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Seperating Blyth's Reed Warbler and Eurasian Reed Warbler by fore-supercilium

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Old Saturday 13th September 2014, 09:52   #1
wolfbirder
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Seperating Blyth's Reed Warbler and Eurasian Reed Warbler by fore-supercilium

Separating these two species is a real tricky business unless they are singing birds in spring, not everyone understands or is able to see emargination on primary 3 or 4 for BRW, and of course call and primary projection are key identifiers though still subjective, as is colour tone which varies according to light and shade.

I have edited 20 photos, a small sample admittedly - taken randomly to try to highlight typical features - 10 BRW's and 10 ERW's to zoom in on face only.

I am not saying this is a key diagnosis of those quieter autumn birds (call is still diagnostic but requires knowledge & often birds are not accepted by authorities on call), but the open expression of the BRW's created by a whiter, broader fore-supercilium may be a helpful indicator in the field. Also look at bill structure and colouration, iris colour (seems slightly lighter in BRW), and eye-ring. Beyond that I fully accept that beyond helping with familiarity of each species, this is not scientific or diagnostic, but if it helps people appreciate the subtlties of each species then that is a bonus........

First 5 Eurasian Reed Warblers....................
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Last edited by wolfbirder : Sunday 14th September 2014 at 07:00.
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Old Saturday 13th September 2014, 09:54   #2
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Another 5 Eurasian Reed Warblers, the last one possessing a slightly broader fore-supercilium..............
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Old Saturday 13th September 2014, 09:56   #3
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First 5 Blyth's Reed Warblers.....................
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Old Saturday 13th September 2014, 09:59   #4
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Second and final set of Blyth's Reed Warbler images...........

the last one shows the variation that can occur, with barely any white fore-supercilium. Infact the last one is more like Reed Warbler, highlighting that even this feature is not always diagnostic. And of course Marsh Warbler is also a confusion species, though it is more likely these 2 species highlighted are the real confusion species to most birders.
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Last edited by wolfbirder : Sunday 14th September 2014 at 07:06.
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Old Saturday 13th September 2014, 18:20   #5
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Worth checking further!

Svensson mentions "Supercilium well-marked in front of and above eye" for Blyth's, but does not mention the supercilium at all for Reed.
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Old Sunday 14th September 2014, 10:53   #6
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The other thing worth noting about colouration is that juvenile BRW's in autumn can look warmer brown than greyer adults, almost matching the tone of Eurasian Reed Warbler. But the white fore-supercilium of "classic" BRW's should still stand out as being whiter and broader. But, not all birds are classic and some ERW's have broader superciliums than normal, and conversely some BRW's have very inconspicuous fore supercilums!
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Old Friday 19th September 2014, 18:20   #7
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I agree but it can be easy to be fooled by the super - the viewing angle can change this.

The tip about the warmer colouration of first year birds is also a good one.

I don't think many BRW are as obliging as this one and I have REALLY struggled with this species on previous occasions.
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Old Saturday 20th September 2014, 04:28   #8
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I think this species is one of the hardest of all, as you say they rarely show well. It would be better asking birders who have seen plenty whether this feature helps them at all.
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