TERNS AND SKIMMERS 255 



The marsh-terns feed mainly on aquatic insects, such as beetles, 

 flies, dragon-flies, spiders, etc., as well as tadpoles and small fish. 

 They will hawk for insects over the land, like swallows. The sea-terns 

 catch insects, beetles, etc., on the wing, especially in cold windy weather 

 when marine food is difficult to obtain. Forster's tern, a bird of the 

 northern marshes of the New W'orld, arrives on the newly unfrozen 

 grounds in time to feed on the numerous dead fish, insects and amphibia 

 released on the melting of the winter ice. 



The visitor to the sea-ternery in the early summer will immedi- 

 ately note the typical "fish-flight." One bird, which may be the male 

 or the female, flies circling about the colony with a fish in its beak, 

 calling loudly with the characteristic grating tern cry which is not 

 unlike the sound produced by a wooden stick drawn rapidly over 

 serrated metal. The other bird usually flies in front of its partner 

 carrying the fish. The foremost bird flies with its neck stretched out; 

 the bird with the fish has its head bent down. The pair may settle and 

 the fish may be exchanged, the bird receiving the fish immediately 

 flying up and performing the same actions, preceded by its partner 

 leading the way with outstretched neck or one flier may be joined 

 by another and suddenly both will rise steeply into the sky, and 

 presently make a long glide to earth. 



A tern carrying fish almost invariably screams with excitement, 

 and it seems as if the fish in the bill is an essential part of the emotional 

 display used by terns. Tinbergen considers that the male may carry 

 the fish in flight over the ternery when he is seeking a mate. Many 

 variations in the "fish-flight" occur. Sometimes strange birds pursue 

 the bird with the fish, and make ineffectual attempts to snatch it. 

 Other chases occur without fish. There is also a curious gliding flight 

 and a circling high over the colony, with darting downward rushes 

 at great speed. The abnormally slow, lazy, deliberate beating of the 

 wings ("butterfly flight"), recorded of other sea-birds (e.g. the auks), 

 is not uncommon, and appears to have a social as well as a sexual 

 significance: Palmer (1941) considers that it results from an attempt 

 to carry out normal display in the air; it is often concluded by a 

 delaying of the folding of the wings on alighting near another tern. 



The pre-courtship of terns at the nest-site follows the pattern of the 

 gulls and many other birds. The overall sexual bond has attracted 

 the female towards a male in possession of a scrape in the ground; 

 but in order to keep this territory and win a mate he must peck at 

 and assert his dominance over all comers. Palmer considers that the 



