Blyth’s Reed Warbler – Portland Bill, Dorset

Over the previous weekend the pager had reported that an unidentified acrocephalus warbler was frequenting the observatory garden at Portland Bill. By Monday the pager was reporting that the bird could possibly be a Blyth’s Reed Warbler and so I texted James for more information. He replied that it looked good and recommended that I should go. By chance I had booked the next day off as holiday (half term week) and so I decided to head off first thing Tuesday morning.

Just after first light I set my scope up 20 yards from the west-facing stone wall and began to watch the ivy-clad tree and wall, this had been its favoured position for the last 6 days. Confusing the bird with a newly arrived Reed Warbler would not be a problem as this bird had lost its tail during ringing!

It was a good hour before the bird showed for the first time, during a brief spell of sunshine. The acrocephalus warbler, which had been trapped and ringed at the observatory on Wednesday 17th October, popped out of the ivy and showed well, albeit briefly. It quickly darted back over the wall to the sheltered side of the bush and disappeared from view. Over the next hour several other birders arrived and the mystery warbler gave numerous good but brief views.

The most significant feature of the bird (other than its missing tail!) was its striking pale sandy-brown upperparts. It had a fairly indistinct buffish supercilium, a pale creamy-white throat and sullied off-white underparts. In the hand, the bird had a clear emargination on the 3rd and 4th primary and the 2nd and 7th primaries were equal. However, the wing length of 66mm and the yellow-soled pink legs caused some confusion that the bird may be an abnormal Eurasian Reed Warbler or a hybrid.

Although the wing formulae was highly suggestive of Blyth’s Reed Warbler Lars Svensson claims a range of 58-65mm making the Portland Bird’s wings too long. BWP, however, publishes a primary range of 58-66.8mm.

Because of the in-hand discrepancies, Martin Cade was reluctant to claim the bird, and was sensibly cautious. Grahame Walbridge was fairly convinced, however, that the bird was a Blyth’s Reed Warbler. Several observers heard the bird give characteristic ‘tac’ and ‘churr’ calls and the bird was popularly identified as Blyth’s Reed Warbler by most visiting birders. The tail feathers were sent away for DNA analysis and on 29th May 2002 the results (for this bird and another putative Blyth’s Reed Warbler also seen at Portland, in November) were received back at the observatory.

‘Both birds were Blyth’s Reed Warblers – or, at least, their mothers were! The gene that we sequenced is transmitted from mother to offspring, so that the male plays no part. Thus, either the birds were Blyth’s Reed, or hybrids. If hybrids, then the mother was a Blyth’s.’

This was confirmation enough for me. To have seen Pallas’s Grasshopper Warbler, Yellow-breasted Bunting, Paddyfield Warbler & Blyth’s Reed Warbler in the space of a month and all in Southern England is incredible, it makes a trip to Fair Isle less necessary although I’d still love to go.

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