I had a quick run up to Khao Yai to do some more testing/proofing/messing with my macro video setup - which was bit of a waste because I saw nothing small except a flower mantis that did not mind being filmed but did not move, not even a mm. But I was befriended by a Male Orange-headed Thrush (Geokichla citrina) and then later in the day I nearly stepped on a Savanna Nightjar (Caprimulgus affinis) - I think that is what it is? I am not very good on birds.
They are just grab shots with the d800 in crop mode and the 80-400, which continues to delight me its all round usability and its speed and sharpness. I could not resist.
Cool to see the nightjar during the day. They are very hard to spot in daytime. Easy with a spotlight at night, especially this time of year they seem to be abundant.
I can't help you with ID. My guess is that either you are right, or perhaps it's a Grey Nightjar.
Other common nightjars in the park are the Great Eared Nightjar, and Large-tailed Nightjar.
The Grey Nightjar is said to be much greyer than the others. Not sure if it is the sunlight, but the bird in your picture does appear quite grey to me. In flight it might have been a little easier to see some distinctive markings.
And they are easier to distinquish by the calls, but well that won't help you either .
BTW, don't know if it has ever been posted on this webpage, but there is a great webpage where birders share sound recordings of birds, free of charge.
Usually even the location is stated, and often I find recordings from Khao Yai, Kaeng Krachan, or other famous Thai parks. For those who'd like to get more into birding, it's a great way to get to know the bird calls, it has helped me a lot.
Talking about birds.
Just a few hundred metres past the Northern Khao Yai entrance... I suppose it escaped from somebody's private collection...
My new discovery in Khao Yai, haha:
And here's another cute bird later that day; a male Banded Kingfisher allowing me to get at almost 2 metres to take a shot: