The color tone of the primaries seems on the pale side - almost or actually within the range for kumlieni? Also the extent and thickness of the primary fringes seems exceptional for thayeri; isn't the slight "pinch" into the dark at the tip a kumlieni feature?

The uppermost tertial seems to fit kumlieni while the other tertials seem more like a well-marked thayeri. There are published pics of kumlieni (by Bruce Mctavish in Birding World 1996, 1997) with less-barred tertials than this bird.

The scapulars are very interesting: there are only two or three thayeri-like juv scaps (dark-centered), with the rest looking good for juv/B1 kumlieni or B1 thayeri:- I think that most (what %age?) thayeri retain their dark-centered juv upper scaps well into late February.

P1 -P3 (perhaps P4) seem to completely lack a dark subterminal mark - isn't this unexpected in thayeri?

The tail has a thick white tip, and R1 (outermost retrix) is fully barred, R2 mostly barred, R4 more barred than R3, and faint barring at base of R5.