Regarding the comment by Dr. Nial Moores in the shanghaibirding.com post Kamchatka Leaf Warbler in Shanghai: I would agree that both Arctic Warbler Phylloscopus borealis and Kamchatka Leaf Warbler P. examinandus may come through late, alongside each other, even in mid- to late May.
Here in Thailand, I think our earliest Kamchatka (diagnosed on wing length, greater than 70 mm) was 14 April, caught on Man Nai Island (12.015924, 102.283475) at the same time (so far) as our only undoubted Japanese Leaf Warbler P. xanthodryas.
While on average Kamchatka may be slightly larger-billed, and the respective sexes larger and longer-winged than Arctics, there is not much in it. Anybody who expects to be able to call birds one or the other in the field on anything other than voice is stretching credibility, I feel. Plumage overlap seems total. And of course on body size and wing length there is overlap between male Arctic and female Kamchatka.
The featured image above shows two individuals, with two photos each. Both were freshly moulted birds caught on northwards spring passage at Laem Phak Bia (13.050000, 100.083333), Phetchaburi, Thailand. Both were picked out as being slightly more brightly yellowish on underparts and on supercilium than typical spring borealis (caught on the same day), and brighter green above.
The bird in panels 1 and 2 was caught 14 May 2011. Measurements were wing 66 mm, bill 14.5 mm. On mt DNA (COI) we determined that it was examinandus. The identically bright-coloured individual in panels 3 and 4 was caught on the same day. Measurements were wing 62 mm, bill 14.5 mm. It was borealis on DNA.
The paper of which this comparison was a part is titled “Addition of Kamchatka Leaf Warbler Phylloscopus examinandus and Sakhalin Leaf Warbler P. borealoides to Thailand’s Avifauna.” It was published by the Japan Bird Banding Journal and is downloadable (708 KB) through shanghaibirding.com.
We record Kamchatka annually in Thailand, with most records in May. On average it is a later migrant than Arctic with (old specimen) records from Thailand and Malaysia even into the last days of May. Based on birds handled (and DNA assayed), I would re-emphasise that there is total overlap on the range of plumage characters with borealis.
Though we selected our first, unusually bright-plumaged “Arctic Warblers” in mid-May for assay specifically because they were a bit more yellow-green than usual, note that, just as in the panels above, some turned out to be examinandus and some seemingly identically bright birds were borealis.
Takema Saitoh at the Yamashina Institute has a little module that uses a canonical discriminant function that will separate most birds on biometrics. But of course, this is only good for birds that are handled. The key parameters are wing length, total head length (head plus bill), length of outermost primary, and length of tarsus.
As Craig Brelsford notes in his 6 June post, Kamchatka on average are a bit longer-billed. But even this did not reliably separate all birds unless one knew the sex (problems of overlap between female Kamchatka and male Arctic).
Call and song—so important!
Editor’s note: shanghaibirding.com has a growing library of resources on leaf warblers. In addition to the paper Phil Round directs us to above, we have the excellent presentation by Per Alström, “Identification of Phylloscopus & Seicercus warblers in China,” downloadable here (13 MB).
For even more of our posts in which Phylloscopus is mentioned, type “leaf warbler” in the search box nearby.