Siberian Chiffchaff (top) and Siberian Stonechat (above) are long-awaited splits. The former’s identification criteria have now been formalised by the Rarities Committee, while the latter may well be split as two species rather than one, owing to the discovery of a discrete eastern clade. Photos by Rebecca Nason (top, www.rebeccanason.com) and Graham Catley (above).
As many birders look to the east as the autumn winds start to blow, and some seek trips to the Arabian peninsula to expand their lists, it seems timely to cover those distinctive Asian and Middle Eastern forms that may yet become acceptable species.
As our knowledge and definitions of what a species actually is develop, it is inevitable that new taxa will emerge from the molecular and morphological mists. However, it must be borne in mind that some taxa we currently regard as distinct may only be so from a western perspective, and some species will be too poorly known in parts of their range to be truly confident of a split.
One such example may be that of Tundra and Taiga Bean Geese. Now often accepted as different species in Britain, the true story may be more complex. If we are to accept this split, then we must also accept Middendorf’s Bean Goose, the distinctively large-billed form found on the taiga of eastern Siberia, north of Lake Baikal. Middendorf’s has certainly been suspected in Britain, and it is conceivable that birds of this form could latch onto a flock of serrirostris, with which it overlaps.
The most recent genetic analysis has shown five taxa in three clades within the bean goose complex, consisting of the long-split Pink-footed Goose, Middendorf’s Bean Goose, and Taiga and Tundra Bean Geese (including the subspecies rossicus). The last two are currently unsplit by the British Ornithologists’ Union (BOU), but generally accepted by most other authorities. However, the less distinct Tundra and Taiga could be re-lumped, as there remains a question mark over the possibility of hybridisation. More research is needed.
A gander at geese
Beans are not the only geese with distinct and geographically separate forms. Brent Goose has been split by the Dutch among others, and British birders will be well acquainted with the dark-bellied Asian and pale-bellied and ‘black’ American forms from their regular British occurrences. However, there is intergradation on the breeding sites, and the situation is also confused by the existence of a ‘grey-bellied’ form in the Canadian Arctic, which has also been suspected in Britain.
Ducks offer a slightly more clear-cut case in that of Stejneger’s Scoter, the Asian form of Velvet Scoter, recently split as White-winged Scoter along with a second American taxon, a decision accepted by the BOU. Stejneger’s has occurred in Finland in summer 1996, while the American form has wandered to Iceland on a few occasions. Clearly both have potential to reach our shores, and certainly any noticeably knob-billed drakes with orange on their upper mandibles should be carefully scrutinised.
Birds of prey also hold a few potential surprises. Black Kite is widely distributed around the Old World, with three forms occurring in the Palearctic. The African Yellow-billed Kite reaches Egypt and Saudi Arabia, and is closest to the ancestral form of both Red and Black Kite. With its obvious yellow beak, deeper tail fork and more rufous underparts, it is very much a potential good species, and the most recent work suggests that it just as differentiated as Black is from Red. The Asian ‘Black-eared Kite’ has occurred in Britain, with a well-watched juvenile bird in Lincolnshire and Norfolk from 16 October 2006 into 2007, and has been considered a probable species after Sibley and Monroe earmarked it in their ground-breaking 1990 studies. However, genetically Black-eared is much more close to European Black Kite, and has a broad intergradation zone in central Eurasia.
The recent appearance of yet another escaped Purple Swamp-hen near Chester earlier this year highlighted the potential tick harvest that might be gathered by well-travelled listers from this species. Patchily distributed throughout the Old World, it has at least three morphologically and geographically disjunct subspecies groups within the Western Palearctic alone. Optimistic Cheshire birders will now be familiar with the blue-grey-headed poliocephalus group from the Middle East and Asia, but the green-backed form found in Egypt and the almost completely royal blue form present in Iberia and North Africa could also provide the tick-hungry with sustenance, having already been split by the Dutch.
The two forms of Lesser Sand Plover found in our biogeographical region, the Central Asian vagrant atrifrons and the East Asian migrant mongolus, have also been widely discussed as separate species groups. In Britain any sand plover is a mega, but either may turn up, with at least one atrifrons-type having been found, at Pagham Harbour, Sussex, from 14 to 16 August 1997. Differences between males are subtle but relatively clear-cut, with a more orangey breast band and more black on the forehead in the atrifrons group, but identification of a non-breeding female or juvenile will prove more difficult.
‘Black-eared Kite’, the Asian form of Black Kite, may yet not be split as there appears to be a wide zone of intergradation in Central Asia, and juveniles of the two forms can be almost indistinguishable. Photo by Aurélien Audevard.
Caspian Gull also has potential to be eventually carved up into two species, barabensis (or ‘Steppe Gull’) possibly deserving recognition. Though closely related to Yellow-legged Gull, Armenian Gull is genetically distinct, despite some hybridisation at one site in Turkey. There is not the space here to go into the finer detail of large white-headed gull identification, but there will be plenty of exciting changes for laridophiles in forthcoming years.
The Asian population of Sand Martin, subspecies diluta, has been split as Pale Martin in the past, and still is in some quarters. Their ranges overlap, but no mixed colonies have yet been found, and diluta is certainly a potential vagrant to the east of our region. Plain Martin has two disjunct and unique populations in Africa and Asia, the former subspecies occurring in Morocco, and the latter having been recorded as a vagrant at least once in Iran, perhaps opening up the possibility of it showing up elsewhere in the Middle East or even closer.
