Saturday 7 February 2015

Field Marks - Fringes, Notches and Tips

Contrasting fringes, notches and tips are a common feature of many species, most typically on the feathers of the wings and tail.  These can form important field marks.  Typically they are a feature of fresh plumage and they wear and vanish over time.  Here is a reasonably fresh juvenile Wood Sandpiper (Tringa glareola) with neat white notching displayed throughout its upperparts.

Below we have two juvenile Semipalmated Sandpipers (Calidris pusilla) photographed in Ireland in September.  The top bird is a classic, fresh bird while the lower bird is heavily worn or abraded.  Presumably the lower bird hatched much earlier in the summer, but even at that it's plumage is in very poor condition for this time of year.  Note how the while fringes of the mantle are almost completely worn away.  The abrasion of the tips of the lower scapulars splits the feather ends causing diffusion of the remaining fringe, so it is far less obvious.  Note the scapulars are cloaked over more of the coverts in the lower bird but the coverts are also heavily abraded and misaligned - almost clumped together and not neatly fanned like the coverts on the bird above.  Interestingly this worn bird also displayed aberrant behaviour, tending to skulk in dense grass much of the time, while the upper bird was content feeding on the open beach with other waders.

Northern Wheatear (Oenanthe oenanthe) is an incredible migrant.  Populations ranging across the entire northern hemisphere migrate over vast distances, including open ocean to winter in Africa.  The fresh juvenile/1st winter bird below is already starting to lose it's buff-coloured fringes through wear.  Initially these are being worn and/or bleached whiter leaving a barely visible subterminal band of buff, which will also fade.  When this bird returns to Ireland in spring most of it's summer plumage will consist of the very same feathers but these fringes will have completely worn away.  Note feather abrasion and bleaching may introduce very fine pale fringes to otherwise uniform-looking feathers.  These are wear patterns as distinct from pigmentation patterns.

Red-breasted Flycatcher (Ficedula parva) is separated from the fairly recently split Taiga Flycatcher (Ficedula albicilla) through a combination of subtle features including the darkness of its uppertail coverts relative to it's tail (lighter than the tail in RBF and darker in TF) and also by the subtle pattern of the spot at the end of its tertials (thorn-shaped in RBF and round in TF).  The subtle buff tones to the fringes and the breast are also relevant.  As feathers fade and wear RBF will naturally more closely approach TF in appearance.

Thanks to feather arrangements, pale fringes naturally contribute to larger patterns and features such as a pale secondary panel (eg. the Northern Wheatear above), but such features are dependent on the bird maintaining a certain posture and therefore need to be interpreted with care.  Take for example this juvenile Red-backed Shrike (Lanius collurio).  Quite a number of species exhibit this subtle secondary panel.  The presence or absence of this feature depends on individual variation but also as we see it depends on feather wear and feather alignment.

As if things were not complicated enough we also have to consider certain artefacts, most notably edge halos, produced due to image sharpening algorithms, and, one of my favourites, moiré.

Most  moiré shouldn't be too much of a problem as it tends to occur perpendicular to the direction of predicted plumage fringe patterns. However as shown by the example above, extremely low resolution can give rise to a cross-hatch moiré pattern.  This has the potential to mirror the impression of scalloping patterns.  I will be looking at this and other false field marks in future postings.

Finally, here is a rare example of a bird showing dark feather fringes (White-throated) Dipper (Cinclus cinclus).  Cormorants (Phalacrocoracidae) are another group of birds that display this field mark.  I wonder if there is a link between this subtle type of dark scalloping and the underwater environment?  And, if so what is the advantage for these species?

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