Great pics! They are migratory I believe, coming here for a few months in the rainy season. I see one (solo) every year at the Chong Klam Bon reservoir at Pang Sida National Park. Saw it again 3 weeks ago - but have never seen one close enough to take a photo - always well off in the distance. I understand sightings are not common as well. Hopefully someone can add something as that's the extent of my limited knowledge on this.
That makes sense, I got this from another website:
"Thailand (formerly widespread, now very rare and possibly no longer breeds, although sightings are increasing in frequency due perhaps to increased protection of breeding colonies in Cambodia),"
We are due west of Pang Sida, which is close to Cambodia. So I guess they may be migrating from there? I've seen this one twice, the pond is close to my house so I'll make an effort to get some more pics.
I have seen it in the past in the Lam Ta Kong river in Khao Yai (Wang Jumpi area) but not often. Indeed not too commonly seen.
Though we saw two on our recent trip to Phu Khieo and have read in a few birding trip reports of Phu Khieo that people saw this species.
Will post a trip report soon. I only got long distance shots of it, so nothing as near as the birds in your pictures.