This doesnt look like an Acro sp but I’ve been caught before! A few necessary features for positive id are not visible but there is still enough I think.. Setting aside Blyth’s for the moment, it doesn’t look to be one of the larger hippos having a relatively short pointed bill and appears to be quite small and compact. The square tail points to an Iduna sp usually but the long UTCs are worrying for Idunas and strongly favors Blyth’s. The bill is almost phylloscopus-like and the supercilium is well pronounced in front of the eye (slightly less distinct behind). The Iduna options would seem to be Sykes as you suggest, or even Booted. Both the bill and the tail looks too short for Sykes and better for Booted (although it seems to lack the dark smudge on the lower mandible typical of the latter sp but I don’t know how reliable that is). The rich buff colouring on the belly certainly looks fine for a fresh Booted and also Blyth’s cf to the paler cooler tones of a Sykes.
However, I am far from confident (as usual!) and so I would hang out for more help - the likelyhood on location favors Blyth’s over Booted (although perhaps just on the northern limit for wintering Booted?) as does the apparently long utcs.
I'm thinking these two can be difficult at the best of times, unless of course one can clearly see ''some'' of the salient features, emarginations certainly help for Blyth's. My first Blyth's seen coming down from a tree (front on) reminded me of Common Whitethroat, until it profiled itself showing short wings, with pp to tertials being half the latter and with longer UTC's.
Never seen Syke's, but very short pp (1/3) of tertial length and long bill could get my pulse racing.
How did it come across to you in the field - Can you describe behaviour - how it moved, any tail movements, call? - was it high in the canopy, or feeding low and then came up higher briefly? What was the vegetation/habitat? Anything might help.
This feels more like an Iduna to me, as the forehead isn't peaked enough, the eye looks black and beady and the tail looks too square-sided. The tail is abraded, so its apparent length could be misleading, and this might also explain the extent of the utcs. However, I don't think this bird can be conclusively identified as it is.
That’s a good point Andy and every time I came back to this earlier, the utcs/tail length kept being a stumbling block for me despite everything else ‘feeling’ like a Sykes/Booted, the latter perhaps better on plumage tones.
I now agree with yourself and Ken and admit defeat in the absence of necessary visual references!