Wednesday, 20 July 2016

Sic transit gloria dunli

The Dunlin are back. Only a trickle, but enough to be interesting.

This post is me trying to spin them interesting, trying to see if I understand what goes on with them at this time of year.

Prior to my return in 2013 I swatted up on autumn numbers for the whole Medway in the autumn, and for Dunlin one September peak stood out:

1993. Now what was that all about? More than 12,000 birds, compared to nowadays reported average of less than a hundred since the turn of the century. Curiosity piqued.

I set out to get counts for each sector every ten days to compare to these published counts and to known migratory trends. And things have started to come together, mostly through understanding those migratory timescales.

As a species, after completing breeding there is a relatively prolonged movement to their wintering grounds that for some spans the period of their main annual moult. So birds stop en route ('stage') to either moult or to just refuel. If moulting, individuals may stick around for two-three months. If feeding up, might stay a day or so, a week or or so.

The choice of where to do what is essentially driven by race. We have three races of Dunlin pass through east-southeast England. With all three, failed breeders move first, then usually successful females followed a short-time later by successful males. Juveniles leave the breeding grounds last of all.

Trawling BWP and several more recent publications, I made a sort of 'Dummies Guide' guesstimate of timescale/numbers for the south-east of England:

An explanation:

Arctica is a small population up in n-e Greenland. From biometrics gained through ringing, some are known to refuel in England and so it is possible that a small number might drop into the Medway on passage.

Schinzii is a bigger population; se Greenland, Iceland, s. Norway, UK and again a race only needing to refuel here if it stops here at all. Whilst early autumn records of juveniles are believed to be the source of many English 'inland' reports, difficulty in differentiating schnzii from the third race, alpina, in the field makes any ruling out of the latter difficult down here in the south, bearing in mind the number of alpina rriving from the west- northwest to moult. More northerly migration hot-spots such as Spurn may well estimate the majority of their birds in July will be schnzii, but we're further south and in line for that other population...

For alpina that winter in Britain and Ireland, from the western part of their extensive breeding range, their main moulting area is in the Waddenzee. Juveniles don't arrive on there until about the time the adults complete their moult and are ready to move off. Why we see the annual big increase in numbers in October, as birds that have staged for both reasons on the Waddenzee arrive en masse.

The trick is that while the bulk of that adult population moult on the Waddenzee, a substantial number (00,000s) use the Wash and a small number are known to use the Greater Thames estuary. They are here early. So any small number of that alpina population could easily swamp any schnzii and arctica present here.

A Dummies guide to routes in early autumn:
Arctica: down through western flyway to Africa
Schnzii: same, from se Greenland, Iceland, s Norway, UK
Alpina: to main moulting ground in Waddenzee, and the Wash

The above map is not based on anything other than my whims. Helps me frame questions coming up as I type this out. Do arctica favour UK west coast more than east? Do Norwegian schnzii cross North Sea to UK, or go via Waddenzee? Do the Wash alpina favour a direct route to the Wash over the southern North Sea or follow the German/Dutch coasts? (I really need to read up more!)

The main thing is, here we're in the firing line for alpina.

Back to now and early autumn- can you tell them apart? Two schools of thought.

(1) On plumage and jizz, yes, some with difficulty. 'Birds of Norfolk' notes "Very small numbers of arctica occur in Norfolk mainly in July and August- they are often mistaken, by inexperienced birdwatchers, for Little Stints as they are halfway between the size of most juvenile Dunlin and a Little Stint." That's a start then. 'The Advanced Bird ID Handbook' tries to nail it; the smallest subspecies with the shortest bill (equal to or shorter than head-length). Schnzii is small as well, with bill about same length as head, while alpina has a longer bill, much more down-curved. That's a clue to what we're playing with. Several texts go on to list other subtle differences, the main point for here is that they're almost do-able with hard work. For those field birders keen to push boundaries, e.g. finding the true status of these transient species here in Kent, further i.d. pointers can be found in this link to a Birding Frontiers piece on arctica. Even if you're not interested in the status, one text goes so far as to say if birders really want to find scarce and rare calidrids, they had better get to know the Dunlin extremes at least.

(2) Of course, if moulting, there will be a whacking great 'hole' in the wing from a week or so after arriving on chosen staging grounds. These are likely to be alpina. Simples (I wish).

What did happen back then in the 90s?

In 2005 the BTO attempted to make sense of changes in Dunlin numbers here. Proportionally, Medway wintering nos. had dropped from 10% to 5% in the decade from 93/94, as well as seeing a continual decline locally (in relation to Thames and Swale). Wintering Dunlin certainly weren't just shuffling over an estuary. Such attempts to make sense of wintering numbers might have a correlation to autumn, but not considered. And no comment was made in the county bird reports at the time.

Presently? Well, numbers are certainly not as low as reported mainly via the uncoordinated partial coverage WeBS figures used since the millennium. Numbers in excess of those reported for the whole estuary are now usually achievable along the southern shore in autumn.

But that September '93 peak was really huge. Certainly shouldn't have been either of the two transient races, did alpina have problems elsewhere at that time? At this point I'm reminded of James's 'Birds of Sussex' when commenting on highly variable October totals for that county- "Clearly much depends on whether the presence of large numbers of migrants coincides with the date of the count!" Did something lead to a slightly early movement?

Well, easy enough to look at the other two estuaries that make up the North Kent Marshes- the Swale and (the Kentish part of) the Thames. That 1993 peak was reflected in both, albeit on a smaller scale. And interesting to see the Thames had an 8,000 influx in an August a few years later that seemingly showed up in the Medway as well:

Probably too late to ever resolve those peaks, but great to be alert to such any such arrival now. For boring old me, a four-figure Dunlin count in August or September here is now high on my 'wants' list.

For drawing any 'normal' picture of occurrence, best to leave those blips out of any averaging. General trends in both the surrounding, more coastal, estuaries are clear, we should, perhaps, be just a little below them. But is there room for improvement on numbers being recorded within the Medway? Are they a true picture? 2016 is now my fourth autumn, and by the end of this September I intend to revisit this post and confirm there's a more 'greeny-yellow' trend along my southern shore at least. Watch this space.

Yesterday, on the rising tide off of Bloors, I had a grand total of... two.

Like most of 'my' birds here they stayed out at 'scope distance. They usually head out to the island complex to roost, so no close up grilling to race for me (phew). All I can do a lot of the time is just dream about where they've come from, or where they're going to.

For most birders, two Dunlin really would not warrant a second glance. I watched them for nearly an hour.

They seemed new in- uncertain of where to go, allowing the tide to lift them and, for a few minutes, make them into passable phalaropes. Then, rather than flight low to roost they were off skywards, higher and higher. Wanting to stick together, one would try to lead north-west but the other would refuse, drawing them back in and then try to lead north-east; they continued in this zig-zag fashion, higher still, until lost from view way off north over Hoo.

What were they, and what were they about? Who says birding is dull this time of year?

No comments:

Post a Comment