SWAMPS AND SHORES 481 



marine creatures and provides a constant daily food supply for birds, 

 which come in hosts to avail themselves of the opportunity. The supply 

 is especially abundant when the wind is on shore and numbers of 

 swimming creatures are stranded. The birds then follow the water's 

 edge, and do not even avoid the waves. Yellow-legs, oyster-catchers, 

 curlews, godwits, sanderlings, knots, and sandpipers all feed on the 

 tide flats and at the water's edge, each in its own way. Various gulls 

 join them, and ducks work over the residual pools. Driven out by the 

 return of the sea, they fly inland to meadows and pastures, and so 

 alternate from seashore to dry land in accordance with the tides. 15 

 In seas and gulfs, as in the Baltic or Adriatic, where the tides are 

 slight, such wading shore birds are only scantily represented, and 

 they are likewise absent from precipitous coasts. Sand and mud shores 

 afford them the richest food supply; gravel offers them little. Shore 

 birds are largely world-wide in distribution so that the aspect of 

 sandy coasts in this respect is a uniform one in widely separated 

 localities. 



The gregariousness of shore birds is literally forced upon them 

 by the limitation of space in their linear environment. It becomes 

 most pronounced in their breeding places in which the concentration 

 of life reaches a degree scarcely equaled elsewhere in such limited 

 areas. These breeding sites differ according to the species of birds 

 grouped together and according to the varying environment, presenting 

 distinctive aspects in the arctic, the tropics, and the antarctic. 



The wealth of marine life in the arctic seas supports an extreme 

 number and variety of wading birds, and still more swimming forms. 

 Relatively few of these live exclusively on fishes; the majority are 

 plankton feeders, notably the murres (Uria) which feed chiefly on 

 Mysis at certain seasons, the terns {Sterna), which feed on small 

 amphipods. Such feeding habits necessitate diving, since the spring 

 melting of ice and snow covers the surface with the less dense fresh 

 water, so that stenohaline plankton animals are absent at the surface. 

 A superfluity of food is afforded in these seas only in spring and sum- 

 mer, and the scarcity of food in winter has driven many arctic marine 

 birds to omnivorous habits. The gulls even devour the dung of the 

 polar bear, and scavenge after the foxes. The fulmar feeds on every- 

 thing digestible offered by either land or sea — fish, plankton, carrion, 

 even plants. 16 Some, such as Larus hyperboreus, become robbers and 

 nest plunderers. 



The longest known and most intensively studied of sea-bird rook- 

 eries are those of the Arctic. At the breeding season the birds are not 

 uniformly distributed along the coast, since they select breeding sites 



