A Wagtail

Topsham 30th March 2013

It's becoming almost a weekly ritual now, I drop by Bowling Green Marsh, usually early morning (when no one can see me?) a day or two after some half decent birds have turned up, fail to see anything and wonder why I bother. I even went in the hide this morning, for the first time in several years, to try and get a better view of a Little Ringed Plover - I didn't and promptly left. I don't like hides, hides are basically sheds. I've nothing against sheds, I have a shed in my garden, it not somewhere I'd choose to sit though. So, distant waders proving unsatisfying - they're always at the far side of the pool - I returned via the Goatwalk and found something more interesting.

I noticed a wagtail and managed to get a handful of shots before it flew off. It was impossible to get more than a cursory look and I clearly didn't have time to take notes or sketch it, but I had the photos. Now I can analyse the bird at leisure. Here is a composite of the most useful photos.

Like most people I used to think White Wagtails Motacilla a.alba* were fairly easy to identify, at least in Spring, but it now seems that things are a little more complicated - which is always interesting. Before I get stuck in, a word about references; the Identification Guide to European Passerines (Svensson 1992) is always a good place to start but for this family the primary source now has to be Pipits and Wagtails (Alström, Mild & Zetterström 2003), a marvelously detailed book with nearly 50 pages on Pied/White Wagtail alone (not counting illustrations and photos). The other important work is White Wagtail and Pied Wagtail: a new look (Adriaens, Bosman & Elst 2010), really nice critical look at variation in the two forms, the safest field characters and some difficult intermediate birds.

Clearly it's not black or even blackish on the mantle which it would be if it were an adult Pied Motacilla a.yarrellii. But what about a first summer bird? In fact, can I age it from these photos? At the risk of overkill, I'll give it the third degree - hey, it's more fun than doing the crossword!

It's got a black crown but there are a lot of grey feathers on the nape - still moulting? The mantle and scapulars are mid-grey, there appear to be some darker marks but these are just the feather shafts and an optical effect. The back and rump are hidden when standing but on the flight view are seen to be the same colour as the mantle, it's the upper tail coverts that are blackish. The chin, throat and bib are black with only a few whitish feathers, the breast sides and flanks are extensively grey - the same colour as the mantle or perhaps slightly paler.

On the closed wing I can't see the lesser coverts so I'll start with the medians; there's a clear difference (both wings) between the inner (black with white tips) and outer (not quite black with narrower grey tips) feathers. The greater coverts show a similar appearance; the inners (black with grey fringes and broad whitish tips) contrast with the outers (shorter, greyer with thinner fringes and tips). The two longest tertials are much fresher (black with broad whitish fringes) than the shortest (quite brown with very thin fringes). The secondaries look a little paler than the longer tertials and there are only thin pale fringes not the solid white patch shown by adults. The primaries are hard to see but the tip of the longest (just appearing beyond the tertials) is brownish grey, not black, the fringes (also those on the primary coverts) are thin and look worn.

In late March both adult and first summer birds will have just finished (or be about to finish) a partial moult. So all head and body feather will be new, also a variable number (none to all) of secondary coverts and tertials and sometimes a few tail feathers will be replaced. During the previous autumn an adult will have had a complete post-breeding moult, a first year will have had a partial moult - i.e. not including it's juvenile remiges and usually not including some juvenile coverts and tertials. So moult contrast/limits in the coverts isn't going to work as it does in Autumn, the key to ageing in spring is detecting the presence of retained juvenile feathers. A bird hatched last year should still be carrying its first set of flight feathers and possibly some juvenile coverts.

Those outer median and greater coverts look very much like juvenile type feathers (as shown in Svensson, and Alström et al.) and the shortest tertials do look convincingly faded and worn. The secondaries don't look black like the longer tertials and they don't show the solid white patch as on , for e.g. this adult Pied from earlier in the week - also showing fairly fresh black primary coverts with neat white fringes.

Bearing in mind all the caveats about ageing spring birds, I'll stick my neck out and call this a first summer. But is it alba or yarrellii? Adriaens et al. rather complicate matters by describing a number of intermediate birds, some of which are certainly hybrids. They also critically analyse those characters useful for identification. Here's how I think their seven characters (A-G) work out on this bird:

1. The colour of the mantle and scapulars and the percentage of blackish feathering shown in these areas (characters B,C and D) - it seems there is some overlap in colour between the subspecies and my bird is not pale enough to be an obvious alba; there's definitely no blackish colour though, other than on feather shafts (an optical effect) which seems to rule out yarrellii.
2. The colour and extent of the flank pattern (characters E and F) - again there seems to be some overlap in colour between forms so unless the flanks are greyish white (alba) or really dark or black (yarrellii) this is less helpful, as is the case with this bird.
3. The whiteness of the belly (character G) - clean and white in most birds of both subspecies but in some yarrellii there are a few dark spots. Certainly no spots apparent on this bird.
4. The colour of the rump (character A) is last as this still seems to be the safest character. There is a little overlap though, some alba can show a fairly dark grey rump (specifically the area above the upper tail coverts, which are blackish in both forms, and roughly between the tips of the middle tertials). This area was difficult to see as the tail is raised on the one blurred flight shot making it hard to judge the exact extent of the grey feathers but my guess is that the rump is the same colour as the mantle.

So, based on mantle colour (no black feathers) and rump colour (same grey as mantle) it seems to be a White Wagtail according to the criteria in Adriaens et al. On the other hand, it's far from a classic pale alba and I have serious doubts; the Dutch Birding paper describes some quite alarming intermediate birds, some certainly hybrids.

* I like scientific/Latin  names, they're not hard to learn, they tend to change less than common names and they're often shorter. It's my blog and I'm going to use them.

References

Adriaens, P, Bosman, D, & Elst, J 2010. White Wagtail and Pied Wagtail: a new look. Dutch Birding 32: 229-250.

Alström, P, Mild, K & Zetterström, B 2003. Pipits & Wagtails of Europe, Asia and North America: identification and systematics. London.

Svensson, L 1992. Identification Guide to European Passerines.
Fourth edition. Stockholm.