Thursday 8 January 2015

Field Marks - Lighting and Avian Anatomy

In 2015 I am going to be focusing a lot on the identification of field mark in digital images.  In the series of postings Birds and Light I have been exploring the broader subject of lighting and it's influence on images.  For the time being I will be looking more specifically at the impact of lighting on field marks in images.

Avian Anatomy
The overall structure of objects determine how they appear when lit.  The contours of a bird are determined by it's musculoskeletal structure, overlaying skin and soft tissue, but probably most importantly of all, by the overlay of feathers covering its surface.

Feather tracts, called pterylae are areas of skin on a bird from which feathers grow.  Feathers are not located randomly on the surface of a bird but are distributed often in lines of feathers within feather tracts.  Between these feather tracts are patches of bare skin called apteria.

Just as the hairs on your hand become erect due to muscle contraction when it is cold, birds can fluff up their feathers to trap air in between as insulation from the cold.  Unlike the autonomic control of hair erection in mammals including humans, birds have considerable control over the erection and relaxation of their feathers and it's mechanism is more complex than the simple hair control in mammals (see HERE and HERE).  Many birds obviously use this adaptation in courtship.  Feathers don't raise and lower randomly but rather in lines and clusters.  There is a certain predictability in contours and patterns and the more we study birds the more familiar we become with these patterns.  Of course the movement of feathers is also influenced by the wind and some feathers are more downy than others so are more likely to remain out of position.  There is some nice detail on feathers and plumage HERE, from where the sketches of the pterylae and apteria of Clark's Nutcracker were sourced for the graphic above.  I photographed the accompanying Clark's Nutcracker in Canada in early July.

Typical Plumage Coutours
When we look at the combination of anatomy and feather tracts we start to see some consistency in the contours of birds.  Interestingly standard bird topography doesn't always coincide exactly with a bird's contours.

The Head and Neck
The head and neck of most birds tends to be well feathered (capital tract).  There often tend to be apteria encircling the eyes and from the eyes out and around the auricular region.  This might explain why the ear-covert feathers (auriculars) are bigger as they need to cover this area of bare skin, and why they are often not as neatly aligned as the crown, nape and throat feathers.  There may be other small apteria on the head and neck as for example in the submalar apterium of the Clark's Nutcracker shown above.  However, with the exception perhaps of the loral area, around the eye and the ear-coverts, the rest of the head and neck tends to be quite uniformly feathered in lines of feathers facing backwards from the bill.  Thus the forehead, crown, nape and throat feathers tend to produce fairly consistent looking shadows, consisting of narrow streaks of fine shadow running along the direction of the feathers.  Then, if feathers are raised, typically we see this as lines of transverse shadow running along the end of these parallel lines of feathers.

The Lores, Supercilium and Ear-coverts (auriculars)
The shade of the lores can represent an important fieldmark for some species and this field mark may not always be so easy to judge in a photograph.  This is due in part to the contour of the head, but also due to the complex feather arrangement in this area.  The same applies to the supercilium, eye-ring and other field-marks associated with the ear-coverts.  So we must pay attention to the fact that feather position can really influence the appearance of a field mark around the face of a bird.

Pseudo-field-marks may be due to shadow and may therefore only manifest under certain lighting conditions.  Take the supra-loral shadow in Booted Warbler (Iduna caligata), only visible when the head is angled in a certain way relative to the sun.  As the composite above shows, if this area of the head is in full sun the field mark is missing.  In this species the field mark seems to be produced by a thick or pronounced tuft of feathers on the forehead.

Throat, Breast, Belly and Vent
As the Robin image below illustrates, the throat is not a uniform shape.  The auriculars, submoustachial and throat areas all represent distinct regions within the ventral tract.  The crop is an important consideration.  If it is full, the bulge can transform the appearance of the features in this area of the bird's plumage.  As the head swivels, and depending on the posture of the neck, a shadow or cleft may form along the throat line, and depending on the posture of the neck some feathers can be partially obscured beneath throat feathers.

Another cleft is formed between the two prominent pectoralis muscles (breast muscles) connected to the prominent breast or keel bone.  There is a large apterium running down the centre of the breast and parting the ventral feather tract. This apterium further enhances this channel.  Surprisingly, standard topography does not give this cleft a name, despite the fact it is quite prominent on some birds and may even carry a distinctive field mark on some species.

As these Robin and Western Bonelli's Warbler photos illustrate, the patterning of shadow on the underside of a bird often consists of characteristic but faint longitudinal shadows running down the edges of breast and belly feathering, plus the central cleft and transverse shadows associated with the delineated throat, breast, belly and vent (not shown).  There may also be shading in the femoral tract area, delineating the edge of the rear flank (also not shown in these images).

Upperparts - Mantle, Rump and Flight Feathers
The mantle and rump feathers of many birds tend to follow the same linear pattern of near parallel rows of feathers as the crown and nape, which is convenient.  The stiff flight feathers of the wings and tail also tend to have consistent order which again makes for consistent lighting patterns.  Of course, individual feathers may be moulting or displaced and this can lead to confusion, so we can never be complacent when it comes to reading lighting and shadow on the contours of a bird.  In case you are confused by the image below it is a hummingbird feeding from a flower with it's back to the camera (Sooty-capped Hermit, Venezuela).  It's pale-edged crown, nape, mantle, and rump, tapering to rusty-coloured upper-tail coverts illustrate a consistent feather arrangement common to many birds.

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