Field Identification

Korea’s First Marten’s Warbler Phylloscopus omeiensis: Baekryeong Island, May 8th & 9th 2021.

Nial Moores, Birds Korea, May 17th 2021 (with sonogram by Ha Jungmoon added on May 23rd).

Marten’s Warbler Phylloscopus omeiensis, May 9th 2021, Baekryeong Island © Todd Hull

At midday on May 8th 2021, in the northwest of Baekryeong Island during team birding for the global Big Bird Day, Nial Moores (NM) watched a small, bright leaf warbler fly across a track and land in a nearby tree.  There the bird quickly consumed a small caterpillar before silently flying off again into the forest. During front-on views of only 2-3 seconds through binoculars the bird revealed a striking head pattern, with blackish subcoronal / lateral crown stripes and an ashy-grey median crown stripe and a clear eye-ring. The upperparts looked green, with a hint of a wingbar; and the underparts looked leaf-yellow.  No white was seen in the tail as the bird flew off. Even in these very brief views, identification was immediate as a “golden-spectacled warbler”, a generic name given to a group of extremely similar-looking and rather poorly-known species of warblers.  Before this encounter, no “golden-spectacled warbler” had ever been claimed in the Republic of Korea or on the Korean Peninsula. However, the sense of excitement was heavily tempered with frustration. Neither of NM’s team-mates had seen the bird; NM had failed to manage an image; and of perhaps greatest concern the bird was not heard to call. Formerly placed within the genus Seicercus, all of these “golden-spectacled warblers”, like many other Phylloscopus, are easiest to identify by their often diagnostic calls (see e.g., Moores & Borzée 2020). Could this bird ever be identified to species?

NM was soon after joined by fellow Birds Koreans Subhojit Chakladar (SC) and Todd Hull (TH), and approximately two hours were spent together trying but failing to re-find the warbler.  To help in our search, the songs and calls of Alström’s Warbler Phylloscopus soror, perhaps the most likely of the “golden-spectacled warblers” to be found in Korea, were downloaded from Xeno-Canto, and were played several times by SC. However, winds were fairly strong and it was unclear whether we heard any vocalisations in response or not. The bird was not re-sighted, so we left to bird a different part of the island – fortunate as we then found Korea’s first Grey-backed Shrike Lanius tephronotus!

Soon after dawn on May 9th, in much less windy weather, the area where the warbler had been seen was revisited.  The trees were full of bird activity, including lots of Phylloscopus warblers, with several singing Yellow-browed P. inornatus and Pallas’s Leaf Warblers P. proregulus, a singing Radde’s Warbler P. schwarzi, and calling Pale-legged P. tenellipes and Eastern Crowned Warblers P. coronatus, in addition to several Chestnut-flanked White-eyes Zosterops erythopleurus. After waiting for more than two hours, SC eventually (and miraculously!) re-located the “golden-spectacled warbler”, feeding low down in some bushes, loosely in the company of a Radde’s and, even more remarkably, a Yellow-streaked Warbler P. armandii (which we likely sound-recorded the day before).

In the fifteen tense minutes that followed, images were taken by both SC and TH and a recording of the bird’s calls was made by NM on his phone, as the bird moved hidden, low through the vegetation.  These calls were quiet, initially repeated every 1-3 seconds and then three times or more a second, and sounded flat and more or less monosyllabic. To NM’s ears, the call somewhat recalled the “Tchep” note given by Radde’s Warbler, before that species usually breaks into more anxious-sounding multi-syllabic calls.

Phone recording made by Nial Moores, cleaned and cut by Paul Holt

Presented in raw, low-resolution sonograms, the calls of this “golden-spectacled warbler” appear as simple vertical lines or streaks between 2.5khz and 4.5khz, with these notes strongest and longest at about 3.5khz.

The sound and structure of these calls does not permit identification as Alström’s Warbler, which typically has a bisyllabic call, somewhat reminiscent of a thin, high-pitched Kamchatka Leaf Warbler P. examinandus.   Bianchi’s P. valentini and Grey-crowned Warblers P. tephrocephalus also seem to be potential vagrants to Korea based on occasional records in eastern China listed by eBird.  However, again based on recordings on Xeno-Canto and in the Macaulay Library, the sound and structure of their calls are also very different from the bird on Baekryeong Island.  Bianchi’s Warbler typically gives an Eastern Crowned Warbler-like piping “Heup”, showing as a stretched out “T” in sonograms; and Grey-crowned Warbler typically gives a finch-like “hrrip”, showing as an “h” or as an unfinished “w”in sonagrams.

Only the archived calls of one species in this group of “golden-spectacled warblers” match well with the calls of the Baekryeong bird:  Marten’s Warbler Phylloscopus omeiensis.  This is a migratory species, wintering in South-east Asia and breeding in southern-central China, reaching at closest about 1500km west of the Korean Peninsula.  Nonetheless, a recording by Ayuwat Jearwattanakanok (ML282673751 ) in Thailand contains 51 calls given over 36 seconds, with each call’s frequency ranging from approximately 2.5khz to 4.5khz, similar to the bird on Baekryeong; while Frank Lambert’s recording of calls given in between song phrases on Xeno Canto (XC 104844) reach slightly higher in frequency than this (ranging from about 2.5khz up to 5khz), but are given with a similar kind of rhythm. The calls of both sound remarkably similar and the sonograms on Xeno-Canto look remarkably similar to the Baekryeong bird.

Fortunately, high-resolution analysis of NM’s recording by Ha Jungmoon provides even stronger support for identification of this bird as a Marten’s Warbler.

