The unsound approach

images and prose by Alex Lees

At 1510 on October 24th 2006 I was about 1/3 of the way along the west cliffs of Lundy, migrants weren't plentiful but the odd Sylvia and Phylosc and an obliging Ring Ouzel was keeping the optimism ticking over. My mind was focussed on Catharus, but on reaching the infamous Ruppell's bushes a warbler flicked out and vanished back into cover - an obviously Acro. As a veteran of late October Scillies, this didn't immediately cause a burst of adrenalin, Lower Moors is often full of Reeds till the end of the month, but when the little beastie showed its face, suddenly the nature of the game changed. It was pretty. Endorphins and cortisol kicked in in equal measure, but within seconds the bird had flashed its primaries at me, it had massive wings. Bugger. Blyth's Reed would have to wait for another year. So two choices remained but one was strictly taboo - late autumn Marsh....(?) I'd sooner claim an undocumented Black-browed Reed Warbler... Still, there was something not right about this bird, so I set to work in digi-immortalising it with a rusty coolpix and trusty Swaro. For an Acro it was pretty showy, just foraging around rather lethargically at eye level in the bushes and its periodic pauses meant I fluked two consecutive ultra-close-up images of its wings. Then after 15 minutes it disappeared off downslope. Leaving me sub-elated with a headache. So what was it? At the time the bill and head pattern and feet really stood out as pro-Marsh, the wings weren't as silvery as I was expecting and I had generally envisaged something a little more distinctive.  It was all going a bit fuscus, so I limped back to 'town' and discussed it with Rob Duncan and team. There were keen but we couldn't refind it, that day or subsequently and well, the rest of the trip was disappointingly quiet. I got back to sunny Norfolk and bandied around some images. At the time I had nominal experience of Marsh, but hadn't seen one for over 10 years and never an autumn bird, so my experience was effectively very limited (cf Ahmed here). I posted some images here and spoke to a few more people and then simply wrote the bird off.

Nearly three years on and I'd got a better handle on Marsh, 4 found and at least 8 seen on a spring visit to Foula, at least one crisp autumn bird on North Ron and then a sf bird on my doorstep Norfolk. On each occasion distant memories of the Lundy bird surfaced. I finally decided to throw the cat amongst the pigeons by throwing a closed wing shot on here under the guise of a competition and opened it up to Surfbirds readers. The result spurred on this little diatribe.....   

The evidence         

Fig 1. Videograbs of Acro sp. Lundy 24th October 2006, full video here.

After getting some documentary video I chanced getting a still image, miracuously the bird, the scope and the camera all came into alignment at once and I got two crisp shots as it paused (perhaps ruminating on why it was sat on a godforsaken rock in the Bristol Channel and not in an Acacia in East Africa). It is true that sometimes field notes supercede taking images but this is not one of those occasions. With a cheap camera and the expensive scope you already own, miracles can sometimes be achieved......

Fig 2.

Fig. 3

So, what do we know about autumn Marsh?

Well, sometimes they can be feckin obvious, like this one, this one and this one, other times apparently less so (and this one!). So they are easy right? Well, no. Wilson et al. (2001) found that 'Published criteria for the separation of Reed Warbler Acrocephalus scirpaceus and Marsh Warbler A palustris in the hand were found to be of limited value in identifying these species in samples captured during autumn migration through the island of Lesvos in the Aegean Sea.'  as ...  'Reed Warbler morphology varies clinally with birds in eastern Europe (likely to be migrating through Lesvos) being more similar to Marsh Warblers than are populations in western Europe from which existing identification criteria were calculated.' But.... 'a modified biometric index did separate a sample of 364 birds captured during August and September 1994-1996 and 1998 into two clear-cut groups along the morphological gradient which is known to distinguish the two species. We were therefore confident that these two groups comprised Marsh and Reed Warblers. Linear Discriminant Function Analysis confirmed that this index could be used to identify approximately 95% of the birds in our sample.' Phew.... best buy a protractor.... One constant remains, find an Acro after October, and you'll be fighting a losing battle trying to convince anyone its a Marsh. High profile mis-identifications of problem birds such as the Kelynack Acro ought to put off anyone thinking about sneaking in a cheeky late Marsh. A search of the Birdguides database reveals few confirmed October reports and nothing later than the 23rd. Passage through Cyprus occurs from late August until mid October (exceptionally early November)  (BWP) and birds are already moving through Kenya in late October (BWP). So at face value we shouldn't get drift Marsh Warblers in late autumn, but we could of course get vagrant Marsh Warblers; maybe an eastern bird with a spanner deviant orientation a 'nominal' reverse migrant.    

