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Rynchops niger
Skimmers have a light graceful flight, with steady beats of their long wings. They feed usually in large flocks, flying low over the water surface with the lower mandible skimming the water (in order of importance) for small fish, insects, crustaceans and molluscs[ caught by touch by day or especially at night. They spend much time loafing gregariously on sandbars in the rivers, coasts and lagoons they frequent.
Migratory R. n. niger breeds on the Atlantic coast of North America, and from southern California to Ecuador in the Pacific.
Your second photo should be put in its own spotting as it is a tern, not a skimmer.