Tuesday, 19 January 2016

Field Marks - Grey Scales and Gulls (Part 3)

Vega Gull, Duncannon, Co. Wexford, 10th January, 2016 (c) Killian Mullarney.

News Flash!  Global Upsurge In Interest In Grey Scales
Doubtless, the recent rise in interest in the blog was sparked more by a certain interesting rare bird than an interest in obscure image quality and analysis tools!  Perhaps I should add more rarity spice to my postings from here on.  In any case hopefully this topic has struck a chord.

In the earlier posting I promised to provide an update on the comments received from North America regarding the Irish Glaucous-winged Gull.  Here is a summary.  While the comments certainly point more towards Glaucous-winged Gull parentage there has been a lot of concern from those familiar with the hybrid zone of the North American Pacific Northwest suggesting the bird might be an American Herring Gull X Glaucous-winged Gull hybrid or back cross.  This is not just on the basis of the darkness of the primary tips but also due to the bird's relatively slight bill, among other subtle points.  Having said that the Irish bird may be a perfectly good match for many Glaucous-winged Gulls wintering in Asia.  Few North American commentators would claim a thorough knowledge of the birds breeding to the far northwest of the breeding range and wintering on the other side of the Pacific.

What I have found perhaps most intriguing from the discussion is the apparent disparity between the quoted grey scale range for the primary tips of Glaucous-winged Gull (eg. 6-8 per both Howell & Dunn and Olsen & Larsson) and those, clearly darker-tipped birds wintering in East Asia.  There is a suggestion that darker wing-tipped birds may represent an older hybrid swarm that is now stable.  This in turn might account for their apparent exclusion from the quoted grey scales in the literature.  But that is just speculation on my part - I have not been able to establish how the range 6-8 was first established and whether it is representative of the whole breeding range of GWG or just a sample, 'pure' population say from Homer, Alaska.

Hopefully the grey scale tool as presented here will provide at least a means to objectively analyse grey scales from photo collections and help develop this debate a bit further.

Vega Baby!
No sooner had discussion of the GWG begun to simmer down here, Killian Mullarney revealed he had found, in his home county of Wexford, what just might be a first for the Western Palearctic - a Vega Gull (Larus vegae or L. argentatus vegae or L. smithsonianus vegae depending on what taxonomy you choose to follow).  After two failed attempts to catch up with this marvelous beast Killian has kindly allowed me to carry out some analysis of his own images using the grey scale method.

While the image at the top of this post is beautifully exposed, the problem with the comparative method is it relies on a reference point.  So, in the absence of a grey card or some other reasonably reliably grey scale value it is not really possible for me to confirm the accuracy of the exposure of this image and therefore to gauge the mantle grey scale of this bird from this image.  

Killian also however provided another image showing the Vega Gull accompanied by a group of Herring Gulls and this is all that is required to verify the Vega's grey scale value.  Vega Gull is slightly darker than American Herring Gull (L. smithsonianus) and more in line with Northern Herring Gull (L. a. argentatus) here in Europe.  The birds alongside the Vega in this case are all of our local race, L. a. argenteus which are within the same tonal range as AHG (4-5).  By adjusting the image brightness to bring the mantle shade of the HGs in line with this on the digital grey scale its then just a matter of sampling and reading the VG's mantle shade.  And the Vega is right where it should be (7-8).  QED.

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