There are three races that occur in Wales - schinzii, arctica and alpina. All three must pass through Wales at migration times.
The birds I suspected were Calidris alpina schinzii were of average size, had short to medium bills but noticably longer than 'arctica', some buffy rufous plumage in their scapulars and were a bit worn due to their earlier moult. The race schinzii breeds mainly in south east Greenland, Iceland with smaller populations in southern Norway and northern Britain and Ireland - this race mostly winters in west Africa too. I suspect many pass through Wales at this time and are certainly the commonest wader on the east coast at present (per Martin Garner www.birdingfrontiers.com)
Many of the birds I saw were strikingly small and short billed - some almost stint like in appearance. They showed extensively black scapulars with pale fringes - more monochrome in appearance and appeared slightly fresher looking due to later their moult pattern. I suspect that these were of the race arctica which breeds in north east Greenland and winters in west Africa passing through Wales briefly on migration.
Thirdly, there is the race alpina which breeds in northern Fennoscandia and western Siberia and some of these spend the autumn and winter in the UK. These have the longest bills of the ones likely to appear in Wales and have a rustiness to their scapulars at this time of year. They should also have more fresh appearance due to later moult. Although they winter in Wales, is mid-August a bit early for them to arrive? As many Sanderling, a few Littkle Stint and even Curlew Sands were in the Dyfi flock, I suspect that alpina also occus here at this time of year.
Coupled with the above, there is also the variation between male and female - males with shorter bills than females, and then variation amongst poulations i.e. runt birds which are known to occur.
I'm no expert on this, just a bit interested. I'd like to thanks to Martin Garner for his advice on Dunlins and the 'odd' wader that I came across. If anyone's got opinions, ideas or observations regarding these races I'd love to hear them. Wouldn't it be great to find a cadidate 'hudsonia' or even a 'sakhalina' Dunlin on a Welsh estuary?
Below are a set of photos of a few of the Dunlins caught with Tony Cross and Paul Leafe on the Dyfi last week. I noted the most 'extreme' bills, weight and size ands took photos of these birds. The presumed 'arctica' are the birds with the smallest measurements, the largest measurements were assigned as presumed 'alpina' and the 'middle of the road noted as probable Schinzii - an extremely crude way of doing it I know, but probably as good a way as any! By the end of the week, I was pretty confident in identifying the most extreme of these birds in the field.
A presumed arctica Dunlin - short straight bill, black scapulars with pale edging.
Another 'arctica'? above and below - both look short billed, small and monochrome.
Above - The dark centered scapulars and pale fringing that one would expect to find on an arctica.
Above and below - middle of the road birds. Average bills, average wight and size, sone rufous in the scapulars and quite worn plumage. I presume these are schinzii
It wasn't just Dunlin races that were on show. I've posted on this forum before regarding northern race Ringed Plovers and the photo below shows this well. On the left hand side of the picture is a small, dark Ringed Plover, almost certainly a northern Scandinavian bird (known as tundrae or more recently psammodrama). The rest are our local hiaticula race birds - being paler and larger. These individuals really stand out.