I had hesitated to comment, one reason being that my personal experience with Red-billed Leiothrix was in Japan. BUT, I see that the site of the photo was in Japan...
I discovered these wintering and then breeding in the spring near my home, which was in a wooded university campus -- essentially a nature reserve. What a surprise it was the year a very large flock of them first showed up (2003, winter; they had not been present for all of the 12 years I had been living there). They have become quite naturalized in Japan, clearly originally escapees, perhaps released long ago by illegal importers. Anyway, while this individual's exact plumage was not familiar to me, I think it not unlikely that it is a very juvenile one, I just never got to see any quite that young (just the time of year of school break when we often traveled), before the bill changed. It doesn't quite match the description in Robson, not having any olive tinge to the underparts, nor white, and that swoop of gold coming around the auriculars seems too extensive onto the neck ... but ....
I was always amazed watching them, at the great variation in plumage and shades of coloring. But their song is heavenly.
Looking forward to more comments.....
Marie