Marsh Harriers: where East meets West.
A consideration of claimed Western Marsh Harrier Circus aeruginosus
in South Korea.
Nial Moores/Birds Korea, January 2006
(with significant insights and improvements made by Dr. Igor Fefelov and Jesper Hornskov)

In September 2003, a group of experienced observers (including the author) initially identified a marsh harrier at Seosan, South Korea, as a (presumed) Western Marsh Harrier Circus aeruginosus, a potential national first record. This initiated a chain of correspondence and a review of available literature, which led to a reassessment of this record as unsatisfactory, as neither a hybrid Western x Eastern Marsh, nor even pure atypical Eastern Marsh could be satisfactorily ruled out.

That correspondence, however, also led to the “re-discovery” of an image of a marsh harrier taken by Kim Hyun-tae, also at Seosan, in October 2000. Raptor expert Bill Clark gave an in lit. assessment in 2003 of this individual (based on the one available image) as a juvenile female Western Marsh. As a result, the species was subsequently added to the KWBS bird checklist covering the Korean peninsula. This decision was not, however, followed by Birds Korea (with our checklist covering only the Republic of Korea).

With much recent discussion about the Korean national checklist, and following the sighting of several other “odd harriers,” Birds Korea is again contacting leading field birders and ornithologists within the region to ask for opinions on the image, and to re-evaluate some identification criteria for these two sister taxa. We are now actively appealing for further information, images of both taxa from all parts of their ranges, and support in developing identification criteria that can be applied more rigidly to age and separate: (a) Western (WMH) and Eastern Marsh Harrier C. spilonotus, (b) perhaps Eastern Marsh (EMH) from the Asian mainland and EMH breeding in Japan, and (c) possible hybrids between WMH and EMH. For the present, this much shorter interim note aims only to introduce some basic information on the identification of juvenile and female-type EMH and WMH with direct relevance to the harrier photographed at Seosan in 2000.

Following email discussion with several experienced observers (including Phil Round in Thailand, and at most length with Jesper Hornskov in China and Dr. Igor Fefelov in Irkutsk, Russia), the following are here proposed to help separate juvenile, immature female and adult female WMH from EMH (NOTE: these criteria are in many ways significantly different to those suggested in several leading field-guides used in the region):

  1. EMH and WMH share most plumage features, but these features tend to be shown by different ages of either taxon (i.e. shown earlier by EMH). Correct ageing is therefore very important in assisting correct identification.

  2. Juvenile EMH are generally dark-brown, though invariably they show some pale in their plumage (i.e. they apparently do not have an effectively all-dark plumage like some juvenile WMH).

  3. Pale areas on Juvenile EMH are often found on the head (though they can very rarely be dark-headed: I. Fefelov, in lit. Jan 2006); on the upperwing (on the marginal and other wing coverts); on the breast, with ca 95% of juvenile EMH seen at Beidiahe/Happy Island showing a pale breast band (J. Hornskov in lit., Jan. 2006), sometimes with this breast band extending round on to the neck sides and mantle (I. Fefelov in lit., Jan. 2006); on the scapulars, sometimes (I. Fefelov, in lit. Jan 2006); and on the rump, where this pale rump band often appears barred or mottled to some extent (perhaps only appearing white in birds that have become rather more bleached by the spring of their second calendar year: I. Fefelov, in lit. Jan. 2006).

  4. Although juvenile WMH can be pale-headed, as juveniles they only rarely show pale on the upper marginal coverts (leading edge of the wing) and they seldom show either a pale breast band (perhaps only ca10% of individuals) or other obvious pale patches (features generally developing from the second calendar year on).

  5. Unlike juvenile EMH, juvenile WMH do not show any obvious pale on the rump, having at most pale tips to the rump feathers, so that the presence of an obviously pale rump in a juvenile rules out WMH (and should be considered highly atypical of WMH at any other age).

  6. Juvenile WMH do show rather obvious pale tips to the greater coverts, which can usually be seen fairly easily in flight (assisting in ageing) as much of the rest of the fresh-looking plumage is typically rather dark and smooth-looking.

  7. There are differences in the underwing patterning of many juvenile WMH and juvenile EMH. Many juvenile EMH (though not all) show rather more extensive pale bases to the undersides of the primaries than WMH, with the five inner primaries largely pale, lacking very broad, very dark tips and therefore forming strong contrast with the rather dusky-looking secondaries. WMH, by contrast typically show largely grayish-brown, rather dark primaries, often with a thin whitish crescent across the primary bases. Many EMH, however, show a very much more restricted pale blaze on the underwing, rather similar to the vast majority of WMH.

  8. As EMH age, many show increasing traces of barring, e.g. on the flight feathers, and on the tail. Such barring is very rare in especially female-type WMH, likely shown only by rather older individuals (and apparently also by some immature males, that can show a combination of juvenile plumage with somewhat barred secondaries and tail feathers: I. Fefelov, in lit., January 2006).

