Common Tern
Sterna hirundo
The nominate subspecies of the common tern is 31–35 cm (12–14 in) long, including a 6–9 cm (2.4–3.5 in) fork in the tail, with a 77–98 cm (30–39 in) wingspan. It weighs 110–141 g (3.9–5.0 oz). Breeding adults have pale grey upperparts, very pale grey underparts, a black cap, orange-red legs, and a narrow pointed bill that can be mostly red with a black tip, or all black, depending on the subspecies. The common tern's upper wings are pale grey, but as the summer wears on, the dark feather shafts of the outer flight feathers become exposed, and a grey wedge appears on the wings. Voice: Common terns have a wide repertoire of calls, which have a lower pitch than the equivalent calls of Arctic terns. The most distinctive sound is the alarm KEE-yah, stressed on the first syllable, in contrast to the second-syllable stress of the Arctic tern.
ID text from Wikipedia, CC BY-SA 4.0
River Tern
Sterna aurantia
It is a medium-large tern, 38–46 cm long with an 80–85 cm wingspan, distinctly larger than any other species in Sterna and more comparable in size to a Sandwich tern (Thalasseus sandvicensis) or a lesser crested tern (T. bengalensis). It has dark grey upperparts, white or very pale grey underparts, a forked tail with long flexible streamers, and long pointed wings with pale grey primaries. The bill is stout for a tern, bright yellow in summer and duller yellow with a dusky tip in winter, and the legs bright red.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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