Wednesday 1 October 2014

Gestalt - A Gestalt Field Guide

'Gestalt' (or 'G.I.S.S.' - 'general impressions of size and shape' - also spelt 'JIZZ') is the name we give to the recognisable 'feel' of an individual species in the wild.  It is a combination of it's structure, how it moves and it's behaviour.  As birders gain experience in the field we quickly become aware of the gestalt of common species we encounter regularly.  When a new species appears in a familiar setting, very often it's presence is first signalled by it's gestalt - something unusual about it's size and shape, or the way it feeds or moves about.  Of course a common species with an uncharacteristic behaviour or shape can fool an observer into believing they are watching a different species.  It is also very difficult to describe a bird's gestalt in any objective or measurable way.  Thus this subtle field craft has it's pros and cons, it's strong advocates and those who are more into field marks.  Most experienced birders would tend to use gestalt a lot in forming an initial impression but combine that with topographic field marks to form a solid identification.

It would be wrong to say that gestalt can be properly captured in an individual photograph but video can go a long way to capturing it.  Then again, if a video is merely a collection of photos, surely a flavor of a bird's gestalt is captured in every single photograph or frame of video.  The question is, how do we reliably and consistently draw out from our photographs this 'essence' (for want of a better description).

Movement and Behaviour
It nearly goes without saying that any effort to try and establish how a bird moves or behaves purely from still images is futile.  Video offers some clues but may be misleading if the video is shaky or the frame rate is off-putting.  For a useful tip in dealing with shaky video see HERE.

Structure and Measurement

Birds come in all shapes and sizes as we all know.  Measurements (avian biometrics) play an important  role in bird identification.  It is this aspect of gestalt which we may find useful here.

Comparative measurements can be taken from photographs but this must be done with great care and should be checked and verified using a number of photographs and from different angles or distances.  There are various ways in which the distance between two points can be distorted in a photograph so nothing should be taken for granted here.

The image below shows how perspective can alter the relative proportions of a subject.  Luckily this is rarely a problem as we don't tend to get close enough to birds for perspective to enter the equation.

We face similar problems when the subject or an object we are trying to measure is not in perfect side profile.  Our measurements are influenced by perspective foreshortening.  This is possibly the biggest challenge we face with measurements from bird images as explored HERE, HERE and HERE.  The problem ultimately comes down to the fact that a photograph is a two-dimensional projection of a three-dimensional space.  For more see HERE.

The image below illustrates lens distortion, a common problem with lenses where magnification differs from the middle to the edge of the lens.  It is not very easy to detect and focus is unaffected.

As if size analysis wasn't difficult enough we have to contend with strange quirks associated with the human visual system.  Size illusion is something we should always be mindful of in bird identification.

As the above examples illustrate, making size or structural pronouncements based on digital images requires a lot of  careful consideration and cross-referencing.  Not only do we have to contend with real image distortions, but we also need to factor in illusory distortions!  For more see HERE.

But lets for a moment assume that we are confident that a set of photos of a bird accurately depicts the bird's structure and proportions in life, without too much distortion.  Where is the means for making a valid comparison between a photograph of a bird and a gestalt standard or guide?  

Gestalt Studies in Field Guides

Well, the fact is, we don't currently have too many such standards.  Conventional field guides only present birds in ideal portrait profile, plus occasionally one or two other compositions.  There are not too many field guides that accurately depict gestalt.  There has been the odd book devoted to the subject, most notably Birds by Character by Rob Hume, and similar works involving artists like Dan Powell, DIM Wallace and others.  

The Crossley ID Guide by Richard Crossley certainly took us a step further in terms of photographic guides, but perhaps the best example of what I am referring to can be found in the pages at the back of Hawks At A Distance by Jerry Liguori.  The shape montage format (eg. Northern Goshawk and Northern Harrier below) is a fantastic tool for field observers and those trying to learn the subtle gestalt of closely related birds.  I also think this type of format is of great value when trying to identify a bird from photographs.

DIY Tools

Unfortunately, this is a pretty new approach, not often replicated in print.  While on the IRBC I occasionally resorted to generating similar montages to assist in making my own ID determinations from tricky photos.  The internet is a perfect reservoir of images for this purpose, with thanks to all the birders who put their images up online for all to see.  So, in the absence of a perfect Gestalt Field Guide, why not make your own!

What I have tended to do is take each image in turn and look for photos of various candidate species captured in similar lighting and composition.  I put these together into a montage.  Needless to say, the more images and contemporaneous notes that are available the better the chances of reaching a satisfactory conclusion via this process.  It may not work for all species but I have found it handy for raptors, swifts and seabirds to give some examples.

No comments:

Post a Comment