Monday, April 29, 2013

Identification of Blyth's Reed Warbler


Over the past few years I have seen three Blyth's Reed Warblers in Britain and the last one, on Fair Isle in 2011, was mist netted and ringed and so I was able to get some in-hand shots of the bird. The photos have been sitting on my hard drive serving very little purpose since then. I thought I'd stick a few on my blog so that they are available for anyone wanting help with identifying putative Blyth's Reed Warblers or who just want to brush up their ID skills.
Blyth's Reed is an un-streaked Acrocephalus and as such is pretty difficult to identify as there are a number of other, very similar, un-streaked Acrocephalus warblers in the Palearctic region. Blyth's, Marsh and Eurasian Reed can all be told apart on song but the chances are a bird in the UK will be a first-winter bird in the autumn and will be silent. If it does call you are listening for a Lesser Whitethroat type 'tuk'  which is unlike the 'tcharrr' of a Eurasian Reed or the softer note of a Marsh warbler. So what do you look for in a Blyth's? They have a combination of relatively short primary projection and clear emargination on two of the primaries. O.K. if you have a bird in the hand but difficult in the field! The emargination on the primaries can be seen on the first photograph but it would not, I suggest, be of much use generally.

1st autumn Blyth's Reed Warbler Acrocephalus dumetorum  Fair Isle October 2011
In this second picture we can see that the wing is quite rounded, more so than in Marsh Warbler or Eurasian Reed Warbler. The bird has a short primary projection, about 50-60% of the length of the exposed tertials. According to Kennerley and Pearson in " Reed and Bush Warblers" Helm 2010, "The wing point is formed by p3 and p4, and p5 is often so close in length that just six closely spaced primary tips are visible on the closed wing, fewer than in Marsh and Eurasian Reed." This feature is clear on this bird and it is a particularly important identification feature

1st autumn Blyth's Reed Warbler Acrocephalus dumetorum  Fair Isle October 2011
In this third picture we can see that the bill is quite deep and broad based and so it appears to be more substantial than on Eurasian Reed Warbler but they tend to be about the same length. The bill of Marsh Warbler is slightly smaller. Guidebooks always point to the long, sloping forehead and the low angular crown, peaking just after the eye as a useful ID feature of Blyth's and again this feature can clearly be seen in these photographs. This bird has a clear open-faced expression that can also be a useful ID feature of Blyth's.

1st autumn Blyth's Reed Warbler Acrocephalus dumetorum  Fair Isle October 2011

Blyth's Reed lacks the hint of warm tawny-brown shading to the rump and uppertail coverts that you would expect on a Eurasian Reed Warbler and it is not as greenish as a Marsh Warbler's rump. The overall colour impression of this first-autumn bird is a cool olive-brown above with hints of warmer, browner tones in the wings and on the tail and pale below with a whiter chin and throat. On these pictures we can see that the supercillium is quite indistinct, especially behind the eye. It is quite poorly defined on this individual. Blyth's tend to have a supercillium that bulges in front of the eye but that is less prominent behind the eye and this gives the bird a bland appearance.



1st autumn Blyth's Reed Warbler Acrocephalus dumetorum  Fair Isle October 2011


This bird is showing very worn rectrices and fresh primaries. It is a classic first-winter bird in autumn: the tail feathers are browner than in an adult and the upperwing coverts, tertials and flight feathers are all brighter than in an adult. The first three photographs show that the upper mandible is grey with pinkish cutting edges whilst the lower mandible is a pale pink.



There are a number of confusion species. As well as Marsh and Eurasian Reed, Large-billed Reed Warbler Acrocephalus orinus is probably the most difficult to tell apart - fortunately not a problem in Britain...not yet!

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