Ask The Experts: The Arctic Warbler Splits

Christian Perez tackles another difficult new split in the WBCP 2013 Checklist. In this article he gathers advice from experts on how to tell apart Arctic Warbler, Kamchatka Leaf Warbler and Japanese Leaf Warbler.
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The Arctic Warbler Splits
by Christian Perez

New Splits in the WBCP 2013 Checklist

Two new species appeared in the WBCP Checklist of Birds of the Philippines 2013: the Kamchatka Leaf Warbler Phylloscopus examinandus and the Japanese Leaf Warbler P. xanthodryas, both split from the Arctic Warbler P. borealis that was described in the Kennedy guide. They are part of the family of Leaf Warblers and allies. In this article I will try to give practical advice on the identification on the three warblers. This is not meant to be a scientific paper, but a compilation of information I have gathered from available sources to give practical advice to WBCP members for the identification of the species.

The split was proposed in 2011 in the following paper: “Alström, P., Saitoh, T., Williams, D., Nishiumi, I., Shigeta, Y., Ueda, K., Irestedt, M., Björklund, M., and Olson, U. (2011). The Arctic Warbler Phylloscopus borealis– three anciently separated cryptic species revealed. Ibis 153(2): 395–410”, an abstract of which can be seen here (the PDF takes a while to download, be patient):

http://onlinelibrary.wiley.com/doi/10.1111/j.1474-919X.2011.01116.x/abstract

http://www.slu.se/Global/externwebben/centrumbildningar-projekt/artdatabanken/Dokument/Personal/Per%20Alstr%C3%B6m/Arctic%20WarblersNY.pdf

Range

The map in the figure above shows the breeding range of the three species. According to the IOC World Bird List (http://www.worldbirdnames.org/ioc-lists/master-list/), the Arctic Warbler breeds in Eurasia and Alaska, with subspecies borealis in northern Europe to northern Siberia and northeast China and subspecies kennicotti in Alaska. The Kamchatka Leaf Warbler breeds in Kamchatka, Sakhalin, Hokkaido and the Kurile Islands. The Japanese Leaf Warbler breeds in Japan (except Hokaido). According to Kennedy the Arctic Warbler (meaning the three species together) winters in southeast China, Taiwan, southeast Asia, Sumatra, Java, Lesser Sundas, Sulawesi, Moluccas, Borneo and the Philippines. Kennedy adds that “each race could occur on every island”.

Physical Features

I quote from the paper abstract referenced above: “The differences in song are consistent and with practice clearly audible, especially the difference between Phylloscopus borealis and the two others. Also the calls are noticeably and consistently different. In contrast, plumage and structural differences are very slight. P. xanthodryas is on average more yellow below, brighter green above and has a longer outermost primary than the others, which are even more similar to each other.”

I would add a recent communication from Per Alstrom (one of the co-author of the paper) to Rob Hutchinson: “Sorry, but the only safe way to distinguish between these three “Arctic Warblers” is by call (and song, in case they would sing on migration/in their winter quarters). P. xanthodryas is on average more yellow below than the others, but that’s of little use.”

And a communication from Phil Round to Rob: “Japanese Leaf Warbler is large and strongly yellow-tinged especially on supercilium and throat. So far as I can see Kamchatka and Arctic overlap totally in plumage, and male Arctic will overlap with female Kamchatka on wing-length. So far as I know, these two can chiefly be identified on call.”

It is clear therefore that the species cannot be safely separated on physical features alone, even if the Japanese is on average more yellow than the others.

Vocalization

That leaves us with calls and songs as the only safe method of identification. Fortunately, the three species are usually noisy and have different calls and songs. While in the Philippines the birds mostly call, they might start singing late in the season in April or May. WBCP members should become familiar with the vocalizations, download them to their smartphones and compare the sounds heard in the field with the records.

Calls and songs of the three species can be downloaded from:

http://www.slu.se/sv/centrumbildningar-och-projekt/artdatabanken/kontakt1/personal-a-o/per-alstrom/per-alstroms-forskning/arctic-warblers-vocalizations/

Here are some examples from the site (calls of the three species followed by songs of the three species):

Please click on the video to listen to the recording:

Phylloscopus borealis call

Phylloscopus examinandus call

Phylloscopus xanthodryas call

Phylloscopus borealis song

Phylloscopus examinandus song

Phylloscopus xanthodryas song

Conclusions

Unless there is positive identification of the species based on vocalization, WBCP members should report “Warbler sp.” in their trip reports. Trip notes should indicate briefly how the species were identified (i.e. recording made in the field, calls compared in the field to pre-recording, or identification by an experienced birder).

There is anecdotal evidence that the Japanese Leaf Warbler is a passage migrant around October (south bound) and April (north bound) and that the Arctic and Kamchatka Leaf Warblers winter in the Philippines. However this is not supported by any published scientific study of the migration patterns of the three species in the country. That study remains to be done. As an example of anecdotal evidence, in a visit to Candaba in April 2013, Rob sighted 16 Japanese Leaf Warbler and no other warbler species. It is important that WBCP members make the effort to identify the warblers during their trips and report the sightings to the Records Committee to build up a database of observations in the Philippines.