I paid some interest in all this Phylloscopus since a wild and I'm quite familiar with Claudia's Leaf Warbler that is one of the commonest bird in North West Hunan where I use to have spent quite a lot of time this past year.
To me Claudia's Leaf Warbler is a warbler that looks very uniform, the head marking have very poor contrast and appart of the border of the lower crown dark stripe (I mean the part between coronal stripe and super) the rest of the "dark" stripe are usually of the same color of the mantle. Claudia's Leaf Warbler looks very dull and quite dark and on the underpart with no clear color differencies bewteen ear coverts and breast. Given an impression of a very uniform dull pale bird.
This bird is to me far to contrasting to be a Claudia's Leaf Warbler.
I have less experience on fokiensis, but from the very fiew pictures from breeding ground available on the net, to me this species have much whiter underparts, and a more contrasting head pattern as it seems to be the case with this bird. I don't know with very worn plumage, but in breeding plumage, it should show much more yellow on the super, but this is probably wearing off at this time of the year. To me fokiensis should looks like a godsoni with whitish underparts and traces of yellow on underparts as well.
I've been abble to compare Claudia's Leaf Warbler and Hartert's Leaf Warbler (godsoni ssp) in the field and to me there is some differencies in their call, Claudia's being more explosive a bit tit like and often given trisyllabic call, where godsoni give more flat call, bysillabic... Also Claudia's Leaf Warbler is a very vocale species, where godson seems to be more silent. fokiensis being a subspecies of Hartert's Leaf Warbler (Phylloscopus godsoni), I suspect their vocalisations to be more close of godsoni rather than claudiae.
Again I think that try to recording vocalisations of all this birds and take pictures of them will help a lot to sort out this complex.
fokiensis breed in Fujian, and claudiae is a very common bird in NW Hunan, so I suspect most of this birds are more likely to belong to fokiensis, but there is good probability that Claudia's LW turn up in Hong Kong sometimes.
I published at the below link recording made in breeding ground of Claudia's LW (ID 100% sure with song sonogram) :
http://www.tragopan-asie.com/en/ ... oscopus-claudiae-2/
and a good set of pictures of Claudia's LW made by a friend of me during one of our trip at Hu Ping Shan NW Hunan (ID 100% sure with sonogram as well) :
http://www.tragopan-asie.com/en/ ... lloscopus-claudiae/
There is two species that people rarely mention is Emei Leaf Warbler that breeds in N Guangdong (and probably elsewhere)and Ogilviegranti Leaf Warbler (see my other post) that also breed very near. Emei's LW is more likely to be misID with Claudia's LW because of their dull underparts and very little contrasting head pattern, but vocalisations should be decisive.
So rather than claudiae/fokiensis if my argument above haven't convinced you, I would suggest something like ogilviegranti/claudiae/fokiensis/emeiensis:-)
All of them breeding in a perimeter of 1000 km from HK, some of them much more close than that.
All the best,
Jonathan
