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BTO Cornish Alder Flycatcher revisited

 
 
This page contains 15 reader comments. Click here to view (latest Fri 15/10/10 16:56).

Below is a copy of an article written last year (by myself and Kester Wilson, the ringer who caught the bird) outlining the identification of the Nanjizal Empidonax flycatcher. Although this bird was accepted by BBRC as an Alder Flycatcher, it has since been returned by BOURC for further investigation. Part of the problem may lie in the fact that the determination of sex by biometrics was at odds with the genetically proved sex using skin cells retrieved from a dropped feather. This further highlights the difficulty in identifying individuals of this group.

So although this article may not help the debate, it should be an illuminating read for all.

Alder Flycatcher
Alder Flycatcher, Nanjizal Valley, Cornwall (Photo: Kester Wilson)

Interestingly, this bird was in remarkably good condition, with a fat score of 4 (on a scale of 0 to 8) and a muscle score of 3 (on a scale of 0 to 3). For more details on these methods, see:

The tail length of this bird was at the top end for Alder Flycatcher, and, coupled with the fact that males tend to have more pointed wings (p10 = p6 on this bird), the bird was presumed to be a male.

Alder/Willow Flycatcher can be difficult to distinguish in the hand, and the key is a set of measurements which produce two discriminant formulae, as described in Pyle's Identification Guide to North American Birds Part I (1997). To a degree these formulae describe the overall shape of the wing, with Alder Flycatcher generally having a more pointed wing shape than Willow.

Formula I

(longest p – p6) – (p5 – p10)

On the Nanjizal bird, the difference between the longest primary and primary 6 (counting from the outside) was 6.3mm (this difference itself suggests Alder Flycatcher, as 95% of Willows have a maximum of 5.9mm). The measurement of p5 – p10 was ‑3.2mm (i.e. p10 was longer than p5, as could be seen in the field). Again, this difference suggests Alder Flycatcher, as 95% of Willows have a minimum value of ‑3.1mm.

So the result for Formula I is 9.5mm. This can then be graphed against the bill length (measured to the nares), putting this bird outside the range for Willow Flycatcher.

Chart
Reproduced with permission from Pyle (1997)

Formula R

[(longest p – p6) + (p9 – p5) + (wing – tail)]/[(p6 – p10) + bill]

In the Nanjizal bird, this came out as:

[6.3 + 8.8 (70 – 59)]/[0 + 8.5] = 3.07

This value falls outside of the range of 95% of Willow Flycatchers (values of 0.98 to 2.91) but well within the range for Alder Flycatcher (2.41 to 4.68).

Alder Flycatcher
Alder Flycatcher, Nanjizal Valley, Cornwall (Photo: Marcus Conway)

The information in this article was believed correct at the time of writing. BirdGuides accepts no responsibility for errors, or for any consequences of acting on information in the article. The opinions expressed are those of the author(s) and are not necessarily shared by BirdGuides Ltd.

hide section Reader comments (15)

