I see - for some reason I thought Jean Francois was referring to xanthodryas by a different name. I'm not sure where the idea of Sakhalin came from, to be honest. The OP bird is clearly one of the Arctic complex - both borealis and examinandus are possible.borealoides is Sakhalin Leaf Warbler...another species forming a pair with yet another species: Pale-legged Leaf Warbler (P. tenellipes)
tough matter these leafers )
Gerben, in terms of status, xanthodryas is a very scarce/rare visitor to Korea (< 10 annually) whereas, borealis is by far the most numerous and examinandus is a regular, but much scarcer, passage migrant (< 100 annually) per BirdsKorea. Since the question was asked earlier, there have been no advancements in field identification of the complex, vocalisations are still key to substantiated claims to species level. JLW is the largest, averaging heavier-billed (deeper and broader-based) and plumage-wise, it's the brightest, with strongest yellow saturation, AW is the smallest and dullest with KLW somewhere in between but, crucially, differences are average so, there is overlap between the 3 species. In the hand, crucial measurements include bill, length of P1 and wing.I don't know if Jean Francois really intended to talk about borealoides. Why though would Japanese Leaf Warbler not be an option too. It may be a rare vagrant in Korea but it is next door and Japanese Leaf Warbler not rare in Japan itself. It is a migrant too; again if Eastern Crowned Warbler is considered a vagrant in Western Europe, why not Japanese Leaf in Korea.
I am not saying it is one, I only suggest that I think a choice cannot be made between these three species (IF it is actually Arctic-warblerish) I would be dishonest if I said that I can decide that. I am too much of a eagerly learning novice in East Asian Phylloscopus matters to take an active stance.
Yes, I upload my records to ebird. I usually try to get a photo and a sound recording to confirm an ID, especially for when it come to leaf warblers! I did manage to get a recording on the BirdNET app earlier in the day which picked up an Arctic Warbler in a different area on the island. I'll put this one down as AW/KLW/JLW as Grahame and Andyb39 mentioned.you're most welcome
@sandwich311 do you upload your records to any of the recording sites?
I generally find that all three I know are very useful (each in their own way): ebird,org inaturalist.org and observado.org (the latter two for all nature records)
I am always interested in seeing records of the East Asian coastal region.