Really quite unsure on the ID of this thing (seen in Suan Rot Fai this morning). Options are Shikra (resident, common), Chinese & Japanese Sparrowhawk (both passage migrants). Besra occurs in Thailand, but is unlikley in Bangkok.
Phil Round's "Birds of the Bangkok Area" states that Shikra is often outnumbered by the migrant secies during passge (now).
My feelingis that it looks good for Japanese - comments welcome!
EDIT: Phil Round commented - he reckons it's a Shikra!