Friday, 17 April 2015

Field Marks - False Malar Stripe

In this series of postings I have been exploring avian field marks from various perspectives.  One aspect which is of particular concern to us from a bird ID perspective is the potential for false field marks.   I touched on various aspects of this HERE.  Shadows give cause for confusion, particularly as the lighting in the field often means the bird bares no resemblance to the image in the book.  Experienced birders get over this and quickly learn how standard avian anatomy tends to result in some standard lighting tricks, which we can account for.  Most of the time light and shade fail to mimic field marks exactly.  But certain field marks are more prone to being 'falsified' by the light and other factors.  One in particular is the malar stripe.

The malar stripe is a fairly common marking.  It falls along the line between the throat and the submoustachial feather tracts and usually consists of markings on feathers belonging to the throat.  There seems to be a trend towards getting rid of the term malar stripe and replacing with lateral throat-stripe as summarized by David Allen Sibley HERE.

There happens to be an underlying apterium (featherless patch of skin) between these pterylae (feather tracts), called the submalar apterium.  So, a cleft will form here when these feathers are parted.  Difficulties arise due to the relative movement of these feather groups, with the result that the malar region may be difficult to properly assess at times, particularly in images.  Not only do we see shadows forming here but we also see the downy bases of feathers and perhaps even the submalar apterium itself exposed on occasion.  The impression of a false malar stripe and other markings is easily created under these circumstances.

Take this singing (Common) Nightingale Luscinia megarhynchos photographed in Morocco.  The deep cleft between the throat and the submoustachial in the right-hand images marks the position of the submalar apterium.  This is one of the iconic avian songsters but it generally sings from deep cover.  A photograph of one in song is a neat challenge.  The nominate western subspecies megarhynchos  differs subtly from the eastern race hafizi (golzii) and from the closely related Thrush Nightingale L. luscinia by, among other features, it's lack of a malar stripe.  But this feature is somewhat obscured when the bird's throat feathers are ruffled, as is the case for instance when it sings.  So, ironically, though it's song is distinctive, a photograph if it singing, frequently isn't!

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