Subspecies identification of Eastern Yellow Wagtail in Singapore

With the recent sighting of a Citrine Wagtail at Neo Tiew, the common migrant Eastern Yellow Wagtails have received more attention too. Despite some common misconceptions, current evidence suggests almost all wintering birds in Singapore are not identifiable to subspecies. Only taivana can be conclusively identified, and Manchurian/Amur-breeding macronyx is not separable from Arctic-breeding plexa and tschutschensis.

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...