Monday 19 January 2015

Field Marks - Lighting and Bare Parts

In Birds and Light - Translucency I outlined the difference between opaque, transparent and translucent  materials.  Avian bare parts are translucent in nature, which means that a certain proportion of the light hitting a bird's bill and legs passes through those structures.  In doing so, these are illuminated internally.  This property can dramatically alter the appearance of bare parts under different circumstances.  

Take for instance the Booted Warbler (Iduna caligata) pictured below.  I twitched and photographed this confiding individual in Ballycotton, Co. Cork in early September, 2004.  It was only the second Irish record at the time and coincidentally was followed by the 3rd Irish in Co. Mayo later the same day.  The day was exceptionally mild and sunny and the bird performed well in this bright sunshine.

The sub-terminal mark on the base of the lower mandible is a relevant field mark in this species but I struggled to get any good photos of it due to the strong light.  In the main image, inset, the bird is illuminated by strong sunlight from the right as evidenced by the sun's reflection on the bird's eye and bill.  There are also shadows from twigs falling on the subject's nape and side.  The bill is brightly illuminated and it's subtle tones are somewhat obscured as a result.  The lower mandible appears to be mainly flesh-coloured.  In the smaller inset image the bird is clearly in silhouette.  The bill colour appears much different in this image, with a richer orange colour to the lower mandible.  Interestingly, the darker parts of the bill, the upper surface of the upper mandible and the sub-terminal mark on the lower mandible are much more visible in this image than in the other, better lit image.  The pattern of darker areas being subdued while lighter areas are illuminated is consistent with translucency.  Light passes through the paler areas more freely and therefore they appear brighter.  What is most interesting however is that this bright orange colouration is probably due to the colour of the bird's mouth, not it's bill.  The internal structures of translucent objects as well as their colours are illuminated by light passing through them.

Birds of the Western Palearctic (BWP) describes the colour of the bill of Booted Warbler as follows.  Upper mandible and distal half of lower mandible dark brown or brown black in caligata.  Base of lower mandible pale flesh, pale yellow, orange-flesh or orange-horn.  Mouth pale lemon-yellow, bright yellow or orange-yellow.  Note the description of the lower mandible and mouth includes the related I. rama (caligata and rama were considered conspecific when BWP was published).  

As the image above shows, the bill colouration varies dramatically due to lighting.  As stated, the bright orange colour when the bill is back-lit appears to be due mainly to internal illumination due to translucency.  The image below illustrates just how vivid orange the bird's mouth actually is.  This undoubtedly influences the colour of the bill when it is illuminated in strong light, and especially when it is viewed while being lit from behind.

These images were digiscoped with a Nikon Coolpix 4500 and Leica Televid scope.  Note the bill graphics above are only an approximation in structure, pattern and colouration, based on various photos and are used here merely to illustrate the specific point.

The legs in many birds are equally difficult to pin down in terms of pattern and colouration.  As this image shows, bright sunlight can reflect more strongly from the bare parts than from plumage, obscuring the true colour of the bare parts.  The description of the leg colour variation of Booted Warbler in BWP is too long to repeat here.  Natural variation only serves to further compound the problem posed by lighting and other environmental factors.

Remaining bare parts include the eyes and bare skin, eg. the orbital ring around the eye, wattles and facial skin in parrots and vultures.  Judging the colour of the iris and bare patches of skin is similar to judging bill and leg colour.  It depends on lighting, exposure and of course white balance.  Occasionally dark eyes may appear paler than they should be owing to a peculiar light angle in which sunlight reflects and illuminates inside the eyeball, but for the most part lighting tends to be fairly predictable.  Of course, one of the great advantages of the surface of the eye-ball is it's facility to allow us judge the sun's position more or less.  Other factors to consider here include include moisture, natural oily or waxy coatings on bare parts, soiling from earth, pollen or other foreign bodies, scaling and abrasion due to the elements, etc., etc.

I have included this close portrait of a Ring-billed Gull (Larus delawarensis) to illustrate the complex colouration within bare part structures when viewed under optimal lighting.

In summary, lighting plays a major role in how field marks are displayed, not least on bare parts.  This posting illustrates how the internal colouration of a bird's bill can actually influence it's external appearance under certain lighting conditions.  All of this compounds an already complex challenge of trying to accurately depict patterns and colours in bare parts.  One needs to be especially critical when trying to gauge colour and patterns in these areas and should not be too concerned or surprised if a field mark appears to be missing.  In all likelihood the mark is simply obscured.  As always for the best results look for the images captured under duller, more diffuse light and be wary of images captured in bright sunshine.  It's also probably best to avoid drawing strong conclusions where translucency is at play, given the potential complexity of internal structures being illuminated.

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