Accepting the Evidence: a Retraction & a Change of Diagnosis

Back on the 8th April I posted about catching a Wood Warbler at Lower Moor Farm. I was very excited about it: I have seen plenty as a birder but had never caught one to ring. As a result, I spent a long time looking into the bird to try to make sure my diagnosis was correct. My decision was made on the following basis:

  • Wing Length: at 74mm it was longer than the longest given wing lengths for any Willow Warbler in any text I have access to (Svensson, Demongin, BWPi)
  • Head size: compared to the Chiffchaff taken out of the net immediately adjacent to it, the bird’s head was at least 50% bigger
  • Bulk: it was just bulkier than any Willow Warbler that I have processed (338 at time of writing)
  • Colouration: the undersides were pure white, the legs and lower mandible were rose brown, indicative of a second year Wood Warbler rather than Willow Warbler
  • Wing conformation: the photograph that Anna took of the spread wing looked identical to the photograph in the second edition of Jenni & Winkler of a second year Wood Warbler wing (Fig.36 in that publication).

I asked on the original post for alternative opinions. Despite a lot of views nobody disagreed with my diagnosis. That information was also shared with two of the senior members of the group, with over 80 years ringing experience between them (no insult intended to the rest of the group: they are a lot younger than us old fogeys, and consequently have less experience). Initially one agreed with my diagnosis, the other said he would do an analysis of the data and come back to me. When he did, his diagnosis was Willow Warbler. His reasons for this were:

  • Colouration: the complete absence of yellow on and around the neck region.
  • Weight: within range for a Willow Warbler
  • Tail: within range for a Willow Warbler
  • Primary 1, when counting descendantly, was longer than the primary coverts, an absolute diagnostic characteristic of Willow Warbler. Wood Warbler P1 is always shorter than the primary coverts.

Of these, the colouration is as shown in BWPi and the Concise BWP for second year Wood Warbler. So the lack of yellow does not immediately disqualify it as a Wood Warbler. The weight and tail are both within the normal ranges for both Wood Warbler and Willow Warbler. However, the weight is mid-range for a Wood Warbler and towards the top end of the range for Willow Warbler. One would have thought that a newly arrived migrant would have weighed lower in the scale. Of the 489 Willow Warblers processed by our group in the month of April, 11 have hit that weight or more, so it isn’t unique. Whilst the wing length is outside of the extremes given in the texts mentioned, looking at our group records there is a record of a Willow Warbler with a 75mm wing caught and ringed at Swindon Sewage Works on 19th August 2012 by an experienced ringer, so I expect the record to be accurate.

However, the one thing that cannot be overlooked is that first primary exceeding the length of the primary coverts. Having reluctantly accepted that to be the case, one of the more experienced members of the team suggested Phylloscopus trochilus acredula. Checking that bird out: it all matches well to the bird that I caught, including primary one exceeding the length of the primary coverts. As a result, I have changed the record to reflect that subspecies of Willow Warbler. Accepting that this is the correct identification one has to put the head size and seeming bulk down to clinal variation.

Having not had any experience of this subspecies, I have had a look at the details of the bird and this subspecies summers from Scandinavia across Siberia to the River Yenisey. Online resources label it as the Northern Willow Warbler. I asked the BTO if they could let me know how many of this subspecies have been ringed in the UK. Including this bird there have been 21 identified as acredula and ringed in the UK and 3 in Ireland. Interestingly, the first record was on the 23rd July 2017 – in Wiltshire. This is the second for Wiltshire. The majority have been caught in Dorset (nine) and there have been two caught next door in Gloucestershire.

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