seldom 

 visited, and which, as far as can be seen, would adequately 

 support a larger stock of birds than that, at present, inhabiting 

 the whole estuary. The destruction wrought each winter on the 

 resident stocks is so slight as to be imperceptible in the mass. 

 This fortunate circumstance is solely due to the fact that the 

 territories, which are inhabited, provide the conditions which 

 make it very difficult to bring the birds within range of the gun. 



IX. — The Optimum Habitat in Summer. 

 About fifty pairs are estimated to breed yearly in the lower 

 reach of the Tummel between Pitlochry and Ballinluig. For 



