Whiskered Tern

The whiskered tern has a black cap, a strong bill and a more positive flight that recall the common or Arctic tern, but its short, forked-looking tail and dark grey breeding plumage above and below are typical of the marsh terns. The bill is 29 to 34mm in males and 25 to 27mm and stubbier in females (Wikipedia, general species account).

The only source read for the Bhutan guide gives this comparison. No range, habitat, diet or breeding information was found, so none is stated, and its occurrence in Bhutan is flagged for checking.