The good run of American Buff-bellied Pipit records in recent British winters has also resuscitated thoughts of the allopatric Asian subspecies turning up. This is one of the commonest pipits in Asia and a great deal of its range overlaps with Olive-backed Pipit, a well-known and frequent wanderer to our shores.
Wagtails are another regularly opened can of birding worms, and the difficulties in identifying the numerous subspecies of Yellow Wagtail and the possibility of splitting has been touched on before (see Birdwatch 215: 35-37). Lesser known, however, is the split of Yellow and Citrine Wagtail into eastern and western groups or clades, both of which have reached Britain before. White Wagtail also has a cross-Palearctic jigsaw of forms, and ‘Amur Wagtail’ has already appeared in Britain, in Co Durham in April 2005. Other potentially good species from this complex to have reached the WP include the Asian ‘Masked Wagtail’ in Norway in November 2003.
Rufous Scrub-robin has been a favourite among birders for a division between the greyer syriaca from the eastern Mediterranean and the brighter galactotes from the western side. Despite clear gaps in its range, the species is too genetically consistent to consider splitting just yet. The resemblance of ‘Eastern Nightingale’, the Central Asian version of our celebrated songster, to Rufous Scrub-robin has been remarked upon, and the cold greyish upperparts contrasting with the rich mahogany tone of the tail, along with the relatively prominent supercilium and wing-bars, would seem to mark it out as an ‘obvious’ separate species. However, birds in the Caucasus appear to be intermediate, so clarification is needed.
‘Middendorf’s Bean Goose’ is both morphologically and genetically more separate than Taiga and Tundra Bean Goose are to each other, making it very likely to be a ‘good’ species. Photo by Ian Merrill.
A long-awaited tick is Siberian Stonechat. Many birders will have this on their list pending a BOU split, and it has always been a fixture on Birdwatch’s Bird of Britain: the Complete Checklist, though it remains a subspecies in Collins. However, Siberian shows a clear division between an eastern and western clade, and the former is probably best split as ‘Eastern Stonechat’. Variegatus birds from the area to the west of the Caspian Sea show a wheatear-like tail pattern and will almost certainly end up split as ‘Caspian Stonechat’.
Wheatears are a hotbed of incipient speciation, with Mourning Wheatear being a particularly complex conglomerate of forms. In Collins, Mourning Wheatear’s patchwork distribution among the arid regions of East Africa and the Middle East is presented as having Maghreb and Mourning Wheatear as representatives of this superspecies. However, recently published work shows that at least one other form, endemic to the Iranian Plateau, is even more differentiated, and is perhaps best treated as a separate species, ‘Persian Mourning Wheatear’. The same study also recommended re-lumping Maghreb and Mourning. Finsch’s Wheatear is closely related to the Mourning complex, and it is possible that the Turkish and Central Asian subspecies may be separable or part of the latter superspecies.
Warblers are the stuff of nightmares for many birders. Some currently accepted species are in fact still hotly debated, particularly those of the Greenish Warbler complex, which is often regarded as an incomplete ‘ring species’. Green Warbler is now accepted as a species by most authorities; many committees in Europe also accept Two-barred Greenish Warbler as a separate species, but it’s not split in Collins. The widely distributed ‘single-barred’ form, an annual scarce vagrant to Western Europe, ranges from the Baltic across northern Asia, reaching down and intergrading with other forms around the plateau until it becomes Two-barred Greenish in eastern Siberia. To Western eyes, the two forms differ by plumage, call and song, but in global terms it is an artefact of geography that we only get the two most differentiated subspecies as vagrants, and in fact, while they behave as different species in their overlap zone to the north of the plateau, the two forms could be better viewed as just the separate ends of a continuum.
Most are becoming comfortable with the idea of Siberian Chiffchaff being a discrete entity. The species has a distinctive call and song, and a full suite of diagnostic plumage features has been published online by the Rarities Committee to enable the true picture of the form’s British status to be assessed. The intermediate form ‘fulvescens’ from the west of its range muddies the waters somewhat, but individuals should still be identifiable in the field.
The Asian subspecies of Buff-bellied Pipit, japonicus, occurs regularly in Israel as a visitor, and resembles Water Pipit as much as its American sister form. Photo by Aurélien Audevard.
Seed-eaters hold some surprises too. Mountain birds are often as isolated as island birds, and the absence of gene flow may mean that Snowfinches from Asia Minor, the Caucasus and Iran and further into Asia may all be incipient species. Twite populations found in the same areas might be at a similar stage. Crimson-winged Finch has well-delineated sister populations in North Africa and the Middle East, appearing to be as divergent as, say, Seebohm’s and Northern Wheatears, and is another long-awaited split.
Lastly, buntings have considerable geographical variation, underlined by the recent separation of House and Striolated Buntings. Rock Bunting is closely related to these, and the Caucasus provides us with a taxon, prageri, to consider for separation. Lastly, a division between the yellow-bellied southern Turkish and white-bellied western Turkish subspecies of Cinereous Bunting has been long considered, and further western European vagrancy may well force more research on these forms too.
This series of splits articles has been intended as a guide to the currently ‘hot’ forms that might or do occur in Britain and in our biogeographical region, as well as hopefully informed speculation on what might yet happen, taxonomically. However, the true message is to enjoy nature’s variations, whether tickable or not, because just as in human culture, it’s the local differences that make the whole so engrossing.
For a full list of references, please click here.