By Raghav Narayanswamy

With the recent sighting of a Citrine Wagtail Motacilla citreola at Neo Tiew, the common migrant Eastern Yellow Wagtails M. tschutschensis have received more attention too. This article aims to address the subspecific identification of Eastern Yellow Wagtail based on a review of the latest evidence.

Our main reference for this article is Alexander Hellquist’s 2021 paper in Dutch Birding (Hellquist, 2021), which also covers more details on the vocalizations of different taxa – for this article we focus only on plumage traits.

Taxonomy and distribution

The taxonomy of this species is very confused and may require review as more evidence emerges in the future. There are four subspecies of Eastern Yellow Wagtails according to the IOC 14.1 taxonomy, and their statuses in Singapore are as follows.

Subspecies (IOC 14.1)	Range (adapted from IOC 14.1)	Local status
Motacilla tschutschensis tschutschensis	Southern Siberia, northern Mongolia, eastern Kazakhstan and northwestern China to northeastern Siberia and Alaska	Likely the most common taxon in Singapore, but status is complicated by uncertain geographical boundary with plexa.
M. t. plexa	North-central Siberia; exact boundary with tschutschensis is contested	Status unclear owing to confusion with tschutschensis and macronyx.
M. t. macronyx	South-central Siberia, northeastern Mongolia and northeastern China	Status unclear owing to confusion with tschutschensis and plexa.
M. t. taivana	Southeastern Siberia to Sakhalin (eastern Russia), and far north Hokkaido	Scarce to rare in Singapore.

Table 1 The subspecies of Eastern Yellow Wagtail likely to occur in Singapore, based on the IOC taxonomy. 

This taxonomy is shared by Clements (2023) although BirdLife (2024) recognizes two additional taxa angarensis (southern Siberian population of tschutschensis) and simillima (from Kamchatka and Alaska, here synonymized with tschutschensis). The status of plexa also remains an open question, with Alström & Mild (2003) synonymizing it with thunbergi, which is now placed in Western Yellow Wagtail M. flava, and more recent authors, such as Hellquist (2021), suggesting it is poorly differentiated from nominate Eastern tschutschensis.

Identification

Instead of considering the subspecies one by one, we address the different types of birds that are observed in Singapore: birds with dark heads or faint white supercilium, birds with a prominent white supercilium, and lastly birds with a yellow supercilium. Conventional wisdom among our birdwatching community is to rely heavily on the supercilium to differentiate the various taxa, although as we discuss here, this is usually unsafe. Most wintering birds in Singapore are best left unidentifiable at the subspecies level.

Dark-headed birds and birds with a faint or clear white supercilium

Birds with dark heads – no supercilium – or with a faint white supercilium, sometimes more prominent behind the eye than in front, or vice versa, have been broadly accepted as pertaining to subspecies macronyx in Singapore, or the “Manchurian” subspecies group on eBird. 

This identification is not supported by the evidence. As indicated by Alström & Mild (2003) and later Hellquist (2021), birds with entirely dark heads occur across much of northern Siberia in both Western (e.g. thunbergi) and Eastern Yellow Wagtails, well outside the geographical range of macronyx. Subspecies plexa of Eastern Yellow Wagtail, which is believed to winter in southeast Asia, is highly variable; birds ranging from completely dark heads (similar to macronyx) to others with prominent white supercilia (close to “classic” tschutschensis) are all found in its geographic range. Even birds found in the core breeding range of subspecies tschutschensis, which usually has a prominent white supercilium, can occasionally show a dark head as found by Hellquist (2021) and corroborated by photos available online (such as these examples from Alaska: 1, 2). 

Identification based on other characters (upperpart colour, underpart colour, hind claw length, wing formula) is complicated by differences between plumage types and extensive overlap across all taxa, including macronyx. Subspecies macronyx is less variable in its head pattern, and it never shows more than a trace of a white supercilium, but the variation of the Arctic breeding birds (plexa, tschutschensis) means that the subspecific identification of birds on plumage traits is not viable. 

Width/colour of supercilium	Subspecies
Supercilium absent, dark head	tschutschensis, plexa, or macronyx
Faint white supercilium, sometimes more pronounced behind or in front of eye	tschutschensis, plexa, or macronyx
Prominent white supercilium	tschutschensis, plexa, or first-winter taivana (see below)
Prominent yellow supercilium	taivana (see below)

Table 2 Recommended treatment of birds based on the different supercilium “types” which are represented in wintering adult Eastern Yellow Wagtails in Singapore.

As shown, birds in the first three groups are not identifiable to the subspecies level. Individuals with a prominent white supercilium, though, can be assigned to the “Eastern” group in the latest (2023) eBird taxonomy, which comprises subspecies tschutschensis and plexa. Note that first-winter taivana, especially females, may lack the yellow supercilium, instead showing a more buffish-toned supercilium. Many first-winter males undergo a partial moult in the winter when they acquire some adult characters, allowing them to be identified (Alström & Mild, 2003).

Birds with a yellow supercilium

Birds with a rich yellow supercilium are safely attributable to subspecies taivana, as no similar taxa occur in Singapore. These birds also have a greener mantle and crown, and often noticeably darker ear-coverts. Intergrades between taivana and tschutschensis occur where the two subspecies meet, for example in southern Kamchatka. These birds have rather reduced yellow on the supercilium, browner mantle and crown, and lighter ear-coverts; however, some pure tschutschensis can show slight yellow tones in the supercilium as well (Hellquist, 2021). First-winter taivana often has whiter, more buffish supercilium, and is thus probably not safely separable from tschutschensis and plexa, although many first-winter males have a rather rich yellow supercilium and strikingly dark ear-coverts. 

Discussion

Subspecific identification of Eastern Yellow Wagtails with a dark head, faint white supercilium, or prominent white supercilium based on plumage is not possible based on current evidence, while birds with a rich yellow supercilium can be identified as taivana. The taxonomic status and geographical distribution of subspecies plexa, which almost certainly winters in southeast Asia, including Singapore, is contested. 

Due to extensive variability in Arctic-breeding birds, we suggest that observers report adult birds with dark heads or faint white supercilia as tschutschensis/plexa/macronyx, and birds with prominent white supercilia to tschutschensis/plexa; further identification is not supported by reliable field characters at present. 

Acknowledgements

I am grateful to the Records Committee team, as well as Frank Rheindt, for discussions and comments on this article. Jared HJ Tan and Francis Yap contributed photos.

References

Alström, P. & Mild, K. (2003). Pipits and wagtails of Europe, Asia and North America. Christopher Helm.

Clements, J. F., Rasmussen, P. C., Schulenberg, T. S., Iliff, M. J., Fredericks, T. A., Gerbracht, J. A., Lepage, D., Spencer, A., Billerman, S. M., Sullivan, B. L., & Wood, C. L. (2023). The eBird/Clements checklist of Birds of the World: v2023. Link 

HBW and BirdLife International (2024). Handbook of the Birds of the World and BirdLife International digital checklist of the birds of the world. Version 8.1. Link

Hellquist, A. (2021). Identification and taxonomy of northern and eastern yellow wagtails – new pieces to the puzzle. Dutch Birding, 36, 333–370. Link