The Eocheong Thrush: An Unexpected ID Challenge!

Nial Moores, Birds Korea, May 12th 2022

On April 24th 2022, a deepening low pressure system started to move rapidly eastward across southern China into the Yellow Sea. Ahead of this storm, at midday on the 25th, Ms Pack Won-Hee found a stunning-look thrush on Eocheong Island (Gunsan). The thrush was unlike any other on the Korean list, with a strong, pale bill; a hint of a bare patch behind the eye; a blackish head and black breast (both with brownish tones), contrasting with the remainder of the slate-grey upperparts; deep-orange flanks, dappled and smudged with coarse spots; and extensive white on the belly and ventrally. At least three more birders saw and photographed the thrush that day. Reviewing images, the “Eocheong Thrush” was identified as a Black-breasted Thrush Turdus dissimilis, a national first record, and images were shared widely.

The Eocheong Thrush. Images strictly © Pack Won-Hee

The bird was clearly a male; and the brown tones on the head and breast combined with spotting on the flanks indicated immaturity – suggesting this was a male in his Second Calendar-year.  Although not widely thought of as a long range migrant, a quick check of eBird and the online Birds of the World stated that Black-breasted Thrush is a partial migrant and the developing storm was setting up an airflow directly from the range of the species, about 2,100km to the southwest. Moreover, vagrancy to Korea looks to be a real possibility as one was photographed in Liaoning in March this year. In summary: right weather, right age for vagrancy, and a birder’s find of a lifetime!

Windy.com App: Forecast cloud and rain, with ground level isobars as of 16:00hrs on April 24th. At that time, the centre of this deepening low pressure system was situated close to Guangxi Province, bringing low cloud and rain showers, and creating a powerful and mild southwesterly airflow – presumably encouraging some species (including the Eocheong Thrush) to initiate migration across the Yellow Sea that night.

The Eocheong Thrush was a short-stayer, unfortunately, and was seen by only a small number of additional observers on 26th and perhaps also on the 27th.  Questions were increasingly raised about features shown in the images that seemed odd for Black-breasted, including an apparently paler edge to the tail and dark feathering at the base of the legs. Some birders apparently raised the possibility that this might be a rare (unknown?)  colour variant of Grey Thrush T. cardis, or perhaps even an Island Thrush Turdus poliocephalus, because of the underpart spotting.

I was asked my thoughts about the ID.  Personally, I have never knowingly seen Black-breasted Thrush. As one of the developers of the Birds Korea Checklist and as a volunteer eBird Reviewer, however, I felt a mix of responsibility and curiosity to dig a little deeper.

On eBird, it was fairly easy to find images of Black-breasted with dark feathering at the base of the legs (e.g., an adult male in Thailand: ML342510971 ); and some that suggested a similarly pale-edged tail to the Eocheong bird (e.g. ML205148271 and ML292012421 ).  These were therefore not issues of obvious concern.  Moreover, the images on eBird also made clear that Black-breasted Thrush, like several other thrushes, is a species that shows substantial age-related, sex-related and individual variation. The closer you look at images, comparing one individual to another, the more likely you will find differences and anomalies. One bird in the Macaulay Library labelled as an immature male (ML79868231 ) even almost lacks orange in the underparts (please see below for a likely reason why)! I therefore agreed with the ID as Black-breasted, asking openly if not Black-breasted, then how else to identify the Eocheong Thrush?

My lack of personal experience with Black-breasted and the persistent national controversy over the ID of the Eocheong Thrush, meant I still needed to email images of the bird to several of the region’s top field ornithologists to ask their opinions.

The first to answer was BirdLife Asia’s Dr Yong Ding Li, who “tended” toward ID as Black-breasted, but also raised the issue of the underpart spotting perhaps suggesting a hybrid origin; followed a few days later by Nick Upton (of Thaibirding.com), who felt that “there was something not quite right” about the bird, adding “The problem for me are the dark spots on the breast, belly, flanks. I don’t really remember BB Thrush having this although I have found one photo where a bird shows a little of this…Having said all of that. I guess this could be a young male that has retained some of the spotting that is present on 1st year birds and females… The pattern of spotting puts me more in mind of Japanese (Grey) Thrush”.

