A couple of days ago I visited a patch of remnant mangroves at Samut Sakorn (the samut Sakorn Mangrove Research Centre) and found a couple of Mangrove Whistlers there. For anyone who wants to see this species and does not have time to travel to the south, where there are extensive mangrove forests, this site probably gives people the best chance of seeing it close to Bangkok (about 1 hour from the city centre).
I wrote about my visit here: Samut Sakorn Mangrove Research Centre
I wrote about my visit here: Samut Sakorn Mangrove Research Centre