254 SEA-BIRDS 



Watson (1908), Tinbergen (1931), G. & A. Marples (1934), Palmer 

 (1941), and others. The Marples observed that "a tendency for the 

 birds to fly in pairs while on migration rather points to a permanent 

 union;" and these indications of permanent matings are recorded by 

 other observers, including Palmer. Terns may however often be seen 

 travelling singly, and the migrating flock does not appear to consist 

 of paired birds; it may be that these records of two, four, six, etc, 

 terns in flight are only coincidental signs of normal sociability, out 

 of the breeding season. 



The fish and crustacean food of terns is secured either by diving, 

 or by snatching without actually wetting the plumage as the bird 

 skims over the water. Terns can swim, but do not do so verv much — 

 or very well; and if confined in a glass-sided water tank without a 

 resting place will drown. When hunting for food the tern quarters 

 the surface, its head down and beak vertical, like the fishing gannet. 

 In diving it drops like a stone, wings closing as it descends; but it 

 makes much less splash than the gannet, being smaller, and also 

 usually diving from a lesser height. Terns are skilful fishers, pausing 

 in mid-air and hovering until sure of a successful strike. Some species 

 submerge partly or completely (royal. Sandwich, common, roseate, 

 arctic, Caspian, and little terns); other species (black, whiskered, 

 white-winged black, gull-billed, sooty, bridled and noddy terns, 

 and black skimmer) do not dive, or dive but rarely. Noddy terns catch 

 minnows as these jump clear of the surface of the sea. Skimmers, feed- 

 ing at night, are even more specialised; they plough the surface with 

 the long lower mandible, and by so doing they attract small fish to 

 the moving phosphorescent water. The skimmer then turns back and, 

 flying over the disturbed area, snatches and scoops up the shrimps 

 and small fish which are attracted to the glistening track. Some 

 terns, including the skimmer, occasionally wade in shallow water in 

 search of food. 



The captured fish or shrimps, etc., may be swallowed immediately, 

 as the diving tern rises, and, with a shivering motion of the wings, 

 shakes the drops of water from its plumage ; or larger fish may be held 

 in the bill for the lengthy display ceremony described below, or may 

 be carried to mate or chicks. In the majority of cases the fish is carried 

 hanging down on the left side of the bill, although there appears to 

 be no ready explanation of this custom. Fish-carrying is sometimes 

 disadvantageous, as when skuas are about, which force terns to drop 

 their catches. 