Fig. 4. Red line depicts length of emargination on the 3rd primary, falling about level with the 7th.

So what is pro Marsh?

The primaries appear to be evenly spaced, and from what can be discerned from the images (Fig. 4) P2 ends well beyond P4 and the emargination on P3 is roughly level with P7 (P3=P10 in Reed). However, estimating precise emargination positions and primary lengths from photos may be a dangerous game as they are influenced by the angle of the wing relative to camera (we assume in this case that the bird is orientated perfectly perpendicular to the camera). Emarginations can be particularly tricky, given that the apparent point of emargination will vary depending on where the shadow falls, and hence how the bird is holding its wing (droopy or tight etc). However, the emargination length can be also seen in the video (Fig. 5). The wings are obviously very long and although the pale primary tips do not seem to be extremely contrasty in the still, the video suggests otherwise. Tertials appear longer than the secondaries. Iris colour is difficult to discern from the images, the legs are quite pale brownish and the feet are yellower still. Claw colour is difficult to discern from the video. The upper mandible is fairly dark, contrasting with a flesh coloured lower mandible, the bill length appears short for Reed and better for Marsh. So, er the Walinder score = A-(B x C) where A= length of bill to skull, B =  width of tarsus and C = the width of the bill at the distal end of the nares. If the final product is: 4.5 - 8 then its a Marsh Warbler and if its  8.5 - 12.5 its a Reed Warbler (Svensson 1992). Actually, it wasn't in my hand, so forget that..... 

Fig. 5.

So that's the new approach stuff, what of old school features? Well, colour-wise it was paler than your average Reed, somewhere between the cold colours of the video and the warmer stills (note how shade and recording media make a big difference). The rump is pretty much concolorous with the rest of the bird (not warm-toned) and the bird's underparts are certainly yellowish. All told, I reckon it adds up to a good case for Marsh, it shows all the expected plumage tones, bare part colours and structure and from what can be discerned wing formula. But is it enough? Will anyone buy into a late Marsh that hasn't had its DNA read by a mystic molecular geneticist? If folk do believe in it then it sets a precedent for non-trapped late autumn Marsh (at least in my poor memory). So county recorders out there, what d'ya reckon? 

Extra gen for in-hand birds by the Italian professionals, link and translation thanks to Rich Billington:


original notes @ Kelly et. al. 2001


Notch Length * Notch Projection



Max. Chord * Wing point – 1st Secondary


        Notch Length * Notch Projection




Notch Length * (internal foot length inc. claws)2


                            (Max. Chord) 2



hind claw length * notch length * notch projection * bill to skull * (internal foot length inc. claws)2


(Max. Chord)2 * Wing point – 1st Secondary * bill width * tarsus width


Adulti = adults

Giovani = Juvs


To calculate threshold values useful for discriminating the two species, an initial data set was analysed comprising 1304 individuals trapped between 2001 and 2005 during migration and breeding periods at 10 ringing stations in the Piemonte, Veneto, Emilia Romagna and Toscana regions. The validation of the results was obtained from a sample of 407 nesting individuals from areas where the two species breed apart from each other

We think that it is best to start the identification using an initial selection by applying the Kelly formula and then, if necessary, IFM for adults and IFM3 for juveniles. All three methods are easily applicable in the field thanks to the attached tables.


Cannaiola Verdognola = Marsh Warbler

Cannaiola = Reed Warbler

Sovrapposizione = overlap


Proiezione Notch 2RP – second primary notch projection

Lunghezza notch 2RP -  second primary notch length

Valori di Corda massima * Punta ala 1° remigante secondaria = Max. Chord * Wing point – 1st Secondary

Valori di IFM (in grassetto sovrapposizione adulti) – IFM values (adult overlap in bold)

Valori di IFM3 (in grassetto sovrapposizione giovani) – IFM3 values (juv overlap in bold)