Birds and Light - Against The Sky (Part Two)

In Part One I highlighted why photographing birds against the sky is one of the major challenges in bird photography.  The sky is always brighter than our subject.  This plays havoc with image  metering and exposure and also challenges the dynamic range of the camera.  The result is often an underexposed image with a limited tonal range.  Lighting also varies greatly throughout the day, resulting in very varied images.  Swifts by their nature are typically photographed against the sky and there is no greater ID challenge here in Europe than a Common Swift Apus apus versus Pallid Swift Apus pallidus puzzle. 

These are classic Pallid Swifts which I photographed recently near their nests at Castro Verde in Southern Portugal.  With good views, the overall brown, prominently scalloped plumage, contrasting dark eye and mask, broad, blunt wings, a slightly contrasting mantle and broad white throat patch, Pallid Swift is perfectly identifiable.

Photographed last month in Ireland, this Common Swift by comparison has none of the paleness of it's Southern cousin, a plainer, more uniform plumage, with subtle scalloping only visible at extreme close range in ideal light, a typically less contrasting eye and mask, slightly narrower more tapered wings, and a typically less prominent whiteish throat patch.  From this image, confusion with Pallid Swift is unlikely.  

The problem of course, as highlighted in part one of this thread is the huge challenge created by variable lighting and exposure.

A White-tipped Swift (Aeronautes montivagus) from Venezuela makes a useful subject to illustrate the variable nature of lighting on a bird viewed and photographed against the sky.  Ideally, birds would be photographed in bright, overcast conditions.  Light cloud cover acts as a light diffuser, minimizing shadows and scattering light to illuminate the bird from all angles.  The problem of metering and exposure control remains but is more manageable.  Dynamic Range is less challenging also. These are the conditions that most closely suit the analysis of subtle or bland field marks (as discussed HERE and HERE).

There are a number of well established ID criteria for separating Common from Pallid Swift.  Just how well do these stand up to the difficulties presented by photographing against a bright blue sky?

This pair of birds are virtually inseparable, such is the nature of the lighting in each case.  Thanks to the angles there is nothing evident in the head or wing shape to assist with the identification.

One might expect the subtle mantle contrast of Pallid Swift to appear prominently in a seemingly uniformly lit image such as this, however light and shadow can be very subtle.  This bird's mantle is angled just right to obscure the feature while instead the tail appears subtly darker.  This kind of challenging tonal distinction reminds me of a similar problem faced when trying to capture and verify the subtle tones in the primaries, secondaries and tail of 1st year Thayer's Gulls Larus thayeri (see HERE).

The composite above is made from a series of images taken in the morning from a balcony apartment where we were based in Southern Portugal.  Field identification at the time proved surprisingly tricky as both species exhibit a similar appearance when light transmits through the wings and tail.  I found the Pallid's lower-pitched and more warbled call to be the most efficient indicator of what was passing overhead.

Discussion and Conclusions

Subtle identification criteria allow the separation of very similar species.  But field guides may not adequately prepare us for some of the confounding factors we face in the field and from the study of photographs.  Observing and photographing birds against a bright sky pushes our limits as well as our camera's capabilities.  Poor exposure control and challenging dynamic range in turn alters contrast and the appearance of subtle tones and field marks.  It's probably best to remain extra cautious when trying to assess all subtle criteria based on images captured against the sky.

Mystery Photo Competitions

For a blog devoted to bird ID from photographs it occurred to me that some of you might be a little confused by the content!

As birders, we all love a good challenge and there may be few things quite so challenging for a birder then to be asked to put a name to a difficult bird from a single photograph.  Not only do we need to possess a thorough knowledge of potential confusion species and their identification keys, but we may be required to consider confounding factors such as lighting, gestalt and size illusion.  This blog, for those still unclear, is devoted mainly to teasing out those confounding factors, and building a knowledge base and solutions around overcoming those problems.  So it's not a blog about ID as such, though as I have recently delved more into the area of gestalt I am finding myself trying my hand at some tough ID challenges from photographs (such as the mystery 'white-rumped' Swift from Dublin, 2002 eg. HERE),

Many a birder in Britain, Ireland and beyond cut their identification teeth with the Mystery Photo competitions and later the Monthly Marathon series of the journal British Birds.

The Mystery photographs series was unique.  It proved not just an enjoyable challenge for birders of all standards, it was also a great learning aid, which today provides an interesting insight into how identification was developing in Europe at the time.  As someone who myself developed my skills  as a birder at a reasonably slow pace, it always astonishes me how much information a new birder has at their fingertips.  The temptation to try and absorb all this vast ID information in a hurry must be hard to resist.  Consider for example the years of research crammed into a modern bible like the Collins Bird Guide (Svennson, Mullarney & Zetterstrom) or The Advanced Bird ID Handbook (Van Duivendijk).  Photographic guides have also evolved in the intervening years, the Crossley series perhaps the latest evolution.

The geek in me has started looking back at the old Mystery photographs competitions with a different perspective.  Many of these mystery photographs were actually pretty impossible challenges that coaxed us into reading on and learning insights from the experts.  Others were breaking ground and revealing ID tips which today we probably take for granted.  Plenty of them of course still catch me out.

It occurred to me recently that those who set modern mystery photo competitions must have a rather different approach to the architects of the early Mystery photographs competitions.  Needless to say all modern competitions involve digital images, many of which are published online so they can be analysed more forensically.  Typical modern competitions (eg. the Masters of Mystery competition run by Dutch Birding typically require birders to have recourse to the modern field guides.  Typically the bird is displayed in an awkward pose, partially displaying key identification criteria.  Most images tend to be pin sharp and high in resolution.  Unlike the photographic competitions of old, there is little margin for guesswork, less reliance on gestalt and certainly far less by way of confounding factors like lighting and shadow, film grain or defocus to get in the way.  Has the exercise become a little too sanitised, less enjoyable?   Or perhaps like everyone who looks back too far I am starting to show my age a bit?  Still, it would be nice to see a few more blurry and grainy competition images, aimed more at coaxing out those subconscious, less certain clues that birders embed deep in their brains while watching familiar species over hundreds and hundreds of hours of observation.  Less of the meticulous and more of the mystery please!