Sonagrams created by Ha Jungmoon. Top Row: (a) the bird on Baekryeong, recorded by NM on May 9th; (b) Marten’s Warbler extracted from Macaulay Library (ML183112) ; and (c) Buff-throated Warbler from Xeno-Canto (XC639210). Bottom row: (d) calls of Grey-crowned Warbler (XC625377) ; (e) Alström’s Warbler (XC569671); and (f) Bianchi’s Warbler (XC61379).

In addition, the three best images of the bird on Baekryeong also support identification as Marten’s Warbler. They show a small and bright warbler, with a prominent pale eye-ring, with a slight break in the eye-ring below the eye.  Much of the upperparts appear leaf-green, though with darker remiges and rectrices.  The bird has well-defined blackish lateral crown stripes and a clean ash-grey median crown stripe, both of which extend more or less unbroken from the nape toward the forehead, petering out short of the bill. In addition, the lores show a crescent of paler yellow; and the grey of the median crown stripe can be seen to bleed diffusely below the lateral crown stripes above the ear coverts.  The closed wings show an extensive lighter blaze, and a very weak wing-bar. The tail is dark with a greenish gloss and prominent white.

Marten’s Warbler Phylloscopus omeiensis, May 9th 2021, Baekryeong Island © Subhojit Chakladar
Marten’s Warbler Phylloscopus omeiensis, May 9th 2021, Baekryeong Island © Todd Hull

In the image of the bird from the rear by TH, this white seems to occupy much of the visible inner web of the outer tail feather, with white also evident on at least two additional pairs of tail feathers.  Alström (2020) notes that Marten’s has “prominent white wedges on inner webs of outermost rectrix (25·5–32 mm) and penultimate rectrix (12·5–26 mm), occasionally also a small white tip on prepenultimate rectrix.” The underparts look yellow, brightest on the throat and weakest on the rear flanks and vent. Of particular note, the bill looks small, with a largely orange lower mandible (though with a dark spot centrally), and a dark culmen; and the legs look clean pink or yellowy-orangey with pink feet, dependent on the angle.

Typical structural and plumage differences between these near-identical-looking warblers can be found in the descriptions by Alström (2020) in the online Birds of the World. For example, Alström’s Warbler looks proportionately large-billed and short-tailed (the opposite of the Baekryeong bird); the “white wedges on inner webs of outermost (14–26 mm) and penultimate (0·5–17·5 mm) rectrices (are) often rather diffuse, and head pattern is less contrasting than in Marten’s. In Grey-crowned, the subcoronal stripes usually almost reach the bill base, the lores are greyish-green (“latter indistinctly yellow-suffused”), and the eye-ring is “usually thinly broken at the rear”.  Images of Bianchi’s Warbler in Birds of the World suggest multiple, likely very subtle or regional (?), differences from the Baekryeong bird, including a longer and more powerful-looking bill, stronger legs and feet; an eye-ring that often seems to flare to the rear (in a similar way to the eye-ring of Dark-sided Flycatcher Muscicapa sibirica); and extensive green admixed into the yellow of the underparts.

In order to test our tentative identification of the bird as a Marten’s Warbler based on the calls, the images and sound recording were shared without specific opinions on identification through email with Paul Holt and Prof. Per Alström, both leading experts of this species’ group.

Both independently identified the bird in question as a Marten’s Warbler, with Per Alstrom (in lit. May 17th) writing, “Congratulations on the first Phylloscopus omeiensis for South Korea!  Its call is very typical, and the plumage structure supports the id (although it’s not possible to eliminate P. valentini just based on the photos).”

As such, Marten’s Warbler will be added to Category One of the Birds Korea checklist in our next update.

Remarkably, this is the third new species of Phylloscopus warbler for the Republic of Korea recorded on Baekryeong Island in a little over 12 months. The nation’s first adequately-documented Chinese Leaf Warbler P. yunnanensis was phone-recorded and very poorly digiscoped on the island in early May 2020 (Moores & Borzée 2020) , followed by the nation’s first Buff-throated Warbler P. subaffinis in late October (Moores & Ha 2020). All three identifications have depended heavily on recordings of calls made through smart-phones.

Occurrence of Marten’s Warbler in the Republic of Korea was not really expected. However, this individual was likely one of several species displaced eastward from Southern-Central China by an especially vigorous low pressure system which moved eastward over southern China into the Yellow Sea on May 6th and 7th, with winds then meeting a similarly vigorous low pressure system crossing northern China on May 8th. As a result, southerly or southwesterly-originating winds on Baekryeong Island gusted to >50km/hour on May 6th and 7th, before winds swung much more westerly at midday on the 8th, before moderating further. 

Air pressure, cloud cover and wind direction at ground level, according to the Windy Website at 13:00hrs on May 8th 2021.

Presumably this shift in wind direction, accompanied by an increase in cloud cover, helped drop birds which were being carried across the Yellow Sea. The Marten’s Warbler was first seen within ten minutes of this change in the weather; and Korea’s first Grey-backed Shrike was found only six hours later.  

References

Alström, P. (2020). Gray-crowned Warbler (Phylloscopus tephrocephalus), version 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.gycwar1.01

Alström, P. (2020). Martens’s Warbler (Phylloscopus omeiensis), version 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.marwar4.01

Alström, P. (2020). Bianchi’s Warbler (Phylloscopus valentini), version 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.biawar1.01

Moores, N. & J-M Ha. 2020. First record of Buff-throated Warbler Phylloscopus subaffinis on the Korean Peninsula. BirdingASIA 34: 121-122.

Moores, N. & Borzée, A. 2020.Chinese Leaf Warbler Phylloscopus yunnanensis in South Korea: confirmation of the first adequately-documented national record through analysis of sound recordings. Forktail 36: 63-67.

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