4G4 THE ZOOLOGIST. 



In 1912, nine pairs on the south shore and the storm heach 

 of Loch Tumniel produced four families of altogether nine young. 

 In 1914, eight pairs produced three families of 4 + X young 

 (prohably seven). On the river Tummel, in 1909, at Moulinearn, 

 eleven pairs had five families of eleven young. On the Logierait, 

 upper island station, in 1910, ten pairs had four families of 

 eight young, and in 1912 eight pairs had three families of six 

 young. The ratios of adults to young in the two habitats are, 

 therefore, approximately similar, being about 2*2 i 1. These 

 two habitats are equivalent in human population and in amount 

 of human intrusion in the territories. But the ratio of stocks 

 in the two areas is relatively high, being as 2'5 on the river to 1 

 on the loch. The control of the Oystercatcher population in 

 the two areas must, therefore, be sought in the factors already 

 given, namely, the size and distribution of the shingle areas, 

 alluvium, and boulder-clay. These three factors are equivalent 

 on the Garry and the Tummel, except in total area. The 

 size of the total area, when continuous as it is on these rivers, 

 ought not to affect the linear distribution of the birds. The 

 cause of the lesser population per kilometre in the Vale of 

 Atholl may, therefore, with a fair degree of probability, be 

 referred to the greater human population. 



Thus, on the basis of population ratio and other considerations 

 already mentioned, the drift-river-valley may be regarded as an 

 optimum habitat in summer, within the area placed under obser- 

 vation. In some, the prevailing conditions of an optimum habitat 

 in summer are: — (1) A breeding area and refuge, of sufficient 

 extent, close to a body of water, rising well above summer flood 

 level, devoid of vegetation, assimilating in colour appearance to 

 the eggs and more especially the young, and fairly secure from 

 human and other enemies. 



(2) A local feeding- ground of sufficient area, to which the 

 young have access on foot, terraced to form two adjacent levels 

 so that, when the birds are on one level, they cannot readily 

 be seen by an enemy from the other, covered with old turf, rich 

 in tipulid and coleopterous larvse, together with at least a 

 moderate supply of earthworms, and fenced in, or in other ways 

 protected from disturbance ; both the breeding area and the 

 local feeding- ground being held as territories, and situated on 

 that account not too near the habitual stations of "barren pairs," 



