Dusky Warbler at Stanford Reservoir

… or is it a Radde’s?

Before packing up this morning, the last bird pulled from the nets of the Stanford Ringing Group proved to be a Northamptonshire first but the subsequent circulation of the in-hand images generated a debate over its identity and a good deal of controversy as a result.

A break in today’s somewhat inclement weather provided a two-hour window of opportunity for Mick Townsend to get some ringing in at the usual site in, and around, the Blower’s Lodge Bay area at Stanford Reservoir. MT was just making the final round of checking the nets when he noticed a dull, brown passerine in the bottom pocket of the last net he came to. Upon approach, and with the bird facing away, Mick thought it was a late Reed Warbler but when he came to extract the bird it was immediately obvious that it was a dark phylloscopus warbler with a hugely prominent supercilium. He quickly identified it as a Dusky Warbler, took some biometrics and then set about putting the news out but – sod’s law – his phone’s battery was virtually dead and he just managed to get a text out to Chris Hubbard, a mile up the road, before it died.

Chris arrived within 15 minutes and took the images reproduced here. Because the ringing station is on private land (and access to the reservoir is by permit only) it was decided to release the bird in the scrub in the car park by the inlet. Sadly, for Northants birders, this release area is the wrong side of the inlet, i.e. in Leicestershire!

The bird was duly released at 11.05 and flew to the nearest cover. It was next seen at 11.35, in hawthorns along the Leicestershire bank, about 100 metres from the car park but this was a brief observation after which it disappeared and was not seen subsequently, despite a search by good numbers of observers, who had gathered there since its release.

The images circulated later in the day produced a mixed reaction, with some birders believing it was a Radde’s Warbler. I have to admit it does show some pro-Radde’s features and it had me wondering at the time.

Had it been seen in the field and heard calling then there would have been no debate over its ID but a bird in the hand can be a different matter altogether and the two species can appear so similar that, in some instances, the only sure-fire method to confirm the ID is by measurement of the depth and the breadth of the bill – neither of which was recorded in this instance.

Pro-Radde’s features exhibited by this individual are:

• very long, strong, broad supercilium, extending along the full length of the ear coverts
• very broad, almost blackish eyestripe, almost as deep as the eye, contrasting markedly with the supercilium and, in some images, appearing slightly darker than the mantle
• ‘mean-looking’ appearance
• pale spotted (not plain) ear coverts
• cinnamon under tail coverts
• large orange feet and, from what is visible, palish orangey legs

Pro-Dusky features are:

• supercilium almost uniform in colour, whitish in front of eye in some images (Radde’s is buffish in front of eye, contrasting with paler behind)
• Mantle is wholly brown – confirmed by MT (olive-tinged to strongly olive in Radde’s)
• Edges to closed primaries are pale brown (not greenish-tinged like Radde’s)
• Underparts sullied and dull, lacking the faint, sometimes ‘clean’ yellowish tinge of Radde’s
• Bill shows dark upper mandible with extent of pale limited to cutting edge only (Radde’s usually shows more extensive pale areas); note the left side conforms to this but there is more extensive pale on the right side!

Having studied a number of photographs, Gary Pullan has today put forward what may yet prove to be a new identification feature for these two species, namely that the bill depth at its base is broader than the diameter of the eye on Radde’s Warbler but not on Dusky – and this fits the Stanford bird. GP went on to state that, having seen this bird briefly in the field, the bill appeared weak-looking and the bird did not look bull-necked like a Radde’s Warbler does.

Many – if not all – the pro-Radde’s features, above, can also be shown by Dusky Warbler. The logical ID conclusion is, then, that this is a Dusky Warbler. So, another amazing bird from the nets of Stanford. What will be next and … will we get to see it?

Of further interest, a number of feathers became dislodged when the bird was ringed. These will be sent off for DNA analysis … to be sure, to be sure …