Silver-backed Needletail

The silver-backed needletail is a swift found in Southeast Asia, Sumatra, Java and Taiwan, as a vagrant to Christmas Island, with natural habitat of subtropical or tropical moist lowland forest (Wikipedia, general species account).

That is all the source says. It does not name Bhutan, and gives no size, plumage, diet or breeding detail, so none is stated. Its place in the Bhutan guide is flagged for checking, and the entry is kept very short rather than borrowing from other needletails.