462 THE ZOOLOGIST. 



which especially as they grow older, seldom retreat to the 

 shingle in the event of human or other intrusion. A later stage 

 of the same kind of area is seen on the Cragan Dubh alluvial 

 cone on the south side of Loch Tummel. Here the local 

 feeding-ground on the cone is dotted over with mossy hillocks, 

 under which, and completely covered by the vegetation, are very 

 ancient stumps of trees. The station continues to be occupied 

 yearly by a pair of birds, and young are usually reared, but the 

 old tree-remains have no evident biological significance to either 

 old or young of the Oystercatcher. The Cragan Dubh cone, though 

 small, is a comparatively rich area, and the young do not need 

 to be led far. The wide range of the young in the Fender area 

 appears to be due to relative poverty of the food supply, and is 

 undoubtedly rendered possible by the protective value of the 

 tree-stumps. When these are covered over by vegetation at a 

 future date, a change may be expected in the distribution or 

 range of the birds. 



Next in the sequence comes the lake-beach habitat, as repre- 

 sented by the south shore of Loch Tummel. The conditions 

 here are, on the whole, favourable to the occupants and to the 

 rearing of the young, but the size of the population is evidently 

 controlled by two factors, of which one is the relative narrow- 

 ness of the shingle beaches which are not greatly favourable, on 

 that account, to the concealment of the eggs or the young, and 

 the other is the limitation of the local feeding-grounds to the 

 discontinuous alluvial cones, for suitable local feeding-grounds 

 do not occur elsewhere. 



The portions of the Garry and Tummel under observation 

 have a higher population per kilometre than any of the other 

 areas. In this respect they show a pronounced difference from 

 the other representative of a river-valley habitat, already des- 

 cribed. Using a salient and ecologically important feature, it is, 

 therefore, proposed to distinguish a river-valley habitat, such as 

 the Orchy, from a drift-river-valley habitat, of which the lower 

 reaches of the Garry and Tummel are representatives. In these 

 areas the drift, or boulder-clay, covers a large part of the ground, 

 and is largely cultivated. With the drift, as of importance for 

 the Oystercatcher, must be associated the large deposits of 

 alluvium which determines the distribution and area of the 



