White-winged Black Tern is thought to be barely annual on the Tonle Sap, but it seems that non-breeding birds might over-summer in some quantities, at least they are doing so in 2013. Most of the birds were in first-summer plumage, but about 20% were adults. The adults were in a complete mixture of plumages, from full summer to full winter with everything in-between.
For added value there were also two adult tibetana Common Terns and one adult Caspian Tern.
Here's a couple of adult White-winged Black Terns