JAPANESE BUSH WARBLER
November 5, 2004: Nial Moores
Are birders still seeing/hearing good concentrations of bush warblers; and if so, what (sub-) species are they? At this time of year there should still be decent numbers of borealis or Chosen-uguisu ("Korean Bush Warbler") on the move. On Ainoshima, Fukuoka Pref., they used to be outnumbered ten to one by cantans (the more typical "Japanese Bush Warbler"), with peak counts there of 10 in a day. Borealis is obviously larger than cantans, much cleaner looking, with a warm rufous cap, buffy tones below, and very strong pink legs. They lack the grey and olive tones of Japanese. Importantly they do not give the single "chak" of Japanese Bush, instead giving a harsh "Trrrt" (a little reminiscent of Red-throated Flycatcher), and have subtly different songs. In South Korea, borealis are a summer visitor, with most apparently leaving breeding areas in September. There is then a second wave of birds in late autumn (Mid-October into mid-November). In Kyushu, borealis peaked in early November. I wonder if these are birds coming down from the Russian Maritimes or Sakhalin, as at least one tape of singing bush warbler made in Sakhalin sounded much closer to me to borealis than cantans.
November 8, 2004: Yoshiki Watabe
In Japan, some birders have recently become interested in Chosen-uguisu ("Korean Bush Warbler"), but I think that there is little information on it. I stayed at Hegura-jima in Ishikawa Prefecture on Sept 22th-25th and Nov 1st-6th, and recorded Japanese Bush Warbler (cantans or sakhalinensis), but did not confirm borealis at all. I saw borealis in May, 1993 at Hegura-jima. But on this island, I think it is rare. On the mainland of Honshu I have never seen it (but I have possibly overlooked it). It is said that some Chosen-uguisu (C. c. borealis) winter in Yaeyama-shoto (Iriomote-jima, Yonaguni-jima).
Uyama (2002) said its song had been heard every day from March 14th to April 11th in 2002; this survey ran from March 13th to May 1st, 2002. Uyama (2003) said its song had been heard daily from March 13th to April 12th in 2003 and on April 27th; this survey ran from March 10th to May 16st, 2002. And Uyama (2004) said it had been recorded from Sept 26th to Nov 26th (max: 6 birds); this survey ran from Sept 12th to Oct 14st and from Oct 26th to Nov 26, 2002.
I guess that very few borealis migrate in Honshu. Some Sakhalin birds migrate through Honshu. For example, Pallas's Leaf Warbler, Dusky Warbler and Radde's Warbler are uncommon in Japan; they migrate almost directly to the continent. Choosen Uguisu maybe migrate to Korea via Tushima and Kyushu. Or perhaps they migrate from Yaeyama-shoto (wintering area) directly to the Asian continent. But I think more research is needed.
November 8, 2004: Nial Moores
On bush warblers, (at once such a familiar and also little known species complex) I hope the following might be of interest. Cantans is resident in the far south of Korea (in the coastal temperate zone, where species such as Japanese White-eye and a form of Large-billed Crow apparently identical to that in Kyushu are also found), with small numbers of migrants also occurring along the south coast, and individuals in summer (and much more rarely in winter) reaching as far north as Gunsan. Borealis, on the other hand, is a widespread summer visitor (April-September). In some areas in spring both can be heard singing side by side, with perhaps both nesting on Eocheong Island (near Gunsan). I have not studied their distribution inland. Typically, earliest borealis arrive in the first week of April here, and are very vocal into early May. In the autumn, as written before, there are two waves of migrants on western offshore islands, with local breeders apparently departing in September and the second wave in late October-early/mid-November.
