Saturday 30 January 2016

Field Marks - Bold and Bland Remiges and Retrices

Recent rare gull activity in Ireland and Britain has prompted a closer look at this question.  While trying to ascertain the grey scale value of the pale wing-tips of Ireland's first putative Glaucous-winged Gull found by Fionn Moore (HERE) it was quite evident that debate continued to rage based on the variable appearance from different sets of photographs. In any sample of online images there didn't seem to be any consistency in the tonal levels.  Compare for instance THESE and THESE images.  Not surprisingly, the question over the tone of this bird's primary feathers has remained in flux.

Then along came a striking Thayer's-type gull from Aberdeenshire, Scotland.  The observer Chris Gibbins was in an unenviable position of having to leave the bird before clinching all it's features to his satisfaction.   Luckily others were able to capture a suite of images to assist with the bird's identification.  Later, as the first flight images of the bird emerged they appeared to show a dark contrasting secondary bar and everyone seemed happy the bird was a classic Thayer's (see finder's account HERE).  Then as more images emerged it appeared the secondaries were more concolorous with the wing (see HERE).  Once again lighting seemed to be playing with the tones of remiges and retrices.

Structure Of the Remiges and Retrices
Flight feathers including the remiges (wing feathers) and retrices (tail feathers) are more stiff than body contour feathers but still flexible enough to assist birds with masterfully subtle and graceful controlled flight.  They play a vital role in flight by both capturing and directing airflow to give a bird lift and accurate, agile manoeuvrability in the air.  While the feathers overlap and often work in relative unison, similar to the control surfaces of an aircraft, during flight they occasionally part or may have gaps due to moult or damage.  This can alter a bird's flight characteristics and efficiency.  Wing strokes are dynamic and variable depending on such factors as air resistance and lift requirements and the type of manoeuvre a bird is undertaking.  Tracts and individual feathers therefore at any given moment may be straight and level, bowed laterally or longitudinally, or even twisted about the feather centre.  The same applies to the feathers of the tail.  We often think of the tail as a straight, level structure.  But, in flight the tail may be just as dynamic as the wing.  Very often the tail is somewhat bowl-shaped for stable level flight, but may be splayed and/or twisted during complex aerobatics.

Here are a series of pencil sketches based on a number of published photographs of the Aberdeenshire Thayer's/Kumlien's Type Gull including images by Hywel Maggs, Jonn Nadin and Stewart Whooley.

The Bold Versus The Bland 
I have spent a great deal of time on the blog analysing the distinction between bold and bland field marks.  Bland field marks are far more challenging to work with.  For more please see the posting HERE.

Lighting Factors
Allied to the complexity of angles with which birds can hold their flight feathers, light and shade on the retrices and remiges can pose an extremely complex challenge.  Angles play a major role in how tones are presented.  As outlined in the posting on Lambert's Cosine Law (HERE) the brightness of any point on the surface of a subject is proportional to the angle of the surface to the incident light direction.  Feathers which are bowed or twisted can have points on them that are angled 180 degrees or more from other points, revealing simultaneously to the camera both the upper and under surfaces of the feather and all angles in between.

Translucency in another oft ignored factor which is particularly relevant to any analysis of flight feathers.  While most feathers of a bird cover underlying body tissue and bones resulting in a surface that is more or less opaque to light, the retrices and remiges stick out from the body.  Light can readily pass through these feathers, and in doing so can influence the appearance of subtle tones on the feathers themselves.  Great care should be taken to establish the qualities and angle of the light.  The translucency of a feather is directed related to it's pigmentation.  Paler feathers are influenced by light transmission to a far greater extent than dark feathers.  For more on translucency see HERE.

To the outline sketches above I have attempted to overlay the key shadow and lighting detail in the published images.   The shadows are relatively easy to interpret but I cant be so certain of the influence of light transmission on translucent feathers.  It was Stewart Whooley's stunning image (HERE) which sparked the debate about this bird and got me thinking about writing this blog post.  The combination of the fact the primary pattern of the right wing seems so strikingly subdued in this image and the rather bright marginal covert area both lead me to suspect that the primaries and outer secondaries are being lit from below, resulting in a dilution of contrast in these areas.  Note also the primaries of the left wing are well illuminated below (less so in Hywel Maggs's image HERE).

