BLYTH"S REED WARBLER (2012 & 2017) 

DISCLAIMER an earlier version (2012) of this article can be found on (in Dutch) and on (in French).

Blyth's Reed Warbler, adult, Meijendel, Wassenaar, the Netherlands, 11 Augustus 2012

The 17th record for the Netherlands. By mid 2017, only 5 years later, there are nearly 40 records.

I long wondered why there were so little Blyth's Reed Warbler (Acrocephalus dumetorum) field sightings in Holland, while there were plenty in e.g. the UK. Since birds are trapped more or less regularly (and increasingly), they were out there waiting to be found! In 2012 I therefore wrote a post about their ID on based on trapped birds, and how the “ringer's stuff” can be useful in the field.


Mind that I don't tell anything new, I just try to explain structural characters in a simple way. I also added one plumage feature, since it seems to be over looked by birders.

Ringing helps a great deal to understand differences in wing structure between similar species. It is however not absolutely necessary to spend countless hours on a ringing sites, suffering from sleep depriviation and neglecting other birding activities, your family and your job. In order to judge several characters, reasonable photographs of a bird in the field is all you need.

I only ever trapped one Blyth's, and some of my ringing colleagues trapped another bird which I helped ID (real time; isn't WhatsApp a great invention?). I've seen dozens in the field on two foreign trips and I've seen a singing bird in Holland. So no, I'm not a true expert on the matter. But that's the whole point: they can be identified with relatively little experience, as long as you know Reed (A. scirpaceous) and Marsh Warbler (A. palustris)!


Useful references
Three books are obliged material:

Reed and Bush Warblers (2010) - Kennerley & Pearson
– a must for both birders and ringers


Identification guide to European passerines (1992) - Lars Svensson
– the classic ringer's bible


Identification guide to the birds in the hand (2016) – Laurent Demongin
– the modern ringer's bible


Note that there's more to say about the ID than posted here (like bill, facial markings, leg colouratoin). But here I focus on the wing, with one exception.

There's a general rule that's easy to remember. In summer and autumn all Acro species can be aged when seen well. Adults are (very) worn and often bleached. Young birds are fresh and usually darker. A second marker is the iris, which is much paler in adult birds.

Overall colouration
Let's start with the exception. I think this plumage feature is often over looked. In general Blyth's Reed Warblers are colder toned, with whiter underparts without buffy flanks. Reed Warblers have a warmer, more rufous tone and buffy flanks, especially in fresh juvenile birds. This is less obvious in adults in late summer or autumn, but still more so than in Blyth's. Marsh is greener toned, especially in adults.


A good alarm bell is that most Blyth's Reeds are very uniformly coloured: both Reed and Marsh have much more contrast. There's hardly any contrast in the tertials and at least the outer flag of the alula has the same colour as the upperparts. It's usually more obvious in autumn adults (since they're worn and bleached), but young birds show it, too. This is what I mean:

Blyth's Reed Warbler, Castricum, The Netherlands, 1 October 2010 (Arnold Wijker). Just note how uniformly coloured this bird is! This is often a striking feature in the field, too but be aware of very worn adult Reed Warblers.

Blyth's Reed Warbler, adult, Meijendel, Wassenaar, The Netherlands, 11 Augustus 2012. Note the uniformly coloured tertials and alula .

The inner flags of the alula and tertials are slightly darker, but on the whole it's a very plain bird compared to both Reed and Marsh Warbler. These show a far more obvious contrast, both in adult and young birds. Take a look at this Marsh Warbler:

Marsh Warbler, adult, Meijendel, Wassenaar, The Netherlands, July 2011. Note there's much more contrast between the alula and tertials and the upperparts. The same applies for Reed Warbler. Note greenish hue on this bird, typical for Marsh.

The wings

Alas, here are the wing structure features. This is a 1st cy Reed Warbler:

Reed Warbler, first calander-year, Meijendel, Wassenaar, The Netherlands, September 2010. Note the only emargination is on the 3rd primary (P3).

The outer primary (P1: ascendantly numbered) is very small in passerines, so they are hardly visible in the field (my thumb covers it in this pic).The red line indicates an emargination on P3, which means that the outer flag of the feather gets smaller so to speak. There's no such emargination on P4 and P5. The same applies for Marsh Warbler. Then take a look at P2. It's long, about as long as P4. In Reed Warblers they can even be as long as P3, the longest feather in the wing that forms the tip of the folded wing.

Reed Warbler, adult, August 2014, Meijendel, Wassenaar, The Netherlands
In this bird P2 is slightly shorter, equalling P5

This is the wing of a Blyth's Reed Warbler (note the worn primaries of this adult bird):

Blyth's Reed Warbler, adult, Meijendel, Wassenaar, The Netherlands, 11 Augustus 2012

There's not only an emergination on P3, but also on P4 and, less conspicuous, even on P5. This is a very important feature for Blyth's Reed (and Paddyfield). Note that Reed Warbler sometimes has a weak emargination on P4, but never like this.

You can also see that P2 is way shorter than P3 and P4. Not visible on this picture, but in the hand we measured that P2 fell between P6 and P7: this should diagnostically exclude both Marsh and Reed Warbler.

Primary projection
With reasonable photos this can also be judged on a bird in the field, as long as the wing is seen from the side. In Blyth's Reed Warbler 6-7 primaries project beyond the tertials, in Marsh and Reed Warbler 7-8

Blyth's Reed Warbler, adult, Meijendel, Wassenaar, The Netherlands, 11 Augustus 2012. Six primaries clearly project beyond the tertials, a 7th is about the same length as the longest tertial.

Marsh Warbler, adult, Meijendel, Wassenaar, Zuid-Holland, June 2015. Note that eight primaries (p3-p10) are visible beyond the tertials.

The relative length is also important. Hence, how long is the projection compared to the length of the tertials? In Blyth's this is 50-70% (meaning that in the case of 50%, the projection is half the length of the tertails). In Reed Warbler it's 70-80% and in Marsh 90-100% (the latter meaning that the projection is as long as the tertials).

Blyth's Reed Warbler, adult, Meijendel, Wassenaar, The Netherlands, 11 Augustus 2012. Measuring on a photograpgh is no exact math (as a wing is hardly ever in the ideal position; the wing is not completely folded here), but it's in the right order. Based on this picture, the primary projection is around 60%: well within the range of Blyth's Reed, but too small for Reed and Marsh Warbler.

Marsh Warbler, adult, Meijendel, Wassenaar, Zuid-Holland, June 2015. This is more or less an ideal position to measure the projection, as you can draw a single straight line between the base of the tertials and the top op the primaries. In this Marsh Warbler, the primary projection it close to 100%.

- Plain appearance (apart from other plumage features)
- Short P2 (< P6)
- Emargination on P4 and sometimes even on P5

- Short primary projection (60-70 %: >70 excludes Blyth's)
- 6 or 7 primaries visible beyond tertials (8 excludes Blyth's, 6 wrong for Marsh and Reed)

On a reasonable photograph of a bird in the field, the primary projection could be measured, on a good photograph, the emarginations should even be visible! A recorded Reed Warbler in 2015 with ok - but not smashing - photographs was unmasked as a Blyth's by precisely the characters described above (incl. the plain appearance), so I really think they can be useful!

Rinse van der Vliet, Diederik Kok and Arnold Wijker contributed to this post in one way or the other.