I visited Suan Rot Fai for an hour this evening, and it seems that migrants have indeed started to arrive (after drawing a complete blank on 2nd Sept), with three BROWN SHRIKES heard (only one seen) and two COMMON KINGFISHERS. I was birding around dusk, which gave me a chance to get reasonable views (and blurred photos) of a couple of ASIAN BARRED OWLETS.
Before seeing all these however I came across a BLACK-CRESTED BULBUL which has presumably liberated itself from Chatuchak Market.
Crap photo, but it's the first migrant of the autumn, so I don't care!
Blurred ASBO (gotta love autofocus in low light)
Enjoying its freedom