Saturday, 25 July 2015

Gestalt - The Limitations of Leg Proportion Analysis

It is within the subject of gestalt that I tackle the question of bird size and shape in this blog.

Primary Projection and the Eye/Bill Ratio analysis so far have proved to be fairly unreliable analytical tools owing to the problems of perspective foreshortening and anatomy.  What about the analysis of the proportions of the avian leg?   

Anatomy of the leg
As we have found with the analysis of primary projection, no discussion about the proportions of the leg can be made without some understanding of the anatomical functioning of avian leg bones and joints.  Below I have illustrated the leg bone anatomy of a Bertholot's Pipit, Anthus berthelotti.  Clearly, like the upper limb or wing bones, a considerable amount of the leg anatomy of birds is hidden inside the body.    It is easy to forget while watching birds that all we are seeing are the equivalent of the shin and foot of the mammalian leg.  In effect birds walk around on the tips of their toes.  Everything above the shin is tucked away inside the body cavity.  For passerines and many other birds the 'thighs' as they are called in birding (in reality the shins) are muscular and feathered.  Wading birds tend to have more of the tibia exposed and it tends to be less muscular and unfeathered.

Movement of the leg
When a bird crouches the motion is akin to a person, standing on the tips of ones toes, simultaneously bending ones knees and ankles.  The 'thighs' lie close to the body, hidden inside the flank and belly feathers. As a bird raises itself tall leg joint articulation is equivalent to a human standing tall.  The knee and ankle joints open and in doing so the 'thighs' are exposed through the flank and belly feathers.  Clearly posture therefore has a bearing on the proportion of a measurement like tibia/tarsus ratio.  As illustrated below tibia/tarsus ratio can be highly variable.  There is more than one factor at play in this example however as discussed below. 

Angle of view and foreshortening
When we look at a perched bird it should be obvious that it's legs are not positioned vertically below the body but tend to be splayed to assist with balance.  While the tibia and tarsus may be aligned in the same plane unless our angle of view is perpendicular to this plane we inevitably have perspective foreshortening.  This makes it impossible to take an accurate comparative measurement between these two structures.

We do have a slightly better chance of comparing toe and claw length as these usually align favourably for the camera.  One does need to be careful however with the articulation of the bones of the toe due to the terrain and the potential for the claw to get snagged and skew the toe.  In short, measurements of structures and objects that can move relative to one another is highly challenging from photographs.

Judging points correctly
We have seen how variable the tibia length can appear simply due to posture and feather position.  Typically legs being relatively small features tend to be obscured more often than not.  Legs and joints are also often uniform in colour and it can be hard at times to judge the position of joints and where we should start to take our measurements from. 

In Summary
At risk of beating the same old drum I find myself drawn to the same conclusions I had reached earlier on Primary Projection and the Eye/Bill Ratio analysis.

No.1  Leg proportions are not strictly measurements because more often than not we have to accept some perspective foreshortening.

No.2  Leg proportions only make sense when we can view our components in profile.  Once again perspective foreshortening is going to mess with our measurements.  It wouldn't for instance be advisable to try and compare the accurate lengths of all four toes based on a photograph.  It simply can't be done correctly.

No.3 Leg proportions are unreliable and at best only an approximation.  I would couch that remark with the point that provided two structures are properly aligned and totally in profile it is possible to make a good comparison.  But for the most part we don't tend to have that luxury so most leg proportions from photographs tend to be estimated at best. 

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