#1
Erm... actually Mark - as I understand it the fact that it was a female further supports the identification as Alder, rather than casting doubt on it. The file was returned to BBRC because Kester only submitted biometrics from one wing. Not sure why he did that but not unreasonably BOURC wanted to see the data for both wings. I understand this has now been rectified and the the measurements for the second wing are every bit as much in favour of Alder. Chris
   Chris Hewson, 30/09/10 11:16Report inappropriate post Report 
#2
However: "The tail length of this bird was at the top end for Alder Flycatcher, and, coupled with the fact that males tend to have more pointed wings (p10 = p6 on this bird), the bird was presumed to be a male."
   Mark Grantham, 30/09/10 12:05Report inappropriate post Report 
#3
How is this relevant to what I have posted? It was initially assumed to be a male on biometrics - the DNA proved otherwise - as it was a female, the biometrics are even more in favour of Alder. This is the story as told by Kes on several occasions, including this morning when he asked me to post the above.
   Chris Hewson, 30/09/10 13:42Report inappropriate post Report 
#4
Can you shed any light on the rumour that only one person took the biometrics (Kester?) and as such notwithstanding his obvious skill this is not resilient enough for a first for Britain as there will always be a discrepancy if several people take biometrics from the same bird.
   Mark Thomas, 01/10/10 09:25Report inappropriate post Report 
#5
I don't know - I'll ask him. I know he processed the bird with Mark Warren - whether they both took biometrics is another matter. However, as I understand it the biometrics are clear-cut, not marginal, in favour of Alder. Measruement error always exists but differences between well-trained observers are usually relatively small. The measurements required for the formulae don't include those which typically have a larger error (e.g. tarsus) but on the other hand, I understand there were small nicks in some primary and tail feather tips which could make measurements less accurate....
   Chris Hewson, 01/10/10 09:47Report inappropriate post Report 
#6
Mark and Kester did the biometrics for one wing each - so not repeated measurements of the same thing but independent measurements of different wings that both came out as Alder. They both took the bill measurement and got an identical length (bill to nostrils is much more repeatable / less error prone than bill to skull). Additionally, photos of the wing(s?) held flat against a ruler were also submitted so the appropriate measurements could be read from the photo (degree of accuracy depending on the plane of the photo relative to the plane of the wing, I guess).
   Chris Hewson, 01/10/10 11:34Report inappropriate post Report 
#7
Crikey, if this is where we are with the Cornwall bird, I think anyone seeing the Blakeney bird won't be adding it to their list anytime soon - not just saying that 'cos I dipped it.
   Mr Lawton, 02/10/10 08:24Report inappropriate post Report 
#8
How is multiple measurement of both wings by several people compatible with putting the welfare of the bird first at all times? And if there was a DNA sample taken, surely that proves the identity as reliably as it proves the sex?
   Michael, 02/10/10 09:21Report inappropriate post Report 
#9
Michael - not sure if you are suggesting that was what the ringers did (as opposed to it being good that they didn't do that!). In any case, to clarify, there were no "multiple measurements of both wings by several people". Kester and Mark measured one wing each, once. On the second point, a feather which it was hoped would provide a DNA sample to prove the identification was collected but the tissue sample was so small that it could only be used to determine sex, not species.
   Chris Hewson, 02/10/10 19:37Report inappropriate post Report 
#10
Hi Chris - yes, I was thinking that the Cornish ringers were behaving responsibly in not doing what it appears BOURC would have wanted them to do.

Thanks for the clarification on the DNA issue. Odd though; I'd have thought one cell was enough (a bit late now, but they could even have cloned a cell, if they needed a larger sample, tho' that might be rather expensive).
   Michael, 03/10/10 14:17Report inappropriate post Report 
#11
There's only so much you can do with DNA and with the two of three skin cells we had to work with, it wasn't possible to replicate it. We tried more than once, but just didn't have the material.
   Mark Grantham, 03/10/10 14:44Report inappropriate post Report 
#12
The difference in wing measurement between ringers is not due to ability to read a rule, but the different thumb-pressure that each applies to straighten the natural curve of the wing. This could easily result in several mm difference. Therefore, a photo of the wing against a rule is not conclusive.
   Chris Bradbury, 15/10/10 16:08Report inappropriate post Report 
#13
The photo of the wing against a rule was to show the relative lengths of the primaries, not the wing cord length. I don't think that wing chord is so difficult to measure / interpret that it requires photographic evidence (and as you hint towards, you could take photos showing all sorts of measures and they'd all look equally valid from the photo...)
   Chris Hewson, 15/10/10 16:15Report inappropriate post Report 
#14
Good point Chris, but (usefully in this case) the American method is to take an unflattened, unstraightened chord, so is easier to measure from a photograph.
   Mark Grantham, 15/10/10 16:32Report inappropriate post Report 
#15
Yep, that is a good point too - the Pyle formula requires minimum wing cord as Mark describes rather than the maximum wing cord usually taken in this country, so no flattening or straightening is required. But I can confirm it was, in any case, relative lengths of the primaries that the photos were illustrating, not wing cord length...
   Chris Hewson, 15/10/10 16:56Report inappropriate post Report 

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