 The great crested tern feeds mostly at sea by plunge diving to a depth of up to 1 m ( 3 @.@ 3 ft ) , or by dipping from the surface , and food is usually swallowed in mid @-@ air . Birds may forage up to 10 km ( 6 @.@ 2 mi ) from land in the breeding season . Prey size ranges from 7 – 138 mm ( 0 @.@ 28 – 5 @.@ 43 in ) in length and up to 30 g ( 1 @.@ 1 oz ) in weight . Shoaling pelagic fish such as anchovy and sardine are typical prey , but bottom @-@ living species are taken as discards from commercial fishing . This tern actively follows trawlers , including at night , and during the fishing season trawl discards can constitute 70 % of its diet . Prawn fishing is particularly productive in providing extra food , since prawns usually represent only 10 – 20 % of the catch , the remaining being bycatch , mainly fish such as cardinalfish and gobies . 
