Monday 2 February 2015

Field Marks - Feather Centres & Subterminal Marks

Feathers can carry a great array of plumage patterns.  Diagnostic patterns form field marks which are used for identification purposes in many species.

Subtle Patterning
The centres of the feathers of many birds are solid or fairly uniform in colour and tone, free of any patterns.  And, that is how they are generally depicted in field guides  However, lighting can give the impression of false plumage patterning or tonal variation.  Because feathers do not sit flat on the surface of a bird there will invariably be a gradient created by the varying angle of parts of the feather relative to the observer/camera and the light source.  Take the Little Stint (Calidris minuta) and Semipalmated Sandpiper (Calidris pusilla) comparison below.  The exposed feather centres of the upper wing, including the scapulars and tertials of juvenile Little Stints are typically a fairly uniform blackish.  In juvenile Semipalmated Sandpiper the upperwing feathers show a very different pattern, consisting of a gradient from pale feather bases towards darker tips, combined with a dark shaft streak.  The effect is like a diffuse, dark arrowhead or anchor shape near the feather tip.  This is considered a very useful difference between Little Stint and similar species, but in this particular shot, due to the sun's angle, there is an impression created of a very similar diffuse gradient of tones from the base to the tip of each feather in this juvenile Little Stint.  This impression is due entirely to lighting and only occurred momentarily from the observer's position.  In the field, as the bird moved about, the true pattern of these feathers would have been clearer.  But, such is the nature of a single photograph - the bird, it's lighting and composition are all frozen in time.  The lighting is similar in the Semipalmated Sandpiper shot but the angle is slightly better.  The primaries give a useful clue to the very different lighting going on in these two images.

Clearly these two birds are at the more typical ends of their respective plumage spectra.  So, there are plenty of additional clues to their true identities.  But, if one were faced with an exceptionally grey or worn Little Stint or a more richly-coloured Semipalmated Sandpiper the challenge posed here might have been more difficult.

Identification of Stints and Peeps by the late Peter J. Grant and Lars Jonsson (British Birds: 293-315, July 1984 - HERE) is a classic.  Peter J. Grant was a pioneer of The New Approach to Identification and Lars Jonsson's plates are simply breathtaking.   It is interesting to compare Jonsson's plates (eg. 113 and 114) with the image above and to note Jonsson's very subtle yet perfectly accurate use of lighting.  The problem of course is that such optimal, neutral lighting conditions, which are so typical of most illustrated bird identification plates, are not often encountered during field observation, and are even more challenging to capture as accurate digital images.  And yet, there is possibly no other way to meaningfully treat these birds on a single plate - I find the results to be mesmerising each and every time I revisit this paper.  Arguably no collection of photographs could have done this subject more justice.

So, it is hard enough to cover a complex identification puzzle without having to consider lighting and other photographic challenges as well.  Of course Grant and Jonsson were well aware of the various challenges with photography and referred briefly to these in their paper (page 300).  This whole blog in many ways is elaborating on the concepts briefly discussed on that page.

The image above roughly illustrates the impact of lighting and exposure on subtle plumage markings. Below I have a worn juvenile Semipalmated Sandpiper photographed in dull light.  The tonal range is good and the contrast low, allowing for a much richer appreciation of the subtle tones in each feather than in the image shown above, however colours are possibly not as saturated as they might have been in brighter sunlight.

Bold Patterning
Detail is much more likely to be captured and interpreted correctly in a photograph when plumage markings display natural contrast.  Juvenile Eurasian Turtle Dove (Streptopelia decoacto) starts out with diffuse-centred scapulars and coverts, moulting to contrastingly dark-centred, broadly rufous-fringed 1st-winter feathers.  The moult contrast is very obvious late in the autumn.

While it might seem like a contradiction in terms, some bold patterning is actually subtle in other ways.  Take the Spotted Sandpiper (Actitis macularius) below.  It combines pale and dark subterminal bars on its upperparts, enhancing the visibility of plumage markings as a result.  But it appears as though these markings are intended to be bold at close range but less so at a distance.  The subtlety lies in the choice of colouration.  Still, these more sharply-defined plumage features are certainly easier to interpret than for example the arrowhead markings shown by Semipalmated Sandpiper above.

However, there is also a flip-side. Digital image sharpening tools and the human visual system use a similar method to enhance contrast and acutance (the apparent sharpness of edges).  This means that if the image is low in resolution it may be hard to distinguish between this form of plumage pattern and similar-looking digital artefacts.  Beyond a certain range, all markings, however bold cannot be resolved.

Markings like these are relevant in some identifications.  In Spotted Sandpiper and its European relation the Common Sandpiper (Actitis hypoleucos) the exact pattern of these markings on the tertials and coverts help clinch an identification.

Here is another example of a bird that utilises bold markings but in a subtle way.  This is a Rufescent Tiger Heron (Tigrisoma lineatum) from Venezuela.  At very close range its bold camouflage markings are apparent but at range they are scarcely visible.

Cryptic Plumage
While on the subject of subtle camouflage I might as well briefly look at the question of cryptic plumage.  There are probably no hard and fast rules for the accurate capture of plumage field marks when it comes to these birds because the sky is the limit as regards markings, colours and tones,

Many cryptic species are nocturnal.  For photographing nocturnal species, obviously light availability is the biggest challenge.  The Great Potoo and Paraque below were both photographed under relatively low torch light.  The birds stayed motionless for long periods, allowing for longer exposure times and a close, careful approach using a vehicle as a hide.  Long exposures can be deceptive - any subtle movement (eg. feathers moving in the breeze) can create motion blur which can mask plumage features.  This blurring can easily go unnoticed especially if the exposure is particularly long.  Shadows can also be deceptive in these artificially lit images.  For shyer, more active species including Owls getting useful images can be very difficult.  Photography under artificial light requires careful white balance correction.

But of course not all cryptic species are nocturnal.  Nocturnal species presumably developed cryptic plumage mainly to avoid being predated upon by diurnal predators as they slept.  Not a bad tactic even when you are awake!  There are plenty of diurnal species with complex cryptic plumage as well, eg. Eurasian Wryneck and Sunbittern shown here.  It was a revelation to see just how well camouflaged this Sunbittern remained even while actively displaying.

For many species with cryptic plumage there are few if any confusion species to be concerned with so accurate capture of cryptic plumage markings may not be a consideration.   But for many (eg. Snipes), identification does often rely on obtaining high quality photographs.  The subtleties of image quality parameters and forensic image analysis don't probably come into the discussion in that instance.  Poor quality images simply will not cut it - enough said.

Sunbittern (Eurypyga helias), Venezuela

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