The third to respond was Prof. Phil Round. Images of the Eocheong Thrush immediately reminded him of a controversial thrush photographed in Guangxi, southern China, which had been identified as a Black-breasted Thrush but which after review by e.g., James Eaton and Paul Leader, was instead “eventually thought to be a T. cardis x T. dissimilis hybrid” as “evidently the ranges of the two meet in Guangxui”.  Images of such birds, taken by Liu Zhiyuan and kindly shared with permission with Birds Korea by Xuky, look remarkably similar to the Eocheong Thrush. Indeed, as suggested by Prof. Round, they mean that a hybrid origin is also the most “plausible fit” for the Eocheong Thrush.

Bird initially identified as Black-breasted Thrush and then re-identified as hybrid Black-breasted x Grey Thrush in Guangxi Province, PR China © Liu Zhiyuan…
And the Eocheong Thrush © Pack Won-Hee. These individuals share multiple plumage features, including the bare patch behind the eye; the darker head and breast than upperparts; the orange breast sides; the dark grey wash along the rear of the flanks; and the heavy flank spotting .
A second presumed hybrid Black-breasted x Grey Thrush, Guangxi, PR China © Liu Zhiyuan

Based on this new information, and in the absence of any contrary opinion, it therefore seems wisest for now (at least!) to identify the Eocheong Thrush as a hybrid Black-breasted x Grey Thrush. No doubt a real disappointment for the finder, but a truly spectacular record all the same, which perhaps raises just as many questions as it provides answers!

Acknowledgements

Sincerest thanks to Ms Pack Won-Hee for her stunning discovery and permission to use her images; to Mr Kim Seog-Min for kindly providing additional information and permissions; to Dr Jungmoon Ha and Dr Sung Soyoung for initially alerting me to the bird and the controversy over the identification; to Liu Zhiyuan for giving permission to post images from PR China, and to Xuky (and Grahame Walbridge) for so kindly sharing these images to post here; and last but not least, to Dr Yong Ding Li, Mr Nick Upton and Prof. Phil Round for their really valuable insights into the identification of the Eocheong Thrush.

2 comments on “The Eocheong Thrush: An Unexpected ID Challenge!

  1. Very interesting article but it still remains 2 kind of questions for me.
    – first the probability of such an hybrid to exist and then to be in Korea in migration. We are speaking here about 2 species with no overlap during the breeding season. The shortest distance between the 2 breeding ranges is more than 2000 km. How can we imagine such an hybrid to exist. An even if it exists, the probability of finding one in migration is quite impossible.

    2nd question, the plumage. It is a 2nd cy bird and at this time of the year both Grey-backed Thrush and Black-brested Thrush are not supossed to be “dappled and smudged with coarse spots”

    That’s why I put forward the hypothesis of a possibility of hybrid between Grey-backed Thrush and Japanese Thrush. The probability of such an hybrid seems for me higher as the breeding ranges are very close and the probability to observe it in migration in Korea logical. And the plumage could fit what we observe on the photo, even the big spots present on the 2nd cy Japanese Thrush. It is only an hypothesis but even if the first feeling when watching the photos is to think of a Black-breasted Thrush, I think we don’t have to remains on this first feeling

  2. Thank you for your comment. This is the same conclusion we reached ( most plausibly Black-breasted x Grey Thrush, based on overlap of breeding range and shared plumage characteristics), as set out in the article.

    There are still at least possible 3 differences between our thinking it seems.

    The first: we prefer to use the name Grey Thrush for T. cardis, for a species that breeds in PR China as well as Japan, esp. as there are other thrush taxa endemic to Japan.

    The second is that male Grey-backed Thrush T. hortulorum (a common summer visitor here) often shows spots on the flanks and breast in spring: presumably these are 2cy birds. In the MaCaulay library, quite a few Black-breasted show some spotting on the flanks and / or breast, from India to Thailand. Are all of these hybrids from what is presumably a narrow hybrid zone? Is it not possible that some spotting can also occur in pure birds – though evidently much less coarse than in the Eocheong bird?

    Third, the record of such a hybrid still seems extraordinary, whatever the parentage. Logically speaking, neither Grey Thrush breeding in Guangxi nor Black breasted Thrush would be expected to migrate NE in spring to reach Korea. However, IF e.g. a Grey-backed Thrush for some reason paired up with a Black- breasted Thrush in late winter in Guangxi, then at least it seems pluasible that hybrid offspring might then migrate NE toward Korea the following spring.

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