A Field Notebook: Marsh warbler conundrum

This Acrocephalus warbler was at Tide Mills in Sussex on 6th September 2018. It's a marsh warbler, but opinion is split, with some considering it a reed warbler. It's superficially a difficult identification, especially for birders too concerned with "warm brown hues" at the expense of all other features, and in this blog post I'll explain why.

It's a marsh warbler for many reasons, but not least because it called several times while I was watching it. On all occasions it's call was a hard tongue clicking note, similar to Blyth's reed warbler, which is sometimes described as a sound similar knocking two pebbles together. Reed warbler does not give these clear cut, hard single notes. At no point did it utter anything like a reed warbler call. Unfortunately this is obviously something which you can't judge from the photos and you need to take my word for it. If you're in the reed warbler camp you'll probably just ignore this vital piece of evidence, however you really shouldn't.....

In 2015 a very useful article was published in Scottish Birds on the identification of 1st winter marsh warbler, and it is this which I have largely referred to throughout this blog post.

Scottish Birds (2015). Marsh Warbler in first-winter plumage - SBRC identification criteria, M.S. Chapman. Available: https://www.the-soc.org.uk/files/docs/bird-recording/sbrc/Marsh-Warbler.pdf. Last accessed 09/09/2018.

Even without hearing it call, it's a marsh warbler because it has a relatively short, blunt bill when compared to reed, pale legs and yellow, strong looking feet. Note the pale yellow underparts, especially the undertail coverts, a real pointer to marsh warbler. Note also in this photo that there is no contrast between the wings and the rest of the upper parts, and what we can see of the rump looks about the same as well.

Pale yellowy underparts, short blunt bill and garden warbler like appearance.

A slightly over exposed photo admittedly, but once again very garden warbler like in appearance with a shortish, blunt bill and a rounded head not a bit like reed warbler, clearly yellowy underparts and from what we can see of the primaries they are long and white tipped. Note also the pale edged tertials.

Much of what I have just said applies to this photo also, but now look at the primaries. Clearly long and white tipped.

A short blunt bill. Another photo showing the pale yellowy underparts.

Finally we come to this photo. Pale edged tertials contrasting with darker centres, clearly white tipped primaries, the palest legs you've ever seen, yellow robust feet and pale yellow underparts including the undertail coverts. The bird is also calling like two pebbles being knocked together. It's a marsh warbler. However if you want to ignore all of these other features and concentrate on "warm brown hues" then consider this; up to 20% of all marsh warblers show warm brown upperparts anyway. My bet is that quite a few marsh warblers are overlooked because they are dismissed as reed on the basis of this one feature alone, rather than actually taking in the whole suite of characters. Personally, I think that the browns look deeper than they were in reality because I have over compensated the white balance because of the glare of the sun. Look how deep the greens and reds of the foliage and berries look in the above photo.

If you don't want to read the full article in Scottish Birds, here is an extract:

"Marsh Warbler is typically paler and slightly to notably less warm toned overall, with upperparts variously described as sandy, beige, or dull greyish-olive, with often a tinge of green or olive, lacking in contrast. Usually no contrastingly warmer tones are evident on the rump/upper tail coverts, and it shows paler less warm edgings to the remiges, including the tertials, giving the latter more contrast. The supercilium is typically slightly paler and less warm-toned, and thus more obvious. Warmer toned birds similar to a subdued-looking Reed Warbler, do occur however (possibly up to 20% of first-winters, Pearson et al. 2002). It also shows cleaner underparts, with any wash tending towards yellowish-buff; obvious yellow tones on the underparts are a good indicator of Marsh. Leg colour tends to be pale, often strikingly so.

Structurally wing length and primary projection are on average longer than in Reed, whereas bill length is on average slightly shorter. Soft parts, (bill, tarsi and even often the claws), tend to be slightly stouter than on Reed, giving a slightly more robust impression overall. The jizz of Marsh thus while still obviously Acrocephalus like, tends slightly more towards Garden Warbler Sylvia borin than the classic slimline Reed. Marsh Warbler can be quite vocal. The call consists of single hard tongue clicking notes, sometimes given several times in succession; ‘chik’, ‘chet’, ‘chuc’ ‘tak’, ‘thic’. Also chirring calls similar to the usual call of Reed (perhaps usually slightly more ‘rattling’).

........ Neither form of Reed is known to give these clearcut, hard, single notes, although Blyth’s Reed does."