Wednesday 6 January 2016

Field Marks - Grey Scales and Gulls (Part 2)

In the posting Grey Scales and Gulls (Part 1) I found an effective way to replicate the famous Kodak Grey Scale in our digital space (sRGB colour space).  The recent exciting discovery of what appears to be Ireland's first Glaucous-winged Gull (Larus glaucescens) in Co. Cork afforded an opportunity to test out this process in the field.  

I had two methods in mind.
(1) Calibrate scene lighting and exposure using a grey card (the preferable method)
(2) Hope to capture the GWG in the company of another gull species and benchmark the GWG's grey scale against a known standard (less ideal, as potentially less exact).

I arrived at Castletownbere Harbour before dawn on January 3rd.  Together with many others, and in great anticipation we began a search for the bird, found the previous evening by Fionn Moore.  After a couple of fruitless hours searching, and just as hopes were beginning to fade, news emerged of the bird's rediscovery at the main pier...and it was coming to bread!

Glaucous-winged Gull, potentially Ireland's 1st, Castletownbere, Co. Cork, 03rd January 2016.

Standard references (eg. Table 3 of Howell and Dunn, Gulls of the America's, 2007) place GWG in the Grey Scale range of 5 to 6.  It is slightly darker-mantled than Herring Gull (both European L. a. argenteus or American L. smithsonianus) and, certainly initial impressions in the field, in direct comparison with our HGs this bird looked right where it needed to be in terms of it's greyness!

Despite the excitement of seeing such a superb bird at close quarters I set up the grey card and tried to calibrate lighting and exposure in order to calibrate the bird's mantle to my sRGB grey scale.  Though I have managed this technique under less stressful circumstances in the past, unfortunately in this case the results were less than perfect.  In reality, torn between trying to observe the bird, photograph it, video it and now analyse it's grey scale, it was probably a bit much to expect a perfect result!

Resorting to method two I have selected an image containing both a HG and the GWG.  Having first confirmed the exposure of the image is reasonably accurate (by testing that the HG grey scale is within the expected range of 4 to 5 I have been able go test the GWG close by and confirm that its mantle shade is also within it's expected grey scale range for this species as illustrated below.

Shortly after I left the area another related species, an adult Yellow-Legged Gull (L. m. michahellis) rolled up beside the GWG on the same patch of rock and Killian Kelly was able to obtain an image of the two together with an L. a. argenteus HG  There is enough in this image to add further confirmation that the grey scale value for the GWG is within the right ball park of approximately grey scale 6.  L. m. michahellis falls in the grey scale range 6 to 7 as shown in the illustration below.

Another useful comparison available between Yellow-legged Gull (upper 2nd from left hand bird) with Herring Gull (left hand bird) and Great black-backed Gull and the Glaucous-winged Gull (right hand bird).  Thanks to Killian Kelly for allowing me to post his great image.

There are a couple of important caveats to consider when applying this relative comparison method.

Sample Point Selection
Lighting on three-dimensional subjects paints a range of tonal levels as we all know, so how do we know where on each subject to take comparable samples from?  As discussed in my earlier posting on grey scales and gulls, the upper mantle seems to be the most consistent place to sample grey from.  Killian's image above is a good example of why this is the case.  Typically, under normal natural lighting conditions where gulls are at rest, lighting on the upperparts is at it's most consistent on the upper mantle area.  This is also coincidentally where many large gulls first develop a large uniform patch of grey scale plumage during their 2nd or 3rd calendar year making it possible to use this method for different gull age cohorts.

Lighting Varying With Perspective
I have spent a good deal of time pondering the question of lighting and perspective in postings HERE and HERE.  There are various reasons as discussed in those postings why the lighting and therefore the pattern of tones will vary between identical subjects positioned in different places across the same image.  Good sample point selection can help with the problem but I think we must allow for a small margin for error in a comparative analysis of samples.  I don't believe this margin is more than 1 grey scale in this kind of analysis.  Some of the factors that influence the result include direct versus diffuse lighting, lens focal length and therefore field of view and vignetting, the subject's posture and relative distances of subjects from the camera etc,

We are helped in this particular case by personal observations in the field which confirmed that this GWG was only slightly darker than nearby HGs and, from experience somewhere in the YLG or argentatus HG sphere of greyness.

Last Word - Primary Grey Scale
There has been a certain amount of disquiet expressed by some who have seen the bird in life that the primaries seem excessively dark in some cases including from some photographs.  Among some of the photos that have appeared of the bird online I sense an overuse of contrast which certainly will make the primaries appear darker relative to the mantle.  This for me raises an interesting question.  Just what is the maximum allowable grey scale value for the primary pattern in GWG?  At what point should we begin to suspect a 1st generation hybrid?  I am going to put this question to the Frontier's of Bird Identification forum and will report back on any useful findings.  This last image shows the left wing primary pattern in reasonably diffuse, low contrast lighting (note P10, the outer primary is missing).  From my own observations of GWG in Vancouver in July 2010 I certainly saw birds that matched the pattern of the Irish bird, but how many of those were 1st generation hybrid and hybrid back-crosses with pure GWG?  Will a DNA sample even provide us with the answer?

Based on a direct comparison between the grey of the wing and the darker primary tip pattern I have arrived at a grey scale value of 11 to 12 for the primary tips of this individual.  The question is where does this sit within the normal maximum pigmentation range for GWG?

Here is a brief video of a flock of GWG's which I took while visiting Stanley Park, Vancouver in July 2010.  Note especially the primary pattern of the incessantly calling adult bird which shows roughly the same level of primary grey as the Irish bird.  Note also, out of interest, the rather bleached immature bird.

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