I asked an experienced Japan bird watcher's opinion.
Here is his comments
"
Your picture demands me to learn more myself on distinguish points between Red-necked and Little.
1) At first, this bird is in it's third calendar year or more, not in second calendar year.
[Shorebirds of the Northern Hemisphere] said First-breeding : Many remain in non-breedign area; most individuals probably do not breed untill their third calendar year. Picture 69d, first breeding: Second calendar year birds remain in non-breedign areas and asquire limited breeding plumage, in this case one or two scapulars,, , ,
2) In this bird, scapular, mantle, black stripes on head, ear-covert and breast are new feather which replaced in this early spring for summer plumage. To the contraly, coverts and tertials are old feathers which is winter plumage, so the edges of each feather are abraded .
3) If this bird is Little Stint, why the coverts and tertials have not replaced to summer plumage by the 20th April? Red-necked Stint do not replace these tracts for summer plumage.
4) wing tip slightly longer than tail, Little Stint has this trait though, sometimes I can see a Red-necked have slightly longer wing than tail aming photos.
5) streaked hind neck and ear-covert are rather Little Stint' trait.
6) throat may change to more red as time pass.
7) this bird is feeding, the body is kept in horizontal, Little Stint tilted the body more.
8) the tibia looks short, the bill does not look so slender like Little Stint.
From these points I think this bird is Red-necked Stint."
