Gestalt - The Limitations of Primary Projection

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

Primary projection or primary extension is a form of analysis commonly used in bird identification.  Despite its widespread use it is not a particularly well defined or well understood analytical tool.  Put simply, primary projection is a comparison between the length of the exposed tertials (A-B) on a closed wing relative to the length between the tip of the tertials and tip of the primaries (B-C).  Before I go into detail here I am going to make a few general, rather pessimistic statements:-

No.1  Primary Projection is not a measurement.  Far from it in fact.  It is as close to a comparison of apples and oranges and one can find in birding.

No.2  Primary Projection really only makes sense when we view a bird in perfect side profile.

No.3  Primary Projection is unreliable and at best only an approximation.

Why so negative?  Well when we explore what primary projection actually involves unfortunately what we end up with are a whole load of questions without any answers!

Anatomy of the wing

Despite the fact that we witness the articulation of the joints of the wing in flight all the time we rarely stop to consider how the bones sit and joints articulate when a bird is at rest.  Below I have approximated the position of the wing bones at rest in this juvenile Common Rosefinch, Carpodacus erythrinus.  I have chosen Common Rosefinch intentionally as primary projection would be one of the key distinctions between this old world species and  House Finch Haemorhous (Carpodacus) mexicanus from the New World.

Straight away we can see the problem.  The tertials and primaries are not connected to the same bones and therefore can move quite independently of one another.  If the humerus bone moves on the closed wing it moves the rest of the wing with it.  If the elbow extends the radius and ulna bones then the bones of 'the hand' are also affected.  If the bones of 'the hand' moves only the primaries and/or other feathers of the outer wing are affected.  So the primaries, connected to the bones of 'the hand' can move independently in various planes relative to the tertials.  Clearly, if the tertials and primaries align differently there can be no such thing as a standard primary projection value for an individual bird, let alone a species.  In other words, primary projection is ever changing.

Movement of the forearm
Note how the position of the secondaries in the juvenile Red-backed Shrike (Lanius collurio) below differs in these images from closed in one to fanned in the other.  Meanwhile the tertials and primaries appear to remain much the same in both images.  This suggests an articulation of the forearm, i.e. a movement between the humerus and radius/ulna.  And yet if the 'elbow/lower arm' moves position so must the 'wrist/hand'.  A lowering of the secondaries equates to an extension of primary projection and visa versa.  This point can easily go unnoticed.  Worth pointing out also that the feathers which we refer to as tertials may actually represent different feather tracts in different species.  In many passerines the tertials join the ulna bone together with the secondaries.  In effect, in these species the tertials are actually innermost secondaries.  On many other species the tertials are connected to the humerus bone and are therefore distinctly separate from the secondaries.  This also means that the tertials articulate differently relative to the other flight feathers on different species depending on which bone they connect to.

In Europe two extremely similar Hippolais warbler species Icterine (H. icterina) and Melodious Warblers (H. polyglotta) differ most notably in terms of their primary projections.  Icterine has a long primary projection while Melodious is typically shorter.  Coincidentally, Icterine has a brighter wing panel but Melodious can approach Icterine in appearance, particularly when the secondaries are bunched, as shown by the Red-backed Shrike example above.  Luckily, by bunching the secondaries primary projection is further reduced.  So, overall the ID in this case is made simpler, provided that is that primary projection can be properly assessed.

Unlike the clearly visible movement of a mammalian elbow and forearm, in birds the elbow is hidden beneath feathers and tissue.  It is very difficult to envisage a way to accurately account for a movement of the hidden forearm and thus it's impact on primary projection.  We may be able to qualify a primary projection analysis with the observation that the secondaries look very bunched, thus primary projection is probably slightly shortened from the norm.  That is about as good as it gets.

Movement of 'the hand'
There are multiple joints in 'the hand' of a bird so primaries can be held in various different ways.  Typically, however unless a bird is preening, cleaning or displaying primaries are fairly tightly bunched.  Despite this wings can still lie in a variety of positions from hanging low, almost parallel to the tertials to high, often crossed together over the rump.  Unlike the forearm we may feel we can visualise the arc that the primaries might take from fully closed to partially open, right?  But the problem this time is we lack a frame of reference for the 'normal position' of the primaries at rest.  What we end up with is a range between typically held low to typically held high.

Note the angle of the wing in this case is approx. 45 degrees off of side profile.  This is more in line with the side profile of the wing as opposed to the side profile of the whole bird and I think this is probably the best angle to actually try and measure primary projection properly, if a case could be made for such a measurement.  But, as explained above the frustration in trying to accurately compare tertial length and primary length is in the fact that they rarely if ever line up properly to be measured.  And if this is the case an accurate measurement doesn't serve a real world purpose.  We are left with the realisation that...

No.1  Primary Projection is not a measurement.  Far from it in fact.  It is as close to a comparison of apples and oranges and one can find in birding.

and...

No.2  Primary Projection really only makes sense when we view a bird in perfect side profile.

Angle of view and foreshortening
The angle of the bird and wing relative to the camera is an equally important consideration.  If the wing is in profile we are only considering the X and Y axis, aligned to the plane of the camera sensor.  However if the bird is not in profile then the Z axis (perpendicular to the lens) comes into play.  We also have the problem of perspective foreshortening.  Put simply, we cannot account for the Z axis properly.  The images below are a good case in point.  The left wing on the right hand bird is quite well in profile and the primary projection measurement matches the left hand bird well.  But in the right hand image if we try and measure primary projection from the right wing, which is not in profile the result is very different.  Note the right hand wing neatly illustrates the problem of wing joint articulation.  When we see the wings crossed like this it should be a strong warning that the primaries are at a very different angle to the tertials and that a primary projection measurement will fall at the short end of the range.  However, as we are typically measuring primary projection from a bird in perfect side profile we cannot see what the angle of the primaries are relative to the tertials.  We are limited by our angle of sight.

In other words...

No.3  Primary Projection is unreliable and at best only an approximation.

Judging points correctly
When trying to measure primary projection in photographs observers typically point out that the location of point A at the base of the tertials is a bit arbitrary.  If the bird is in profile typically the greater coverts will obscure the base of the inner tertial.  However, if viewed from above the base of the tertial is visible inside the greater coverts so the tertial length measures longer.  so typically A is defined by the point where the tertial becomes visible past the inner most greater covert.

Solutions
Having thought long and hard about this it pains me to report that I don't think there are any solutions here.   Primary projection is what it is.  It is an interesting comparison that can be made between the visible length of the tertials and visible length of the primaries past the tertials on a two dimensional image of a bird viewed in side profile.  It is not accurate and any attempts to turn it into a scientific measurement are probably pointless.  Above all, it is important not to take primary projection too seriously and avoid the temptation to dive in and measure primary projection from every single image.  Best to take note of the hazards identified above...and probably others I haven't even explored.  Primary projection should be used sparingly and in combination with other identification pointers.