When it comes to bird identification I am sometimes dismayed at my own ignorance. Mostly I get by just fine, but occasionally a bird comes along which shines a glaring light on a gap in my knowledge. One such bird was the
Blyth's Reed Warbler that turned up at Beer Head on 31st May.
Listening to it sing that afternoon, I knew opinion was divided. Some thought the song fitted
Blyth's Reed, some thought
Marsh Warbler. Steve Waite's account
here nicely outlines the debate. From my own experience I couldn't contribute much - a few anciently historic
Marsh Warblers, but no
Blyth's at all. And anyway, when it comes to song, everything is so
subjective; people hear different things. No, the only way this bird was going to be unequivocally nailed was through very good views, and preferably photos.
Which was where my ignorance caught me out...
By the evening, when it was absolutely clear that an identification on song alone was going to be controversial at best, I realised the onus was on the few of us remaining to clinch it from whatever views the bird would give us. So, given the chance, we needed to look for its diagnostic suite of features. And what
were those features? Er...
All I could recall was that
Marsh Warbler had a longer primary projection, and that the tips of those primaries should be neatly edged whitish. And that was it. How
much longer? I didn't know. Could the primaries on
Blyth's have neat pale tips too? I didn't know. I didn't know
anything! Why hadn't I genned up on it before heading over? Slack!
Anyway, it all came good in the end, and the photos were sufficiently detailed to clinch the ID. But it was a bit of a lesson. I was fortunate to get quite a decent shot of the bird's wing, and the seemingly short primary projection and lack of pale tips appeared to indicate
Blyth's, but was that sufficient? I didn't know. One birder on the local WhatsApp group helpfully sent me comparison shots of
Marsh and
Blyth's Reed, another sent me a screenshot from Nils van Duivendijk's
'Advanced Bird ID Handbook' - the whole page (that's a
lot of text!) which deals with
Blyth's Reed and its separation from
Marsh and
Eurasian Reed Warblers. Let me tell you, trying to analyse all this stuff on the screen of your phone, in the bright sunshine of a late spring evening, while at the same time trying not to miss another photo opportunity, is impossible! I do not wish to repeat that frustrating experience, and the purpose of this post is to make sure it never happens. The process of putting all this together and writing it down is hopefully going to etch it all indelibly upon a part of my brain that will automatically clank into action should the need ever again arise...
So here is a bit of photo analysis, highlighting some (not all) of the [collectively] diagnostic features which make the Beer Head bird a
Blyth's Reed Warbler, rather than
Marsh...
And to begin, here are two photos taken at Portland Bird Observatory just three days ago, when a
Blyth's Reed was fortuitously trapped and ringed there. Posted here by kind permission of Martin Cade, they illustrate some key features...
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| © Martin Cade |
As well as the
short primary projection, with
6/7 visible tips, the photo above shows the
lack of neat, pale tips.
Marsh Warbler would typically show a primary projection approximately equal to the length of exposed secondaries, and generally 8 evenly-spaced tips.
This image also shows
the alula, which on Blyth's Reed has a dark inner web and contrastingly pale outer web, whereas
on Marsh Warbler it is wholly blackish, with a thin, pale outer edge. Admittedly, this feature doesn't look particularly eye-catching here, but the first photo on the relevant
PBO website page displays it at a much better angle.
Finally, note also the
rather plain tertials, with diffusely darker centres.
Marsh Warbler would typically show quite contrasty tertials, with strongly dark centres and pale edges.
On Blyth's Reed, the longest tertial falls slightly short of the tips of the secondaries,
on Marsh it is slightly longer than the secondaries.
 |
| Showing primary emarginations. © Martin Cade |
Blyth's Reed has emarginations (narrowing of the outer web) on P3 and P4, and sometimes also a bit on P5, as here.
On Marsh Warbler it's P3 only. In addition,
P2 falls well short of the tip of P3;
on Marsh Warbler P2 is more or less the same length as P3.
Unbeknown to me, quite early on I had caught several of these features in what is otherwise a rather poor photo. It was the last shot in a burst of seven, taken as the bird darted past a gap in the foliage. It is a salutary lesson in what can sometimes be gleaned from even a 'rubbish' image...
And this is one of the most helpful images of the wing...
So there we go. There are other features which help the case for
Blyth's Reed rather than
Marsh - and of course the song ruled out
Eurasian Reed - but along with a recording I would imagine these few alone are quite sufficient to ensure the identification is sound.
Several years ago I learned how to separate juvenile
Yellow-legged and
Herring Gulls, simply because we realised they must be turning up on the Axe and so I figured I
ought to learn. My reasoning was similar with
Caspian Gull. Not suspecting I would ever see one locally I guess I never had the same sense of necessity with
Blyth's Reed and
Marsh Warblers! Plus I'm bone idle.
Ah well, I've made the effort now. So probably I'll never encounter another
Blyth's Reed Warbler in my life...