Tricky IDs - Making the Effort

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...

© 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...