Friday 24 June 2016

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.

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