As for Japan, although most of my notes from my time there were lost, in Fukuoka both cantans and borealis were regular migrants (peak day counts on Ainoshima, Shingu-machi, Fukuoka Ken, in spring of about 50 cantans and 2 or 3 borealis; in autumn of 100 cantans and 10 borealis ), with cantans also resident and breeding. Borealis could be heard in Fukuoka in early April in song especially, and again calling in autumn (late October-early November). Birds were heard several times in Ohori Park in downtown Fukuoka for example, and also on the mainland in Nagasaki-ken. When visiting Ishikawa-ken in 1997, I met a Japanese bander who said he often caught "very large" bush warblers in autumn, some 30% larger than "typical". Allowing for the obvious sexual dimorphism shown by Cettia bush warblers (males being rather larger than females), it is possible I suppose that this was simply the very extreme difference shown by cantans males and females. However, based on my own experience in Japan and here in Korea, and on the bander's very great experience, matched with the fact that he was only catching these birds during autumn migration, that seems rather unlikely, and perhaps more likely borealis were involved.
Based on differences in plumage, measurements, song, call, behaviour, distribution and migration strategies, it seems rather obvious (to me at least!) that the two are a good split. For a very brief review of features:
Korean Bush Warbler is larger, better-proportioned, with a heavier bill and more sturdy legs. It has a rufous cap, warm brown upperparts, a paler throat and buff-washed underparts. For an excellent image of a fairly typical individual, please refer to the 550 series photo guide. Its call is a harsh "prrrt" or "trrrt" (it never to my knowledge gives that distinctive "chak" of cantans ), and its song sounds like a poor version of Japanese Bush, with a fore-shortened opening whistle, and much greater emphasis on the second, louder, more warbled and garbled part. There seem also to be several other consistent differences (based on 100s or perhaps now 1000s of encounters) eg in choice of song post; in posture during singing; in habitat preference etc.
Further to this, I twice saw Manchurian Bush Warbler Cettia canturians in Japan (also on Ainoshima, both in April). It is larger still (looking like a rufous-washed version of an Oriental Reed Warbler, with a greyer nape); gives a softer "trrt, trrt" call than borealis, and has a very distinctive song: a kind of spiralling piping "tul-tul-tul-tul-tul-tu" (or variation thereof). In South Korea, I have only seen Manchurian Bush a very few times, invariably in the far southwest in spring, hinting at a wintering area in southern China? This species (already recognised by many authorities as a good split) perhaps even also winters very occasionally in the Ryukyus?
November 9, 2004: Yoshiki Watabe
Old Japanese publications, for example, Yamashina (1941, A Natural History of Japanese Birds) treated borealis as a subspecies of Japanese Bush Warbler (Horeites cantans), but stated that it also could be treated as a distinct species (H. canturians). He described it as follows: borealis migrates the longest distance for Japanese Bush Warblers, and its breeding distribution is the Ussuri region, Jilin, and Northern and Middle Korea. Its wintering area is the coast of Eastern China and Taiwan. Borealis appear in Korea from mid-April and all disappear by mid-November. Many borealis appear in Taiwan in mid-November and they pass the winter in this region.
But Wild Birds of Japan 590 (Maki & Ohnishi 2000) treats borealis as a subspecies of Manchurian Bush Warbler (Cettia canturianus ). Birds of Japan 550 (Iozawa et al. 2000) treats it as a subspecies of Japanese Bush Warbler. But both publications published borealis photos, so many Japanese birders became preoccupied with borealis, I think. The former states that a few borealis winter in Iriomote-jima and Yonaguni-jima (these islands belong to Yaeyama-shoto in southwestern Japan) and rare records exist in Rishiri-to, Tobi-shima, Hegura-jima (these islands are in the Japan Sea). The latter states that borealis has been recorded in Rishiri-to, Hegura-jima, Tobi-shima and Yonaguni-jima.
It is strange that the Check-list of Japanese Birds 6th revised edition (OSJ 2000) does not include borealis among the Japanese birds. (Editor's note: borealis is included as a ssp. of diphone in the 7th edition, 2012)
Recently, borealis records seems to have increased only in Yaeyama-shoto (Yonaguni-jima, Iriomote-jima etc.). But if its ID points become known by many birders, this taxon's record will increase in other locations. By the way, the Japanese name for borealis is Chosen-uguisu (Chosen = Korea and Uguisu = Japanese Bush Warbler). Yes, Chosen-uguisu means 'Korean Bush Warbler'.
