Key featured species
- Whimbrel Numenius phaeopus phaeopus
- Hudsonian Whimbrel Numenius phaeopus hudsonicus
The appearance of a ‘Hudsonian’ Whimbrel at Goldcliff, Gwent, on 6 May 2000 sent me beetling across the Severn Bridge to see what was, amazingly, only the third British record of this American form of our Whimbrel. The previous two were both from Shetland: on Fair Isle from 27-31 May 1955 and the Out Skerries from 27 July-8 August 1974.
I enjoyed good views of the Goldcliff bird feeding with 43 Whimbrels, and although I had seen hudsonicus on various trips to North America, I now had the opportunity to make a direct comparison between the two races. Up to that point, I had regarded Hudsonian Whimbrel as “just a Whimbrel with a dark rump,” but I was surprised to discover just how different it was. In a nutshell, the American bird looked slightly smaller, plainer, paler and buffer than its companions, and it showed a neater and more contrasting head pattern.
Key differences
The head pattern, with its very strong contrast, was perhaps the most obvious difference: the dark stripes were darker and the pale stripes paler and buffer. In fact, the whole ground colour of the head was paler than that of Whimbrel, with both the thick crown stripe and the supercilium being conspicuously creamy-white, and there was no obvious streaking. At times, the fore-supercilium in front of the eye appeared almost as a whitish spot. To highlight the contrast, the lateral crown stripes, and the eye-stripe in particular, were very dark brown, appearing black at any distance.The much neater appearance of the Hudsonian’s head pattern differed from that of Eurasian Whimbrel in much the same way that the head patterns of Aquatic and Sedge Warblers differ.
The body of the Hudsonian Whimbrel was also paler and less brown, the underparts in particular appearing more creamy-buff. In addition, they were very neatly patterned, the markings on the breast being less coarse while the upper flanks were finely barred, and both areas were less prominently marked than on the accompanying Eurasian Whimbrel. The overall effect was that, from a distance, the breast and flanks appeared more uniform.
In flight, the Hudsonian was even more distinctive.The rump was completely brown, as was the tail, with no white tip. The other major difference was the underwing, which was quite obviously cinnamon-brown and appeared completely uniform in flight. Bosanquet (2000) noted that at closer range it was strongly barred. Unlike Whimbrel, whose underwing is white in ground colour, there was little contrast with the darker under-primary tips. Bosanquet also noted that there was no contrast between the lesser and median upperwing coverts, unlike the accompanying Whimbrel where the former were contrastingly darker.
I saw what was presumably the same bird again at the same site two years later, on 4 May 2002, as it flew past with 15 Whimbrel. Despite the relatively brief view, it stood out from the others, being slightly smaller with completely dark brown underwings and dark brown upperparts. Interestingly, there is a trend among North American waders to have dark rumps and/or dark underwings: American and Pacific Golden Plovers, Long-billed Curlew, Bristle-thighed Curlew, Wilson’s Snipe, Marbled and Hudsonian Godwits and Solitary Sandpiper all differ from their Eurasian counterparts in one or both of these characters. There must surely be a common reason for this.
A separate species
I was so struck by the bird’s distinctiveness that I began to wonder why it hadn’t been split as a species in its own right. I remember saying at the time that it was as different from nominate Whimbrel as American Golden Plover is from European. So why haven’t the two forms been split?
To examine this question, we need to look at Palearctic and Nearctic ‘species pairs’. There is in fact a long list of pairs linking the Old and the New Worlds: Eurasian and American Wigeon, Pochard and Canvasback, Black-throated and Pacific Divers, Little and Snowy Egrets, Grey and Great Blue Herons, Pacific and American Golden Plovers, Black-tailed and Hudsonian Godwits and Common and Spotted Sandpipers, to name a few. Recently, the American Ornithologists’ Union split Wilson’s Snipe from Common Snipe, treatment that has been followed in Birdwatch's Birds of Britain checklist (Mitchell and Vinicombe 2006). In contrast, Whimbrel and Hudsonian Whimbrel remain lumped, as do a number of other Palearctic/Nearctic species pairs such as Hen and Northern Harriers, Herring and American Herring Gulls and Black and American Black Terns.
Whimbrel evolution
So how did these species pairs evolve? Firstly, they are geographically isolated by the Atlantic and Pacific Oceans, but the other major influence at work is undoubtedly the long succession of ice ages. Strictly speaking, we are currently still in an ice age which began 40 million years ago. When the ice advances, it is known as a ‘glacial period’, but when it retreats – as at present – it is referred to as an ‘interglacial’.
Imagine the ancestral Whimbrel breeding right across the tundras of Eurasia and North America. It may at first have appeared fairly homogeneous, but the populations would at some point have been gradually pushed southwards by the spreading ice. The Palearctic population would have retreated into mid-latitude Eurasia, the Nearctic population into mid-latitude North America. If the last glacial period is anything to go by, the two forms would have been separated for about 60,000 years. But there has of course been a whole series of glacial periods. During the past 800,000 years, a glacial period has occurred about every 100,000 years. Prior to this, the glacial cycle was considerably shorter at only 40,000 years. As a consequence of these regular southward shifts in their breeding ranges and the ensuing long periods of isolation between the Old and New World forms, it is easy to see why many Holarctic taxa show marked genetic divergences.
