Saturday, 15 April 2017

A north-west Kent passage- White Wagtails and the land of Ice

After the broadbrush strokes of the last post on spring passerine migration, a post looking at just one species, the White Wagtail.

Already setting up a game there, in that short opening sentence. What is a White Wagtail? Some birders will be on auto pilot for the continental version, not 'our' subspecies that we call the Pied Wagtail. But they are the same species, and that is the 'correct name' for the majority of the range.

Similarly, when a birder reads the code 'alba wagtail' on another birder's blog then they might have one of two ways of interpreting that, The species is Motacilla alba. So, interpret that- a bird, perhaps a flyover, they cannot readily assign to race, or a bird the observer can confidently assign as M.a.alba. Often it is left to the reader to apply their own interpretation.

Recently played a game in a facebook group. A Wagtail pic there was called as White, then said to be Pied, based on the flanks. I was prompted to comment (after some more brief definite replies) on attributing a particular bird to subspecies and at the same time I deliberately avoided saying whether I felt the bird in question was M.a.yarrellii (Pied) or M.a.a. (White). For the poor souls who suffered that, I'm sorry, I'll be doing that again here- but I promise there are elements I didn't include before, bear with me. And for those who want to see the pic to judge it and say deffo one or other, no link- not really the point of this post.

I wanted to put over the idea separation is not as simple as many of the guidebooks make out, even in spring. For those going 'pish and tosh' right now, have a look at this excellent article from Dutch Birding- some of the simplest diagnostic features we use are not as clear cut as we hope- 'kodak grey scales' used in it to show the back colouration does overlap, and illustrations of dark washes on flanks with percentage scores to race- what we go around saying is 100% diagnostic, may only work in 80% of the birds.

Having muddied the water with that, I went further with another link to an article where a series of photographs of M.a.a. rumps taken in Belgium in spring were also nowhere near as clear as the guide books make out.

Identity shouldn't really be nailed with just one character, it needs a suite. And even then, it should only lead to a conservative call of 'showing the characteristics of' because there is a clinality between the races. Yet we often never pause to think about that. Heck, even a coasting yarrellii might not be the British race- these darker birds are also found breeding on the near continent and some nest up in Norway.

Of course, 'showing the characteristics of...' is a phrase we don't like to use in our everyday 'it is' or 'it isn't' dialogues. But we've all had the birder in the hide pronouncing 100% certainty on one feature, some might even have had certainty claimed from features not usually listed (I've known some who nail M.a.a. on flight call alone).

The other question I avoided throughout the debate was this- if the characteristics are a tad muddier than we like to believe, and some identify birds with certainty on briefer views than others, what is the true status of White Wagtail here in the south-east? And the other Kent chestnut- have all the Whites we see have really been overshoots over from the near continent?

Like some keyboard superhero I found myself muttering 'Robin, Alfred, to the Bat-library!'


The avifaunas

Sussex (James) "...birds showing the characteristics of the nominate continental race M.a.alba are recorded locally and have bred. Most records... are of 1-3 birds at coastal localities in spring..." Goes on to mention a couple of noteworthy falls (up to eight) and then documents a 'pure' pair breeding in the Cuckmere valley, and some mixed pairings. The update from Thomas starts 'Few Pied or White Wagtail are noted on passage...'
(Oh James, you may have been updated and replaced by Thomas, but this is why I love you still, you go with 'showing the characteristics'.)

Wood (Essex) mentions some mixed pairings. Mentions (as others also do) a lack of autumn records down to problems with in the field identification with juveniles and that females of both races can be difficult to distinguish from each other (not even risking going there in this post!) The Essex spring peaks were mid-April, with individuals reported from end February to 'right throughout summer' And numbers had increased in recent decades thanks to increased observer coverage.

Taylor et al state that Norfolk spring peaks are in April, with movement between mid-March and mid-May but that numbers were generally small, involving 'single figure counts at coastal sites' First breeding of a pure pair of M.a.a. was in 1997.

Suffolk (Piotrowski) goes for a status of 'common'- 'often occurring during the second week of April with Yellow Wagtails'. Earliest arrivals from 21st February. Numerically, common is then refined as 'around 50-100 occur in good passage years'.

Not exactly a consensus then. The lesson? The status we hold in our heads might well just be a cognitive bias. Individuals we've nailed may well not be as definitive as our subconscious would lead us to think. And we could have a lot more fun with these birds if we just bore that in mind.

Many authors go on about the nominate continental race, which makes us southern softies always think of birds overshooting the channel. Most birders north of the Watford Gap realise this isn't always the case, but us down here..?


G'wan, for a laugh, let's call some 'islandica' for a minute

Yup, here among Kent birders the immediate thought seems to often be that any coastal M.a.a. has drifted over from the near continent. Could be, but most probably not. What? We tend to overlook the fact that the large population in Iceland, and the smaller numbers from Greenland, are highly migratory. They are the ones that move through our islands on a definite, purposeful migration.

Go to the BTO's birdfacts webpage for Pied Wagtail and click on the two links for maps of ringing recoveries and from 'Time to Fly' (an excellent read by Kent's own Jim Flegg). Kent birders should then never again think immediately a White Wagtail is suffering continental drift. Stumble on a small flock, you'll now be able to imagine them as long-distance refuelers, all doing the Icelandic wing clap before taking off northwards again. Joking aside, this could well be why some coastal sites, with suitable feeding habitat, sometimes have flocks of tens and twenties. Or why Dungeness Bird Observatory records many fewer whites than the Bird Reserve a little way inland. These birds might well be tired long-distance migrants putting down in suitable habitat preparing for that next leap home.

North Kent could well have a put down incentive for Icey Whites. As a mainly diurnal migrant they can see a chunk of the greater Thames and decide to coast west then north, perhaps drop in to feed?

Of course, observer bias sneaks in to the official public records, often these sites have the same observers listed against such reports each Spring. The main point is all other Kent birders should never feel reporting their M.a.a. sighting is not really necessary in this day and age. We are fooling ourselves if we think we know what's going on, or if we think we know how many birds pass through Kent.

Yes, there have been records of 'pure' M.a.a. pairs breeding in recent years that are more likely of near continental ancestry. But is a spring bird be more likely to be a longer-distance passage migrant?

The more we find out, the more there is to know.

Records along the south Medway? Nowhere near as many birds showing the characteristics of M.a.a. turn up as seen at either, say, the Hoo Peninsula or Reculver, both closer to that Greater Thames coasting flightpath. Here? Not an obvious flightline. Any passing over would need a reason to land and much of the better feeding habitat here is private. Even so, footpaths around places such as Chetney never turn up many. Any bird seen showing characteristics of M.a.a. will always get an underlining in my notebook. I hadn't had one this spring up to a week ago when I started drafting this; at the time of posting, finally on a score of 'one'. Must look harder. For them, and at them.


Bibliography

James, P (ed.) The birds of Sussex 1996 Sussex Ornithological Society (Over Wallop)
Pennyworth, A. A catalogue of the Wayne Manor library vol. XXVI 2000 (Gotham City)
Piotrowski, S. The birds of Suffolk 2003 Helm (London)
Taylor, M. The Birds of Norfolk 1999 Pica Press (Robertsbirdge)
Thomas, A. (ed) The Birds of Sussex 2014 BTO Printer Trento (Trento)
Wood, S. The Birds of Essex 2007 Helm (London)

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