Friday, 1 May 2015

Field Marks - Shadow Topography

In various postings to date I have referred to contour shadows which form in consistent places on the surface of birds.  While this cannot be considered an exact science I think there is a certain merit in assigning terms to some of these consistent shadows for the purpose of furthering discussion.  In the posting lighting and avian anatomy I discussed the typical contours and pterylosis (the arrangement of feathers in definite areas) that give birds recognisable form.  I noted that in certain topographical areas of the avian body feathers are aligned neatly into near parallel rows, including for example the feathers from the bill and forehead, back over the crown and right back to the tail.  Meanwhile in other areas, such as around the eyes and ears (auriculars) feather alignment is less straightforward.   

With consistent feather alignment we start to see consistent shadows running longitudinally, like diffuse streaking along the length of feathers and pterylae.  We also see transverse shadows frequently running along the bases and just beyond the tips of feathers and pterylae.  Lastly, as illustrated in the posting false malar stripe we find shadows and the occasional exposure of feather bases and bare skin consistently wherever apteria are found on the body of a bird.  When these shadows and other features coincide with the position of typical field marks understandably we have the potential for confusion and misidentification.  In this posting I am going to focus on some consistent areas for confusion concerning field marks and shadows on the heads of birds, particularly passerines.  

Using European Robin Erithacus rubecula as an example below I have illustrated the typical topographical features of a bird's head.  Leafing through a variety of field guides it isn't often made clear that the throat wraps around beneath the submoustachial (malar) and ear-coverts (auriculars)  to meet the nape.  This indeed may not even be obvious in the field until a bird raises it's head and stretches it's neck.

Many birds of course have strongly marked heads and there are a range of commonly used field marks, as illustrated in most field guides.  Terminology varies and is sometimes in a state of flux (eg. the recent trend to replace malar stripe with lateral throat-stripe).  Topographical keys tend to be lacking in some areas, most notably in terminology for markings lying within the ear-coverts (auriculars) and throat (gular).  Below I have tried to fill in some notable gaps, such as the markings bordering the lower and rear of the ear-coverts (eg. common in many passerines), the auricular spot (eg. in some pipits and small gulls), the suborbital stripe (eg. in many larks) and the gular stripe (in some seabirds and the occasional passerine).  There may be better, more accurate terms in use for some of these and if so I'd appreciate some feedback.

The suborbital stripe is found in many lark species including Common Skylark (Alauda arvensis), often in association with a moustachial stripe, yet it rarely if ever features in the topographical keys in field guides.  Even more surprising is the lack of terminology for markings bordering and within the ear-coverts (auriculars).  Normally field guides only illustrate the eye-stripe and moustachial within the ear-coverts.  There is also typically no distinction made between a short moustachial (consisting of a row or two of the small orbital feathers) and a fuller moustachial extension as it were, encompassing the lower, or subauricular border of the ear-coverts.  I think there is some merit in making some clearer distinctions between various portions of the ear-coverts border.

It should be obvious that many of these markings follow specific pterylae, either covering the whole feather tract, or perhaps just the median or lateral portions.  These also align with contours, and therefore the typical alignment of shadows that follow these contours.  I have already explored the supraloral shadow and false malar stripe (perhaps more aptly named lateral throat shadow).  The loral shadow is also a commonly encountered lighting trick.  In fact most of the typical field marks of the head are often mirrored by highly localized and therefore potentially misleading shadows.  Below is a non-exhaustive key to some of the more frequently encountered ones.  I have added suggested terms for illustrative purposes.

False lateral crown-stripe (false coronal stripe)
Created by the protrusion of all or part of the crown from above, or alternatively by protruding lores or supercilium from below, shadows can mimic all or part of a lateral crown-stripe.  The European Robin is not a good candidate to demonstrate this.  Booted Warbler Iduna caligata is a better candidate.  Owing to bulging feathers on it's forehead it shows a prominent supraloral shadow, bleeding into a full false lateral crown stripe (or false coronal stripe to use a related though less frequently used term).

False eye-stripe and loral stripe
The Booted Warbler sequence above illustrates these shadows also.  In many species the lores are lightly feathered resulting in a slightly hollowed contour which frequently carries a shadow within it.  Note for example how hollow looking the loral area of  the Booted Warbler looks when compared with the European Robin.  This effect is even more striking in the Skylark image.  The loral feathers are also frequently misaligned and parted resulting in streak-like shadows, formed between the feathers.  For this reason it can be a real challenge to accurately assess the colour and tone of the lores.  We can also see in the Booted Warbler image how a bushy rear supercilium can cast a shadow over the top portion of the ear-coverts creating the appearance of a false eye-stripe.  So, depending on the angle of observation relative to the light source, the supercilium may cast a shadow above, onto the lateral crown, or below, onto the ear-coverts.

Gular shadow, auricular shadow, false moustachial stripe and false malar stripe
The shadow cast by the bill onto the throat often resembles a gular stripe.  Meanwhile the ear-coverts might cast a shadow onto the nape and throat side, particularly when a bird's head is turned or if it is looking up. These shadows resemble the dark streaks that frame the ear-coverts including the moustachial, subauricular and postauricular markings.  I have already looked at the false malar stripe separately HERE.

In summary I believe there is a benefit to assigning some terminology and order to an analysis of lighting and avian anatomy.  Starting with basic avian topography and standard field marks I have here overlayed some of the recurring and consistent localized shadows that give rise to confusion.  Light and shade is far from simple and this is not a remedy for the challenges of birds and light.  But I think it helps frame a discussion about a subject that isn't often considered.  Another tool for the toolkit.

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