Black-fronted Tern
Chlidonias albostriatus
Black-fronted terns are considered medium-small in size. Adults measure 28–30 cm in length, with a 65–72 cm wingspan and an average weight of 95 g. They have short orange bills, short orange legs, and a body that is dark grey above and light grey below with a white rump and a distinctive black cap. During the non-breeding season, the black cap recedes and becomes flecked with white.
ID text from Wikipedia, CC BY-SA 4.0

Black Tern
Chlidonias niger
Adults are 25 cm (10 in) long, with a wingspan 61 cm (24 in), and weigh 62 g (2+1⁄8 oz). They have short dark legs and a short, weak-looking black bill, measuring 27 mm (1+1⁄16 in), nearly as long as the head. The bill is long, slender, and looks slightly decurved. They have a dark grey back, with a white forewing, black head, neck (occasionally suffused with grey in the adult) and belly, black or blackish-brown cap (which unites in color with the ear coverts, forming an almost complete hood), and a light brownish-grey, 'square' tail. The face is white.
ID text from Wikipedia, CC BY-SA 4.0

Where you might meet both
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Play today's Daily BirdPhotos and recordings are credited to their authors under the licence shown. Full profiles: Black-fronted Tern and Black Tern.