This is an adult female Pied Harrier.
This individual is densely brown streaked on underparts and makes it resemble female Eastern Marsh Harrier, but the grey cast on upper-secondaries/primaries, barred white uppertail coverts and white ground on chest, belly and vent point towards female Pied.
Certain female Pied, in Thailand that I have oberved, have lighter/narrower brown streakings on underparts, making it "whiter ground" more obvious and looking almost white at a distance and I suspect that may indicate an older-aged female Pied.