A third race of Whimbrel
At the end of each glacial period, the two populations spread north again and some species meet during the interglacials, perhaps facilitating a certain amount of gene flow between them and watering down the differences, at least at the edges of their ranges. In this latter respect, it is surely significant that the Siberian race of Whimbrel, variegatus, is somewhat intermediate between the Western Palearctic phaeopus and the North American hudsonicus. Western variegatus has a white back and upper rump, but the lower rump is blobbed and barred with brown. Variegatus from north-eastern Siberia has the entire back and rump marked in this way, appearing dark from a distance. From my own limited experience in China, however, variegatus looked much more similar to nominate phaeopus than to hudsonicus, and photographs in A Photographic Guide to the Birds of Japan and other books that I have checked would seem to confirm this. These illustrate rather brown and coarsely streaked birds, rather like our own western phaeopus.
Interestingly, Grahame Walbridge tells me that there was a bird on Fair Isle on 18-19 September 1979 that resembled variegatus, having the pale oval confined to the back, but Cramp and Simmons (1983) state that some western phaeopus can also occasionally look like this. At present we are basking in an interglacial and the different forms of Whimbrel are once again in close proximity, but it is important to realise that this current situation is neither permanent nor the norm: the interglacials are considerably shorter than the glacial periods. When the next glacial period hits – which could be in anything from 18,000 to 40,000 years’ time – the Palearctic/Nearctic species pairs will once again become isolated by thousands of miles and thousands of years.
While there may be some gene flow during the interglacials, these long and regular glacial periods must have resulted in a fundamental genetic rift between the Old and New Worlds. If we accept this premise, it follows that species pairs on either side of the Bering Strait, which separates North America from Asia, should be split and not lumped. Like Common and Green-winged Teal and Green and Solitary Sandpipers, the two subspecies of Whimbrel, as well as Hen and Northern Harriers and even Black and American Black Terns, should be split, or at the very least considered for splitting.
There is in fact some scientific support for this idea. In 1995 a joint Russian/ North American team headed by Robert Zink analysed the mitochondrial DNA of hudsonicus and variegatus and found “striking mtDNA differences” between them that “suggest strongly that two species are involved”. Zink et al concluded that the whimbrels “are highly differentiated relative to other avian species” and that they “have clearly had relatively long independent histories on either side of Beringia [the geographical region either side of the Bering Strait]” (Zink et al 1995). Despite these conclusions, they stopped short of formally recommending a split because, in terms of sample size, they considered that their data were insufficient to be used as the basis for any taxonomic changes.
Since the study by Zink et al, further backing for a split has emerged. Engelmoer and Roselaar (1998) undertook a more extensive analysis of whimbrel morphology and suggested that plumage and metrics separate the whimbrels into two allospecies (geographically isolated taxa with no possibility of gene flow and differences akin to known ‘good’ species). They recognised six races in total: phaeopus, islandicus, alboaxillaris and variegatus comprising Eurasian Whimbrel, and hudsonicus and rufiventris comprising Hudsonian Whimbrel. Incidentally, British breeding birds are, not surprisingly, closest to the Icelandic islandicus.
The whimbrels were in fact one of six species pairs studied by Zink et al, the other five being the Palearctic and Nearctic forms of Three-toed Woodpecker, Long-billed and Marbled Murrelets, Asian and American Buff-bellied Pipits, Eurasian and Black-billed Magpies, and Common and Mew Gulls. Of these, it is interesting to note that the woodpeckers, the murrelets and the magpies have since been split by the AOU. As far as the remaining species are concerned, it would be nice if the research could be concluded so that this whole question of the ‘trans-Beringia species pairs’ could be laid to rest once and for all. To facilitate a comprehensive comparison, however, more extensive molecular data may be required, both in terms of the number of birds and the number of taxa studied. This would be required right across the species’ ranges, including the gaps where this data is either lacking or has not yet been analysed (David Parkin in litt).
Acknowledgements
I am extremely grateful to David Parkin for taking the time to read an earlier draft of this article, and for commenting extensively on the taxonomic matters discussed in it.
References
- Bosanquet, A. 2000. The Hudsonian Whimbrel in Gwent. Birding World 13: 190-193.
- Cramp, S, and Simmons, K E L (eds). 1983. The Birds of the Western Palearctic. Vol III. Oxford University Press, Oxford.
- Engelmoer, M, and Roselaar, C S. 1998. Geographical Variation in Waders. Kluwer Academic Publishers, Norwell, Massachusetts.
- Mitchell, D, and Vinicombe, K E. 2006. Birds of Britain: the Complete Checklist. Solo Publishing, London.
- Zink, R M, Rowher, S, Andreev, A V, and Dittman, D L. 1995. Trans-Beringia comparison of mitochondrial DNA differentiation in birds. Condor 97: 639-649.
