Little Tern

The little tern, in breeding plumage, is mostly white, has a yellow bill with a black tip, yellow legs, and a black cap with a white forehead chevron . In non-breeding plumage, the bill turns black, the legs are duller and the forehead is much more white. 

It breeds along the coasts and inland waterways of temperate and tropical Europe and Asia and winters in subtropical and tropical oceans as far south as South Africa and southern Australia. 

Range from Wikipedia. Light green is breeding, blue is non-breeding and green is resident. 

Part of the courtship display involves the male offering a female fish. We saw them in Portugal, in breeding plumage, in both the Tagus and Sado Estuaries. Our best view was along a dirt road where we saw at least three laying on eggs in the middle of a dirt road and one, presumably a female, had another bring it a fish while we watched.