Another New Identification Challenge: Warbling and Swinhoe’s White-eyes

Nial Moores, Birds Korea, August 2019

Late last year, the IOC revised their taxonomy of white-eyes, which included recognizing the simplex subspecies of “Japanese White-eye” and several other taxa as Swinhoe’s White-eye; and reorganizing “Japanese White-eye”, which they now list as Warbling White-eye.

This decision has now been followed by eBird in their eBird/Clements 2019 Taxonomy Update (August 7th).  Introducing this revision, their entertaining recipe-style update states that:

“Asian Zosterops have undergone a comprehensive, extensive, and confusing (make that, confusing as hell!) reorganization.  Much of this is based on several studies that include an extensive genetic phylogeny for the taxa in south and east Asia…”

Birds Korea follows the IOC in our checklist, so we will revise these names in our next update. Several Birds Koreans, including two of us who compiled the 2018 Checklist, are also voluntary eBird reviewers for the ROK and the DPRK.   We were therefore contacted by eBird Project Co-leader Marshall Iliff, who asked about multiple claims of simplex in the ROK, adding that there does not seem to be much information online on how to separate Swinhoe’s from Warbling White-eye in the field.

We have therefore put together this note, based entirely on field observations and images and sound recordings of all three white-eye taxa made in the ROK (all copyright of Nial Moores / Birds Korea): Chestnut-flanked White-eye Zosterops erythropleurus (monotypic); nominate subspecies of Warbling White-eye Zosterops japonicus japonicus; and nominate subspecies of Swinhoe’s White-eye Zosterops simplex simplex.

Of course, many of the comments below will not apply to different subspecies of Warbling and Swinhoe’s White-eyes which do not occur in Korea.

  1. Chestnut-flanked White-eye Zosterops erythropleurus.

Status on the Korean Peninsula

Chestnut-flanked White-eye is a common, sometimes abundant,  migrant especially through the northwest islands of the ROK in spring and autumn, especially in May and again from mid-September to mid-October, with flocks also regular during migration in suitable mixed temperate or coniferous forest in the NW, including in Seoul. The species is much scarcer southward, and seems to be genuinely rare in the southeast. There are no mid-winter or mid-summer records in the ROK known to us.

Chestnut-flanked White-eye also appears to be very common in western DPRK during migration, with breeding presumed in the northeast (e.g. in Rason).  


Chestnut-flanked White-eye has the dullest, grey-green tones to the upperparts, and a rather stronger facial pattern than Warbling. There is more black on the lores, with black also often running below the lower edge of the white eye-ring and sometimes slightly above it too. This species is also the only one of the three that usually shows an extensive dark or light plum-wash on the flanks (invariably so in Spring) contrasting obviously with the rest of the underparts.  Most of the rest of the underparts look white or grey-white, apart from the throat and the vent which are bright green-yellow. The throat especially looks neat and very well-defined (lacking the more diffuse edge shown by Warbling or the “bleed” towards the upper breast shown by Swinhoe’s).

An adult Chestnut-flanked White-eye in May on Baekryeong Island . Note the greyish wash to the upperparts; the deep flank coloration; the black lores; and the tree species.
An adult-type in October. Note the clean look to the underparts and the confined coloration on the throat. The bill base often seems to show some pink-grey tones.
Chestnut-flanked White-eye, Baekryeong, October. Note the clean lower edge to the bright yellow-green throat, and the black edging to part of the top of the eye-ring.

Birds during southward migration (presumably all First Calendar-years) can lack much flank wash. These individuals usually otherwise resemble adults, though sometimes seem to have more extensive pinkish tones to the lower mandible.

The typical “jing” call of Chestnut-flanked White-eye is somewhat cleaner, less slurred-sounding, more pingy  and admixed with higher notes to my ears than equivalent calls given by Warbling. These calls are given repeatedly by migrant flocks, as in the recording below made on Socheong Island in September. The song, perhaps only heard in spring in Korea, seems more confident sounding, with repeated mimicry (including of species like Northern Hawk-cuckoo), and is often given communally by flocks.


Large flocks of migrating white-eyes seen or heard flying rapidly over any Yellow Sea island in the northwest (e.g. on Baekryeong south to Eocheong), are most likely this species.  Chestnut-flanked White-eye also occurs, but much more scarcely, south to e.g. Gageo and Marado off Jeju, where Warbling is very numerous.

2. Warbling White-eye Zosterops japonicus.

Status on the Korean Peninsula

Warbling White-eye is common to abundant in the southern warm temperate zone and subtropical(-type) areas of the ROK, including in lowland forest throughout Jeju Island. The species occurs regularly with decreasing abundance northeast to Pohang and even the coastal belt of Gangwon (where quite uncommon), north to Seoul (where increasingly reported).  In the west of the ROK, the species also breeds commonly on offshore Yellow Sea islands north at least to Eocheong and Weiyeon. 

