Sunday 25 January 2015

Field Marks - Shaft-streaks and Tramlines

Of all the typical patterns found on feathers, shaft-streaks can be among the most subtle.  And yet, such a simple pattern may be all that is needed to create a complex and effective camouflage.  If the feather shaft or rachis contrasts with the surrounding feather barbs, the rachis itself forms a very fine shaft-streak.  Such subtle markings are rarely detectable in the field.  However, add even the faintest, diffuse pigmentation along the edges of the rachis and we now have shaft-streaking which we can actually see, perhaps even at long range.

When we combine these marks with the typical anatomy of a bird (HERE) we start to see complex yet consistent patterns emerge.  Feather patterns as subtle as these allow a bird to blend with its surroundings to evade detection by predators.  Wherever feathers run in consistent lines these fine marks begin to aggregate into more prominent features such as crown streaks and 'tramlines' on the mantle.  Wherever feathers are less perfectly aligned, these marks are often far less prominent and appear as finer streaks.  The problem of course, is that these aggregate field marks (eg. tramlines) are subject to a bird's posture and may quickly vanish if feathers are not aligned perfectly.  Under such circumstances a bird may appear unrecognizable.

This image of a Buff-bellied Pipit (Anthus rubescens) illustrates a lot of the subtle points relevant to this discussion.  On the scapulars there are some very subtle diffuse shaft streaks.  One would have to look very closely to pick out any dark pigmentation in the centre of these feathers.  At range these feathers are going to look uniform.  On the crown, mantle and flank the shaft streaks are more prominent, but note how much more prominent the streaking is where the feathers are neatly aligned, such as on the crown and mantle.  The streaks on the upper flank tend to be more dispersed.  I have attempted to illustrate these points more clearly below.

In order for shaft-streaks to aggregate into larger marks the streaks must reach the feather tip and of course the feathers need to align perfectly.  This approach works best on the upperparts and is more hit-and-miss on the underparts, owing to the greater size, downiness and mobility of underparts feathers.  I guess if, as a bird your concern is typically a bird of prey quartering overhead, it is probably more important that the tight upperparts camouflage works well.

This particular Buff-bellied Pipit was joined by a second bird, remarkable in being only the 3rd and 4th Irish records at the time, but proving to be part of a significant influx of six birds in 2007 (more HERE).  As an aside, it was interesting to compare the pattern of the two birds that occurred together in Lissagriffin, Co. Cork.  One couldn't imagine two more different-looking individuals.  The four subsequent birds also differed significantly enough to be easily separable from these and one another.  Among all the intra-specific variation however the mantle pattern appeared fairly constant in these birds.  Together with the distinctive call, the fine mantle shaft-streaking has proven to be perhaps the most useful field mark for birders searching for Buff-bellied Pipits over here.  

Lighting and Composition
I have written generally about this pair before (HERE).  Lighting affects the contrast of field marks.  Shadows can easily be mistaken for streaking and some shadows consistently follow the contours of feathers (as illustrated HERE).  So what is the best lighting and composition for accurately gauging and analysing shaft-streaking?  The best viewing angle for any feature is one in which the feathers are viewed in profile, perpendicular to the line of sight and where the sun is behind the observer.  Diffuse light is far preferable to direct sunlight as contrast is less so there is a greater potential to preserve tonal range.  Tonal range is really important here, because we need a range of tone to depict the full gradient of tonal levels in a diffuse field mark.

Lighting Tools
If lighting or exposure has been less than optimal lighting tools (HERE) can help to unmask field marks that might otherwise be hidden in an image.  Shaft-streaks can be problematic however as they are often diffuse in nature, which means that they require a broader tonal range for accurate display than more clearly defined field marks.  Dynamic range limitations may obliterate these subtle gradients and similarly overuse of contrast tools, unsharp mask or other tools that impact on image contrast and tonal gradients need to be used with care.

Other Factors
Feather displacement, water and soiling can all dramatically alter the normal pattern of feathers.  We just need to be on the lookout for these and factor them in to our analysis. All of the standard image quality parameters are also at play here.  Clearly, distance from the subject is very relevant so image pixel resolution is important.  Fine detail requires high resolution, pure and simple.  Focus dissolves fine marks and reduces contrast, so subtle, diffuse pigmentation is also dissolved.  Under and over-exposure also both reduce contrast and may therefore obliterate diffuse edges of shaft-streaks.  Poor white balance introduces colour casts which again can mask subtle, diffuse patterns.  Lastly various image artefacts can either mimic or mask shaft-streaking.

In summary, shaft-streaking is a simple but highly effective means of forming complex patterns and camoflague in plumage.  In some species shaft-streaking may be an important field mark.  That field mark may consist of an individual feather streak or an aggregate of feathers in which shaft-streaks are aligned to form larger streaks or tramlines.  At a micro-level, accurate capture and analysis of shaft-streaks requires good lighting and exposure owing to the tonal range required.  All of the standard image quality parameters are at play, making this one of the most challenging field marks to accurately analyse in digital images.

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