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
Forster's Tern
Sterna forsteri
Forster's tern is a medium-sized tern with a slender body, deeply forked long tail and relatively long legs. In its non-breeding plumage, the crown is white and a black comma-shaped patch covers the eye and the ear-covert. The wings are gray with the primaries being dark silvery gray, while the underside is white. The bill is black and the legs are a dull brownish red. When breeding, an intense black cap extending down the neck appears. The wings and the back are pale gray while the underside is bright white. It has a black-tipped orange bill and bright orange legs. Voice: The common call of the Forster's tern is a descending kerr. The threat call used in defensive attack is a low harsh zaar. A succession of kerrs is used by the female as a begging call during courtship.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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