458 THE ZOOLOGIST. 



TPIE RELATION OF THE OYSTERCATCHER TO ITS 

 NATURAL ENVIRONMENT. 



By J. M. Dewar, M.D. 



(Concluded from p. 431.) 



VIII. — The Optimum Habitat in Winter. 



On the south side of the Firth of Forth three areas carry- 

 large stocks of Oystercatchers. These areas are chiefly, or 

 entirely, mud-flat habitats. The remaining four areas, coming 

 under observation, are rock-beach habitats, and they carry small 

 stocks. As it has not been possible to determine the total 

 quantity of Mussels available in each area, and as periodic 

 estimations of the numbers in each stock, excepting those of the 

 small stocks, proved disappointingly variable, the direct method 

 of comparing the numbers of the birds with the size of the 

 Mytilus areas, where they feed, could not be used. Recourse 

 was had to other means. 



I have elsewhere * stated, from fairly extended observations, 

 that the Oystercatcher is unable to open tightly-shut Mussels, 

 unless it can reach the byssal cleft, or the Mussels are small 

 enough to be crushed. The former possibility occurs infrequently, 

 Mussels opened through the straight border forming only nine 

 per cent, of the opened shells examined. The latter is apparently 

 avoided, whenever possible, and the birds reject Mussel-flesh 

 mixed with fragments of shell. Dryness of the surroundings 

 brings about a tight closure of the shells, and follows shortly 

 after the tide has receded from the scalps. The birds are then 

 confined in their search to the tide-line, pools, under seaweed, 

 and under mud near the scalps ; a hunt for Mussels presenting 

 the straight border, and the hammering of small Mussels being 

 evidently the last resort. Mussels that become covered with 

 mud are soon killed, and are never numerous ; their presence 

 under seaweed is accidental ; while Mytilus is predominantly not 

 a pool-dwelling form in the littoral zone. The principal search 

 must, therefore, be confined to the margin of the tide. 



From these considerations it is evident that the ultimate 

 bearing capacity of a Mytilus station is directly determined by 



* « Zoologist,' 1908, p. 201 ; 1913, p. 41. 



