Friday 21 October 2016

Birds and Light - Ghosts

In the spirit of the season I recently had a close encounter with a ghost.  A very washed out Western Bonelli's Warbler Phylloscopus bonelli on Cape Clear Island, Co. Cork last weekend got many pulses racing when the finder reported it called just like it's colder, eastern counterpart P. orientalis, a potential Irish first!  The following day it was heard to call like a western and by it's eventual trapping, biometrics left no doubt as to it's true identity.  

The recent focus on grey scales on this blog has re-awakened my fascination with birds and light.  While there is little doubt that the camera is no match for human vision, when it comes to these ghostly birds I often wonder is it the human eye that falls a bit short?  Typically the camera fails to truly convey just how pale and striking these ghosts appear in life.  And yet it seems that, in the hand these birds often don't quite match their shockingly pallid appearance in the field.  I have spent a bit of time exploring various elements which I think contribute to this phenomenon.  

Brightness Illusions
In the posting on brightness illusions HERE I explored a well known optical illusion called the checker shadow.  Unlike a camera's exposure which delivers a uniform correction across an image, human vision is much more sophisticated, allowing for varying degrees of correction at different locations throughout the scene.  For instance, objects which appear to be in the shade may receive a local tonal boost, making them brighter and easier to observe.  

This is proven in the case of the checker shadow when we draw lines of equal tone between checker squares A and B.  The brain is forced to reconcile the fact that these squares are actually of the same tone and our perception finally catches up with reality.  Incredibly its actually possible to witness this alteration of perception in real time as can be shown by moving between these two illustrations.

 In the case of the Cape Clear Western Bonelli's, on the evening this bird was found it was seen to move in and out of a dense, shady area of scrub.  Most observers were both agog and aghast at the appearance of the bird.  It is fair to say that the bird's mantle shade was significantly faded when compared with the typically warmer, honey-colour of autumn Western Bonelli's, such as the bird illustrated below.

Western Bonelli's Warbler, Mizen Head, 30th October, 2004

Also, the typical crisp white underparts of Bonelli's is always much brighter than even the palest of Chiffchaffs (eg. P. colybitta tristis types).  But I think there is a bit more going on here.

Foliage Canopy Edge
In another posting HERE I delved a little more deeply into the domain of these ghostly figures.  While our eyes are mesmerised by the sight of pale birds moving through the deep shadows our cameras find it extremely difficult to obtain representative images.  So much so in fact that it generally takes shooting in RAW and subsequent tone mapping to approach how the bird looked in it's dark environment.

Here, in late autumn birders live for the chance to see an exotic ghost from the east.  Last week's birding on Cape Clear was awesome.  The bird may not have proved to be an elusive Irish first, but it certainly hit the spot for me.

When I finally got home and managed to look at my images more closely with a copy of Lars Svensson's Identification Guide to European Passerines to hand I was able to carefully and properly interpret this open wing shot.  Primaries P2 in bonelli is typically shorter than P6 (longer in orientalis).  Primaries P3,4 and 5 are all roughly of equal length in both.  Generally P2 is hidden and therefore not easily interpreted in the field.  Even allowing for a certain amount of error in this image the short appearance of P2 clearly seems to strongly point to bonelli.  Wing formula and biometrics confirmed this when the bird was captured a day or two later.  Remarkably there have been two more Western Bonelli's in Ireland this week.  For more on the benefits of Ringer's (Bander's) reference guides see HERE.

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