Since an analysis of DNA from the various races of the Lesser Whitethroat was published in 2013¹ which indicated that blythi is not as closely related to curruca as previously thought and could be a candidate for a future split to form a species group with three other subspecies; althaea, halimodendri and margelanica as one species and curruca and minula in the other there has been increasing interest in identifying these birds, both in the hand and in the field.
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| Figure 1 blythi in profile, note brown mantle and buff flanks |
The general impression was of the nominate race of Lesser Whitethroat (which I'll refer to as curruca and blythi for Siberian Lesser Whitethroat) in terms of size, structure and behaviour as it moved slowly through the bushes generally keeping low down. In brighter light the brown mantle was the most obvious feature and on closer examination the mantle colour continued up the nape fading to grey on the rear crown unlike curruca where there is a distinct division between the grey/brown of the mantle and the grey nape. This feature was most obvious when looking at the bird from the rear or when looking down on it (Figure 2), it was not so obvious when viewing the bird side on as in Figure 1 above. See also Figure 4 sent by Trev Charlton which show nominate curruca with the mantle colour extending on to the nape although it is less obvious as there is less contrast between the mantle and nape base colouration. Figure 4 also shows the colder, greyer tones of curruca compared to blythi.
The next distinction from curruca was the colour of the underparts. In this bird the throat was off white but the flanks and lower breast were a warm buff separated by an off white lower belly and undertail coverts but the buff flanks looked to meet behind the legs whereas in curruca the whole of the underparts are off white. Slightly darker brown ear coverts and lores gave a masked effect but how strongly this appeared varied with the light and the viewing angle.
The tertials were dark centered with pale fringes which combined with the mantle colour brought to mind Common Whitethroat. Primary projection is said to be less than in curruca but there is apparently considerable overlap.
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| Figure 2 viewed from behind, showing mantle colour extending on to nape |
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| Figure 3 showing rear flanks and undertail |
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| Figure 4 Lesser Whitethroat of the nominate race curruca © Trev Charlton October |
A consistent distinguishing feature for blythi from curruca is the position of the tip of p2 against the other primaries in the closed wing. In blythi p2 is short and thus falls further away from the wing tip. This is a straight forward measurement for trapped birds but in the field the wing needs to be spread slightly to reveal p2. Fortunately watching this bird on and off over 3 days I did manage a photo with the wing slightly spread.
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| Figure 5 with wing slightly spread showing approximate position of p2 |
Clearly, there is quite a bit of room for error here depending both on the angle at which the bird is viewed and the extent the wing is spread as well as any damage to the tip of p2.
The further p2 is out of alignment with the remaining primaries the shorter it will appear. Also the angle at which the line is drawn will also have an effect, I've tried to draw the line perpendicular to the primary shafts. Having said all that I think p2 falls around p6 which excludes curruca where p2 falls between p4 and p5 or at p5 in the majority of examples.
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| Table 1 Position of p2 in relation to tips of other primaries in closed wing. Figures are percentage of each sample (adapted from BWP) |
The further p2 is out of alignment with the remaining primaries the shorter it will appear. Also the angle at which the line is drawn will also have an effect, I've tried to draw the line perpendicular to the primary shafts. Having said all that I think p2 falls around p6 which excludes curruca where p2 falls between p4 and p5 or at p5 in the majority of examples.
If you are not able to obtain DNA the final evidence required is the pattern of white on the outer tail feathers.
I was unable to see any evidence of a moult limit (mix of juvenile and post juvenile feathers) in the greater coverts which would identify the bird as 1st calendar year but I think the pointed tail feathers are juvenile thus ageing the bird as 1st calendar year. The age is important as there is considerable variation reported between 1cy and adult birds. In its first year the race halimodendri has an all white outer tail feather (t6) so on that basis we can confirm that this bird is blythi. The nominate race curruca can have a similar outer tail pattern to blythi in its first year and both races may show white tips to the neighbouring feathers (t5 and even t4 and t3 in curruca).
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| Figure 6 in flight showing spread upper tail |
I was interested to see if the pattern of the outer tail feather could be seen from the underside of the tail. It looks less distinct but the pattern can be discerned (Figure 7).
Bare Parts
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| Figure 7 Pattern of the underside of t6 from below, note also off white belly |
The iris was brown, similar in colour to the mantle. The bill was dark with a bluish base to the lower mandible extending just over half its length. There was some reddish discolouration around the bill base from the blackberries it was occasionally seen feeding on. The legs and feet were black with paler bluish grey soles.
Call
It was heard to give a tacking call several times which sounded like curruca. No other calls were heard.
Behaviour
Over a four day period the bird was seen frequently in a hawthorn hedge and two areas of bramble extending to about 150m. Most of the time it was within 1m of the ground but occasionally climbed the hawthorns to over 2m. It was very active creeping through the low bushes and appeared to stick to one direction of travel making it possible to predict where it would appear when lost to view. Several times it climbed to the top of the brambles or low hedge and flew 20 - 30m when it would again set off in one direction and repeat the process. The tail was regularly flicked down, much like Common Chiffchaff although it was windy and this may have affected it behaviour in exposed positions. The tail movement emphasised a rather long-tailed appearance. On a couple of occasions it sat on, or close to the top of low scrub to preen.
Photographic Note:
The photos have been cropped and in some instances lightened but there has been no colour adjustments made either by the camera or in post processing.
Acknowledgements
Thanks to Roger Barnes, Mick Cunningham, Paul Doherty, Richard Hart, Vaughan Lister and Johnny McLoughlin and for input whilst watching the bird at various times and comments on its identification. Also Trev Charlton for his comments and use of the photos in Figure 4.
References
1. New insights into the intricate taxonomy and phylogeny of the Sylvia curruca complex Urban Olsson, Paul J.Leader, Geoff J.Carey, Aleem Ahmed Khan, Lars Svensson, PerAlström
2. Handbook of the Birds of Europe the Middle East and North Africa, the Birds of the Western Palearctic Vol VI
3. The Challenge Series - Autumn by Martin Garner
4. Handbook of Western Palearctic Birds Vol 1 Shirihai & Svensson
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