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
Sabine's Gull
Xema sabini
Sabine's gull is a small gull, 27 to 33 cm (10+1⁄2–13 in) in length and weighing 135 to 225 g (4+3⁄4–8 oz). The wings are long, thin and pointed with a span of between 81 and 87 cm (32–34+1⁄2 in). The bill, which is black with a yellow tip, is around 2.5 cm (1 in) long. This species is easy to identify through its striking wing pattern, though at long range it can be confused with immature black-legged kittiwakes. The adult has a pale grey back and wing coverts, four black outer primary flight feathers, and white inner primaries and secondaries. The white tail is slightly forked.
ID text from Wikipedia, CC BY-SA 4.0
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