OYSTERCATCHER AND NATURAL ENVIRONMENT. 459 



the quantity of Mussels available in the water margin during 

 successive phases of the tide, and not by the total number of 

 Mussels on the station ; and that, other things being equal, the 

 longer, that is, the more sinuous the contour lines, and the more 

 shallow the gradients of the station — the greater the number 

 of Oystercatchers a given area will support. These requirements 

 are fulfilled better by the mud-flat than by the rock-beach 

 habitat. In the former, the presence of banks and the extreme 

 shallowness of the gradients cause a comparatively large area 

 of the Mussel-scalp to be kept in a moist condition at all states 

 of the tide within the Mytilus zone. As a result, the birds have 

 not only a long contour line along which they can extend, but 

 also, owing to the great breadth of shallow water, they can work 

 over a greater area, and in several rows, or even in masses. In 

 general, contrary conditions prevail on the rock-beach. The 

 greater steepness of the gradients and the shortness of the 

 contour lines narrow and reduce the length of the zone available 

 at a given time, so that it is unusual to observe the birds in 

 more than single file in the tide-line, after drying of the exposed 

 Mussels. Other circumstances favour the mud-flat habitat. The 

 Mussels run to larger sizes than on the rock-beach, and provide 

 a greater proportion of those sizes most usually taken by the 

 Oystercatcher. The exposure to the effect of storms is negligible 

 on the mud-flat. Wave-action here does not interrupt the 

 operations of the birds. The rock-beach has mostly a great, or 

 severe, exposure to storms, and in bad weather, which in winter 

 may last for days, wave-action interferes with search in the tide- 

 line, and in some places may render whole feeding-grounds 

 entirely inaccessible. Exposure of the rock-beach to severe 

 wave-action is a necessity for the existence of Mytilus, for 

 wherever the rocks occupy sheltered positions, the growth of 

 weed is excessive and Mussels are absent. The mud-flat habitat 

 is not drawn upon by Diving Ducks, which devour large quan- 

 tities of Mussels on the rock-beach habitat ; there being here a 

 different time- distribution in the community, the Ducks operating 

 towards high water and the Oystercatcher towards low water 

 on the same feeding-grounds. Owing to the great area exposed 

 on the ebb and the uniformity of the surface, the mud-flat is 

 more favourable to the Oystercatcher in regard to human in* 



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