Sakhalin Leaf Warbler Phylloscopus borealoides
One or two heard; one watched in song, April 28th 2000, Daeheuksan Island, Jeollanam Province (though see below for an update).
The Sakhalin Leaf Warbler is listed in the official OBC checklist and in many other recent checklists covering the region as a separate species from the Pale-Legged Leaf Warbler Phylloscopus tenellipes, primarily on the basis of its song. Sakhalin Leaf’s song is described in Brazil (1991) as a simple clear three-noted whistle repeated on and on, "hee-tsoo-kee" or " hi-tsu-ki", (somewhat suggestive of a thin-noted American Sparrow rather than a phylloscopus warbler, pers obs).
The Pale-legged Leaf Warbler’s song is strikingly different, and much more similar to that of Asian Stubtail Urosephena squameiceps, an insect-like "si-si-si-si-si", sometimes given with the rhythm of a Yellowhammer Emberiza citrinella, with greater emphasis on the first part of the song, ending with a slight upward inflection (pers obs).
Differences in plumage and behavior are confused by the lack of certainty surrounding the identification of birds not in song, and the dearth of correspondent literature describing differences, so most Sakhalin Leaf Warblers probably pass through the Korean Peninsula unnoticed.
Although described as an uncommon migrant south of Honshu by Brazil (1991), the Sakhalin Leaf Warbler is in fact a common migrant through northern Kyushu, especially from mid-April to early May (pers obs). The Pale-legged Leaf Warbler is, however, a very scarce species, heard by NM on only two dates in 7 years in Fukuoka Prefecture: 4 on May 23 1993 and 1 in late April 1997 (unpublished data).
The Sakhalin Leaf Warbler nests in Japan and on islands to the north of it, though the exact range has perhaps not been clearly described since separation from Pale-legged Leaf Warbler. There is at least one claimed spring record from coastal Hebei. Due to confusion with Pale-legged Leaf Warbler, its wintering grounds are perhaps not well-known?
After publication of this report, Andy Stoddart and G.M. Cresswell kindly sent us the following:
Mosulpo, Jeju, 4 May 1994
Having seen a good number of 'Pale-legged Leaf/Sakhalin Warblers' in May 1993 and the first few days of May 1994 around Mosulpo and Gapa Do, Jeju we finally caught up with a bird singing.
All the birds in both years had appeared identical (ie with quite a bright green tone to the upperparts) and, given the known occurrence of Sakhalin Warbler in Kyushu and the rarity there of Pale-legged Leaf Warblers, it seemed likely that these birds would prove to be Sakhalins.
This was confirmed when we came across a singing bird in the pine belt at Yakpo, Mosulpo early on 4th May 1994. The bird's plumage was identical to that of all the other individuals seen but the song was very distinctive and is apparently diagnostic of borealoides – a high-pitched clear three-note whistle.