The Role of Flight Feather Tonal Patterns In Identification
For a birding community well versed in subtle bird identification it has been interesting to observe some of the recent debate around issues like the primary tip pattern of the Irish Glaucous-winged Gull and the Scottish Thayer's-type gull.  If you think about it, there aren't too many identification challenges that rely on such a critical analysis of subtle tones in the flight feathers of a bird.  Primary pattern is important in the identification of many adult gulls but generally we are dealing with bold (black and white) patterning and not the subtle contrast created between midtones, or an exact measure of tones along a standard scale.  Perhaps we are somewhat unprepared for the kinds of challenges posed by the Aberdeenshire gull.

To the rough sketches I have overlayed what I think may be the natural pigmentation level of the remiges and retrices of the Aberdeenshire gull.  This is how the gull might have looked under perfect, neutral light.  Of course I cannot be totally certain of this as all the published images have their own lighting characteristics.  This is a matter of trying to piece together a puzzle without all the pieces available.

Top 10 Tips For Judging Tones In Remiges and Retrices
Having been pondering this question for the last couple of weeks I have come up with a list of top tips for assessing subtle tones in remiges and retrices.  There is no 'magic bullet'.  These tips only help to reveal clues to the true nature of a bird's plumage field marks.  As always it is best to have as wide a range of images as possible to work from.

(1) Always assume that lighting is variable across the remiges and retrices and that a proper forensic analysis of tones is fraught with difficulties.
(2) For the best results the light must be diffuse and not too bright.
(3) Be especially conscious of the translucency of flight feathers and the potential for transmitted light to influence the appearance of tones on flight feathers.  Note that translucency can sometimes make feathers appear darker than they actually are - a white feather viewed against a dark background can appear grey under certain light.
(4) Ideally the background should not contrast too strongly with the subject.  Images captured against the sky (or water reflecting sky) can be problematic as they often challenge the dynamic range of the camera.  The result may be an underexposed subject and/or inaccurate contrast between tones on the subject.
(5) Individual tones cannot be assessed in isolation.  As the posts on Greyscales and Gulls revealed (eg. HERE) totally accurate exposure is very difficult to achieve.  Despite this, comparisons can often be made between tones recorded within the same image.  Tones can also occasionally be compared across images provided that the brightness and contrast settings are matched.  But this can be a risky exercise.
(6) Tones within the same image should be judged by comparing surfaces which are totally parallel to one another, and therefore lit in the same way.
(7) Always be mindful of where shadows are falling, especially when trying to compare tones on different parts of the subject.  Remember a shadow can be created by an object outside the frame of the image.
(8) One should never attempt to compare tones in different parts of an image by eye alone.  Human eyesight is not good at global tonal comparisons (see for example my posting on brightness illusions HERE).  I would recommend the use of image editing software to make objective tonal comparisons.  The simplest method is to use something like MS Paint to grab a sample patch and drag it across an image to directly compare alongside another patch.  Human eyesight is very good at making local tonal comparisons.  For a more objective analysis Color Quantizer is an excellent free software programme which I have found very useful for mapping the tonal distribution across an image.
(9) It often helps to compare surfaces which are more or less parallel to the camera plane.  But be especially mindful of translucency, particularly where lighter-pigmented feathers are concerned.
(10) Working in greyscale often helps.  Note the human eye is more sensitive to green than it is to red or blue.  People also have individual varying sensitivities to different colours.  If the colour hue or saturation isn't particularly relevant to your analysis it makes sense to transform the image to greyscale (luminance alone).  Greyscale reduces the comparison of tones to one of tonal luminance (more accurately termed luminosity).  For an explanation of the important distinction between luminance and luminosity see HERE.

The Scottish Thayer's/Kumlien's Gull
I think the published images of this intriguing gull highlight just how potentially misleading initial impressions can be.

Here I have combined my analysis of lighting in various images with my analysis of the overall pigmentation by overlapping the two sketches.  The results seem to match the available online images quite well.  Whether 100% accurate or not the purpose of the exercise is to highlight the complex interplay between variable structure, variable lighting and subtle pigmentation.

The Irish Glaucous-Winged Gull
While the debate around the exact tonal level of the primary-tip pattern of the Irish Glaucous-winged Gull may continue I think it is worth putting my 10 Tips to the test.  I think the analysis below stands up to scrutiny.  The lighting at the time the image was taken was dull and diffuse as the morning was overcast.  Though the image is taken against water the backdrop is not overly contrasting or bright.  There is a rich range of tones in the image which strongly suggests the exposure control was quite good.  The grey scale method relies on a tonal comparison either with a grey card or between tones within the same image.  In this case the comparison is with the grey of the wing.  The wing is reasonably in parallel with the camera plane and shading is minimal.  The tonal comparisons were made by directly sampling different regions of the primary tips and directly comparing with a standard grey scale.  For more see HERE.

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