November 9, 2004: Nial Moores
At present the OBC checklist surprisingly calls Cettia diphone "Oriental Bush Warbler" (perhaps not really fitting taxa that very largely nest outside the true Oriental region). Their list does not go to the level of subspecies, and as it does not include either borealis or canturians as separate species, it can be assumed that all three are considered part of Oriental Bush. If, however, all of these Cettia (or Horeites!) warblers are to receive 'good' English names, and if appropriate, alternative historical names do not yet exist for them, then perhaps "Japanese" and "Korean Bush Warblers" might well be acceptable to most birders for cantans and borealis respectively? For birders in Korea, the name "Korean Bush Warbler" would likely be very warmly welcomed: consider how many species are prefixed with Chinese, such as Grey Shrike, Egret and Grosbeak, etc, and most especially by Japanese, from white-eyes to wood pigeons, cranes to thrushes, bush warblers to skylarks, and as yet no Korean anything! If country names are to be avoided in bird names, in light of political sensibilities as well as to better reflect true distributions, a decent alternative for borealis might instead be Boreal or Northern Bush Warbler, while canturians could perhaps become Canturian Bush Warbler or instead Large or even Great Bush Warbler (both names still not yet taken it seems).
For further complication, I would also like to check with Kantorians: is it true that the type specimen of Cettia diphone is actually one of the extraordinary long-billed island taxon (from Tokyo Bay?) that is not even found on the mainland of Japan? If so, Japanese Bush Warbler (or whatever it might be called) would also perhaps need to lose "diphone" and receive again a new "simple" ('Kantan') scientific name as well: Cettia cantans.
November 10, 2004: Yoshiki Watabe
The taxonomy of Japanese Bush Warbler (Cettia diphone) is very confused in Japan at the moment. It is said that 4 subspecies (not including borealis) live in Japan. They are the races sakhalinensis, cantans, diphone, and riukiuensis. Another subspecies, restricta (Japanese name is Daito-uguisu) once lived on Minami-Daito Island, but it has been considered extinct since 1920 (OSJ 2000).
Korea was under the Japanese Mandate before the end of the Second World War; therefore the race borealis was included in the list of Japanese species (Hachisuka et al. 1932). But after the war, borealis was no longer included among Japanese birds.
Sakhalinensis is paler and grayer than cantans (Yamashina 1941). As mentioned below, it migrates to Okinawajima, so it may be seen on Honshu and Kyushu. I looked for cantans and sakhalinensis on Hegura-jima this November, and I saw both a brownish one and grayish one, but I do not know whether this difference was caused by race or age or light conditions.
On the other hand, race ijimae (on Izu-shichi-to and Tanega-shima), race panafidinicus (on Tori-shima) and race iwootoensis (in Kita-Iwo-to and Naka-Iwo-to) were once recognized as subspecies of Japanese Bush Warbler, but they are no longer recognized by OSJ(2000). If you want a summary of their taxonomy and distribution, you may read the Check-list of Japanese Birds 6th revised edition (OSJ 2000) . It is written in English and Japanese. However, its description is partially wrong (see below).
Recent research on Okinawa Island has shown two distinct plumage forms ( Kajita et al. 2002). One form is characterized by having deep rusty upper parts, while the other has grayish olive upper parts. The former (the brown form) is resident on Okinawa, and measurements and plumage characteristics comform with race restricta, which means that the extinct race resricta has been rediscovered. The latter (the gray form) was identified as race riukiensis. It is a wintering bird, not a resident, on Okinawa. Perhaps race riukiensis and race sakhalinensis are synonymous. In that case, sakhalinensis will be an invalid name, because riukiensis has priority.
As mentioned above, it was said that restricta was once distributed on Minami-daito-jima. Last year, breeding of Japanese Bush Warbler was discovered on the island again, but it was not any known race, including restricta. Its taxonomic status is unknown, but its plumage colour and mt-DNA was similar to cantans, and measurements conform with the population onNakano-shima ( in Tokara-retto, southwest Kyushu) . The subspecific name of the population of Nakano-shima is unknown.