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While this is certainly a Childonias tern, and I have no reason to doubt that the identification isn't correct, I don't think that it can be identified with certainty from a photograph of this quality.
The apparently white upperwings and tail are something that can appear to be shown by Black Terns in some photos (when the pale upperwings and tail are emphasized by lighting conditions).
(I think that you do have a W-w B Tern here though)
Lat/Lng: 53.647, 22.541