OYSTEBGATGHEB AND NATUBAL ENVIRONMENT. 463 



local feeding-grounds. The drift and the alluvium are more 

 favourable to the existence of the animals on which the Oyster- 

 catcher feeds, than the poor soil of the denuded river-valley. 

 Cultivation follows the distribution of the drift and further 

 increases the food-supply. But cultivation means human 

 occupation, which is generally destructive. Cultivation, there- 

 fore, operates in two opposite directions in controlling the ratio 

 of Oystercatcher population. Cultivation tends to increase the 

 numbers of the stock an area may carry, and, at the same time, 

 it is constantly tending to reduce the stock by means of human 

 interference. Evidently, a balance has been struck, as the 

 Oystercatcher population shows little or no change from year to 

 year. The ratio of stock on the Garry to that on the Tummel 

 is about 1 : 2. The most evident differences between the two 

 areas are the smaller size of the Garry and its shingle areas, and 

 the larger agricultural population. The smaller shingle-beds 

 on the Garry are less favourable to the safety of the eggs and 

 young than is the case on the Tummel. But, even on the 

 large shingle areas at Moulinearn and Ballinluig, there is seen 

 a tendency to desert the shingle for the river-bank, and the 

 connection between this phenomenon and human interference is 

 well established. Nesting in unusual places is general in the 

 Vale of Atholl, but it must be fairly successful, as no diminution 

 in the local stock has been recognized. Here the abnormal 

 behaviour is probably connected with the semi-public character 

 of the shingle and the large human population, the Vale of 

 Atholl being the most populous district in the area under 

 observation. It would appear as if, under the present secondary 

 conditions of the river-valley, a too large human population 

 outweighs the advantages which drift, and cultivation by a 

 moderate population, confer upon the Oystercatcher. 



Owing to the erratic nesting behaviour in the Vale of Atholl no 

 reasonably complete figures could be obtained for the ratio of fami- 

 lies to nests, and hence no comparison of the breeding results in 

 the Garry area with those on the Tummel is possible. The results 

 obtained at Loch Tummel, however, show that the relatively 

 moderate stock on the south shore of Loch Tummel, as compared 

 with the relatively large stock carried by the lower part of the 

 river Tummel, is not due to greater difficulties in rearing young. 