  9. As WMH age, they also develop varying areas of pale in the plumage, with the development of a pale breast band typically during the 2 calendar year, and golden-cream marginal coverts, but they still appear generally “monochrome” (I. Fefelov, in lit. Jan. 2006), or two-tone and clean-cut, while EMH often appear rather “blotchy.” Of note, the pale breast band does not extend round to the lower neck sides in WMH, though it can in EMH (I. Fefelov, in lit. Jan 2006).

  10. Older female EMH can even develop a more female Hen Harrier C. cyaneus-type plumage, in being brown, with extensive streaking, and rather prominent barring on the flight feathers and on the tail, with the “brown colour predominating on the belly versus female Hen Harrier's streaked belly” ((I. Fefelov, in lit. Jan. 2006).

  11. Rarely, older adult WMH can show white streaking on the head and nape and some paler area on the rump and upper secondary coverts (I. Fefelov, in lit. Jan. 2006), and even more rarely, some barring on the flight feathers and on the tail.

  12. The most consistent difference between the two taxa appears to lie in the exact details of the head pattern. EMH have apparently not been shown to have a (dark) eye-stripe, reaching from the nape, through the eye, to reach towards the bill base. This feature is, however, typically, but not always, shown by (female-type) WMH. Its presence, again in the absence of any other anomaly, should be enough to confirm identification as WMH. However, some WMH can also show apparently rather paler lores, with the dark eye-stripe not reaching the bill base, and it has been suggested that some old WMH females, especially in West Siberia, can also have a very much reduced dark eye-stripe, so resembling immature EMH. This is considered likely due to regional variation, but could also be due to the influence of e.g. EMH genes (I. Fefelov, in lit. January 2006).

  13. The Japanese population of EMH appear to show less barring than mainland populations. Some individuals seem to show a rather narrower, smaller head than WMH (NM).

  14. In addition to a great range in individual variation, some marsh harriers apparently show a range of characters that appear to suggest a hybrid origin, and mixed pairs have been documented.

In summary

  1. An all dark juvenile marsh harrier, or a juvenile or immature with a paler head, full eye-stripe (reaching from the nape-sides through the eye and across the lores), lacking a pale rump, pale patches on the upperparts away from the marginal coverts, or extensive pale on the under sides of the primaries, barring or streaking, can be safely identified as a WMH.

  2. A juvenile marsh harrier showing a pale breast band (extending round towards the mantle), pale on the rump, extensive pale on the upperwing coverts, an extensive blaze on the underside of the primaries, and/or traces of barring can be safely identified as an EMH.

  3. Correct ageing is key to confident identification of non-juvenile, female types.

  4. The existence of hybrids, and the difficulty of identification of many individual marsh harriers due to extensive individual variation, complex ageing and incomplete knowledge, means that it is not possible to identify all individuals confidently. Moreover, firm identification of an extralimital WMH or EMH needs to be based both on the presence of several positive features, and on the absence of negative anomalies.

The Seosan Harrier, October 2000

"The Seosan Harrier", Photo © Kim Hyun Tae

This single image (presumed the only one taken of this bird) shows a back-on view of a harrier, with its head turned to the side. The plumage is rather dark and “monochrome,” although the bird is obviously pale-headed. It also shows obvious pale tips to the greater coverts, and a fresh-looking plumage which should confirm the age of this individual as a juvenile. Dark extends up the neck sides, strongly reminiscent of WMH, but close examination of the head pattern shows that the dark eye is more or less isolated, with a very thin dark line (if at all) reaching from the neck sides to the eye, and wholly pale lores. This is a pattern considered sometimes shown by EMH, and not known to be shown by juvenile WMH. In addition, although there is no obvious pale on the closed wing or on the rump, there is a paler area that seems to extend (round?) from the upper breast sides, with some paler feathering on the mantle. This again is a very strong pro-EMH feature.

In addition to the author's original opinion that this bird was possibly a hybrid (at the least, that it could not be safely identified as a WMH), three expert comments have so far been received on this image:

  1. Bill Clark, in lit., 2003, “"looks like a straight W. Marsh juvenile female”.

  2. Jesper Hornskov, in lit., January 2006: “It’s certainly a juvenile type. The amount of pale on the head would be somewhat troubling for a Western, but maybe not impossible. It certainly doesn’t seem to have the pale rump which would clinch it as an Eastern! … The relevant question, I think, is - can we be sure a straight E Marsh can’t look like this?”

  3. Igor Fefelov, in lit., January 2006: “I think it is nearly impossible to say definitely ‘this is a hybrid’ or ‘it is not a hybrid’, having only this photo…note there is a trace of pectoral band seen, going to the hindneck and to upper back. This is a feature of young EMH, never seen by me in same-aged WMH.”

When asked by NM: “In your opinion, based on all that you have seen and know, would you consider that this is safely identifiable as a Western Marsh Harrier?” Dr. Fefelov's answer: “No.”


As a result of this correspondence (and unless very strong contrary evidence comes to light), the identity of the Seosan 2000 marsh harrier remains for now undetermined/indeterminable. Considering the lack of other adequate records, Western Marsh Harrier will not be added to the Birds Korea checklist of birds of South Korea. The list therefore remains at 504, with 25 of these species remaining unsupported by images or full documentation by multiple observers.