Saturday 7 February 2015

Field Marks - A Summary of Field Marks

To borrow from gaming and virtual reality parlance, field identification is an immersive experience.  Experienced birders use all of their senses plus their experience when out in the field.  Identification of birds from photographs is a far more limited sensory experience.  But what it may lack from a sensory perspective can be more than made up for using extra-sensory skills - namely the forensic tools that help us get more from our images.  So, photo identification is also an immersive experience, but a rather different one than that to which we are accustomed.  When we compare these two 'realms' we find that certain aspects of field identification and photo identification are common to both.  The majority of my postings over the last few months as it happens have fallen within this area of overlap including of course my recent focus on identification through field marks.

I started looking at field marks first from the perspective of typical avian anatomy, feather tracts and the anatomical contours of a bird.  This developed into a discussion about lighting and how light and shadow normally interacts with the anatomy of a typical bird.  Afterall a photograph is merely a light map.  To develop that analogy further, if we understand topography and can visualize contours and land marks we can read a topographical map.  Avian topographical features might be considered akin to topographical features on a map.  The contours of a bird are like the contours on a map.   Field marks are the land marks we are looking for to help us figure out where we are.  If we misinterpret what we see we will be led astray.  By extension, if a map is bland and featureless and we have no landmarks to guide us we cannot find our way.

Having discussed avian anatomy I took a look at avian bare parts, again with a particular focus on lighting.  The translucency of bareparts (and indeed plumage) introduces an interesting additional dimension and potential source of confusion.  I then looked at a range of typical plumage field marks working from the centre of feathers outward.  I started with shaft-streaks and tramlines,  Next I considered the body of the feather in feather centres including subterminal markings.  Lastly to feather fringes, notches and tips.  The final posting in the series touched on colour field marks.

So, what are the conclusions from this brief overview of field marks?  Well, firstly it is clear that the environmental lighting factors which confound field identifications also largely impact on identifications based on photographs.  Lighting and shadows can play tricks on the observer.  When faced with an individual photograph, lighting can really throw the unwary.  But, with a bit of care and experience it is possible to read the scene lighting and take account of it when analysing image content.  It is hard to best a good quality photograph when it comes to critical, forensic analysis.  A single image captured in a moment can reveal more information about a bird than hours if not years of field observation.

Having reviewed different types of plumage field marks what probably stands out most is that it is the tonal complexity of any given field mark that possibly matters most.  Image exposure and dynamic range are key to the capture of any field mark in an image.  It helps if the field mark is quite uniform in tone.  If there are a gradient of tones there is increased risk that the marking will be at least partially obscured in an image.  The worst conditions for capturing many field marks are bright sunny days with broad dynamic range.  The best conditions for capturing all field marks but most especially subtle markings and colours are in bright but diffuse (i.e. overcast) light.

It is clear that the correct ageing, pattern of moult and wear and even feather alignment can all play a key role in many identifications.  Being able to analyse this information relies obviously on a good understanding of the criteria and field marks first and foremost.  It also relies on being able to patiently study and read an image correctly.  Last, but not least, the image needs to be of good enough quality to allow for such a careful and critical analysis.  In other words, there are no short cuts!

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