Much of the Korean population appears to be either resident or partially migratory, with the species preferring broad-leaved evergreen forest, but also tolerating mixed woodland and more open habitats, as long as there are orchards or stands of ornamental berry-bearing plants. Some birds also appear to be summer visitors to mid-level elevation cold temperate forest in North Chuncheong and Gangwon Provinces (e.g. up to c.750masl in Odae National Park).

There are no confirmed records for the DPRK, though several – presumed to be this taxon – were heard in northern Gangwon Province in November 2017.


While still an attractive species, Warbling White-eye is rather less “sharp-looking” than either Chestnut-flanked or Swinhoe’s. The bill is rather long and heavy (with a paler greyish-blue base), and the irides often look quite pale brown. The face pattern is the weakest of the three species, with variable amounts of (black or) blackish-green on the lores A few individuals perhaps show some yellow-green on the forehead. In such individuals, the yellow is not as clean or as striking as in Swinhoe’s, however.  Because the upperparts are brighter green than Chestnut-flanked, and the throat has a rather diffuse edge, the yellow tones on the throat usually do not stand out strongly. The underparts are variably washed through with a warm brown or grey-brown, which is slightly stronger on the flanks or the breast sides, sometimes with yellow weakly running down the centre of the belly. Very young birds (in Jun-Aug) often look brighter yellow-green above and are very pale below.

Two Warbling White-eyes: one in Changwon in February (above) and one in Busan in December (below). Note the heavy and long bill, the poorly defined dark head markings and the fairly uniform brown look to the underparts.
Warbling White-eye in Busan in December. Again, note the bill length, and the diffuse look to the lores and throat, which at some angles can look almost concolorous with the side of the head.

“Ping” calls are the most slurred of the three species; and the rather variable song – which can be heard even on mild days in mid-winter – is rather more jangling and distracted-sounding than that of Chestnut-flanked. The song below, with a few “ping” notes given at the end, was recorded on Gageo Island in March.


Any white-eye seen in winter or mid-summer in the ROK; and any white-eye seen or heard in broad-leaved evergreen forest is most likely to be this species.

3. Swinhoe’s White-eye Zosterops simplex.

Status on the Korean Peninsula

This is the scarcest of the three species in Korea, with perhaps the first adequately-documented record in the ROK one digiscoped on November 8th 2004 on Socheong Island. Up to the time of writing, there had been only a handful of subsequent records, involving singles and small groups of up to ~8 individuals on Yellow Sea islands (with records at least on Socheong, Gageo and Hong islands), mostly in late April and again in October and November.  Although it seems likely that Swinhoe’s White-eye are regular in the ROK in small numbers and are often overlooked, any record of this newly-recognised species submitted to eBird (please!) requires an image, sound-recording or suitably full description to be properly accepted. (Update: based on observations between 2019 and 2022, the Swinhoe’s White-eye is now known to be a regularly occurring migrant through offshore islands, especially in the northwest, with flocks of 30+ observed on Baekryeong Island).


Swinhoe’s White-eye is rather smaller and much “sharper-looking” than Warbling, with a rather less heavy bill. The upperparts are a quite bright, yellowish-green (especially on the head and rump) and the underparts rather clean-looking, and largely white, grey-white or very pale brown, sometimes with a yellow line running down the belly. This species lacks the flank markings of Chestnut-flanked, but has an even stronger head pattern, with usually quite thick black on the lores and around the lower edge of the white eye-ring and a well-defined yellow band on the forehead which is more or less concolorous with the throat.  The yellow on the throat is bright and extensive. The greeny-grey diffuse tones at the sides and lower edge of the throat shown by Warbling are replaced by yellow-green, which extends further down the breast than Chestnut-flanked.

Swinhoe’s White-eye, Socheong Island, November 8th 2004.
Swinhoe’s White-eye, Baekryeong Island, October 19th 2019.
Swinhoe’s White-eye, late April, Socheong. This individual looked really small in the field.

The “jing” calls given by this species sound higher-pitched, softer and less slurred to my ears than the similar call given by Warbling. The recording below is of a group of eight fast-moving birds together on Gageo Island in late April (with a crooning Black Woodpigeon clearly audible in the background!). These birds were first picked-up on call, and after a 15-minute wait, were seen well in open forest/ forest-edge, allowing for their identification to be confirmed on the plumage features described above.

We welcome your comments and to learn if there are additional features to look for.

Again, please note that the copyright of all images and recordings used in this online ID note belongs to Nial Moores / Birds Korea. Thank you.

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