Huge black and white birds (well, off-white) feeding on the apple snails in the rice paddies, or soaring, with necks outstretched. Usually in flocks. The name comes from the gap between the upper and lower mandibles, but you have to be fairly close to see that.
There seems to be a bit of an explosion of these this year. I've heard of them as far north as Uttaradit, which is most unusual, and am beginning to hear of records in Isan and Laos.
If you see them, please post here, or give me a pm, with as exact a location as you can.
They're really big; you're not likely to make a mistake.