I've just gotten a hold of Paul Leader's article and the photos suggest that Yellow-streaked is rather similar to Dusky in structure but closer to Radde's in plumage. I agree with John that vocalizations should be the key feature. In Hong Kong, my experience has been that Dusky Warblers are very vocal while Radde's Warblers are very quiet. I've watched several Radde's Warblers for extended periods with out hearing any call notes. So I wonder how often Yellow-streaked Warblers give call notes? It is a rare occurrence indeed to be able to hear the song of Yellow-streaked.