This is in fact a juvenile Greater Coucal. Note that the body is blackish with pale bars - juvenile Lesser is brownish with obvious pale streaks (adult winter Lesser has a similar plumage to the juveniles):
http://www.hkbws.org.hk/BBS/view ... ght=Lesser%2BCoucal
http://www.hkbws.org.hk/BBS/view ... ght=Lesser%2BCoucal
http://www.hkbws.org.hk/BBS/view ... ght=Lesser%2BCoucal
http://www.hkbws.org.hk/BBS/view ... ght=Lesser%2BCoucal
The habitat is also a useful clue - Lesser is very much a bird of dry grassland/shrubland (mostly in the hills) which would be very unusual in mangroves, whereas Greater is one of the more regular species in mangrove habitat.
