Identification : Colour

Introduction

Colour in birds  is produced in different ways.  An understanding of the different mechanisms involved is key to understanding how colours might be observed and captured within digital images.

Some objects can simultaneously exist as two or more colours.  The tail feather of a Magpie is essentially black unless viewed from a certain angle, in which case it may appear blue or green.  When we scratch a little deeper we discover that colours in many birds are created by a combination of pigments and a phenomenon known as structural colour.  In the case of pigment colours, a portion of the incident light hitting the pigment molecule is absorbed and a portion is reflected.  In the case of structural colours, light refraction, diffraction and/or wave interference alters reflected light, giving the object the appearance of being a radically different colour under certain light conditions.

Pigments

There are mainly three classes of pigments in the plumage of birds.

Melanins

These pigments are produced in the body, resulting in colours from black to dark red and pale yellow. Varying levels in the production of melanin give rise to birds which are abnormally dark or light and result in melanism and albinism.

Carotenoids

Produced from nutrients, these pigments produce vivid yellows, reds and oranges and combine with melanins to produce greens.

Porphyrins

These pigments are produced by modified amino acids which fluoresce a bright red when exposed to ultra-violet light, e.g heme in red blood cells.  The vivid colours produced range from pink, browns, reds and greens.  These pigments are found in some owls, pigeons and gallinaceous species and are also responsible for the brilliant greens and reds of some tropical species.

Aberrant Dunlin (Calidris alpina), Ireland, September, most likely caused by Leucism, a reduction of some or all pigments.

UV Reflectance/Absorption

Many many plants and animals signal by means of ultraviolet (UV) reflectance and absorption patterns.  These patterns are invisible to the human eye but can be observed using a modified digital camera.  Most birds can see in the UV and some even have a fourth cone cell (photo-receptor) in the eye which appears to be designed to see in UV.  Having dabbled a bit in UV photography, HERE I have explored this area in more detail.  This posting contains links to other related postings.

Structural Colour

In this case, colour is not produced simply by the reflection of wavelengths of light from pigments but by a more complicated manipulation of light by micro-structures on the surface of feathers.

The mechanism, termed IRIDESCENCE, is an optical phenomenon in which hue changes in proportion to the angle of observation and illumination.  

Virtually all birders are familiar with iridescence having observed the strange shifting of colour of the feathers exemplified in Hummingbirds, Peacocks and Pheasants, Ducks, Starlings, Crows etc.

The rich chestnut and violet tones bordering this Common Pheasant (Phasianus colchicus) tail feather appear to fade depending on the angle of the feather relative to the angle of the camera and sun.  These are structural, iridescent colours overlaying the rich golden and olive pigments of the feather.

Tail spots of Indian Peacock (Pavo cristatus).

Among various other animals displaying iridescence, the various Butterflies in the Genus Morpho are some of the most spectacular.  The image above is of a Blue Morpho (Morpho peleides) from Costa Rica.

Hummingbirds are the ultimate in terms of structural colour in the bird world, but if the sun isn't shining just so, like in the case of this Violet-chested Hummingbird (Sternoclyta cyanopectus) from Venezuela, many of these birds are not much brighter than crows.

Though there may be more than one type of iridescence in nature, in birds the primary mechanism is called THIN-FILM INTERFERENCE and is based on a combination of reflection and refraction of light.  Light hitting and reflecting off the upper surface of a thin transparent covering on a feather mingles with light which has reflected off the under-surface of that layer.  As one set of light waves has travelled fractionally further than the other, both sets are out of phase and the result is wave interference.  Some colour wavelengths are cancelled out and become invisible (destructive interference) while other colours are enhanced and become more vivid (constructive interference).  So effectively the light and the colour are amplified by these feather micro-structures.

Not all structural colour in birds is due to iridescence.  Some brightly coloured blue birds including Bee-eaters and the blue plumage characteristic of many Nearctic species (eg. Jays, Bluebirds, Indigo and related Buntings and Grosbeaks) use a similar mechanism to enhance the colours in their plumage.

Eurasian Jay (Garrulus glandarius) only shows a splash of structural blue pigmentation, unlike it's more brightly-coloured Nearctic and Neotropical brethren.  For some reason the America's hold the Lion's share of the world's blue-coloured birds.

Colour Standards and Colour Nomenclature

In this blog I have spent considerable time looking at colour standards and colour nomenclature.

It came as a surprise to discover that the internet lacks any standard colour pallet and an appropriate colour nomenclature for birders.  The nearest thing to an ideal nomenclature for our needs is Ridgway's Color Standards and Color Nomenclature (1912).  Sadly, this standard is all but lost and, as of yet, it cannot be digitised properly (for more see HERE).

                                           

The colour standard which applies to the internet, as well as being the default for most digital devices, including cameras, computer monitors, scanners and printers is called sRGB colour space.  While this standard cannot display all the colours we see in life,  it is reasonably good for our purposes - the study of digital bird images.  I spent some considerable time trying to reconstruct Ridgway's colour standard in sRGB colour space, without success.  But, in doing so I came to the realisation that a museum-based colour standard was probably not the most appropriate for studies of birds in the field and in field-based digital images.  The end-result of my efforts was the development of a smaller, more practical colour guide for birders working with digital images in sRGB.  Here it is.

Blue-grey Tanager (Thraupis episcopus).  HERE I outline the rationale and methodology behind this colour standard in more detail.

Colour Accuracy and Colour Sampling Techniques

There are a number of significant challenges involved in obtaining accurate colour representation from digital images, as outlined HERE.  Among these is the difficulty involved in sampling colours accurately.  I came up with a sampling methodology (HERE) to address that issue.

Because digital cameras are all different, any meaningful representation of colour requires reference to a common standard.  The standard adopted by many professionals is the X-rite colorchecker (formerly GretagMacbeth).  I purchased an X-rite colorchecker passport in order to create DNG profiles for my camera equipment and thus bring them in line with that standard.  I think it would be important for anyone hoping to capture colour as accurately as possible to consider doing likewise.  For more information see HERE.

My animated gif below gives a representation of the changing nature of daylight over the course of 24 hours here in Ireland during both the summer and winter solstice.  A number of things are particularly striking.

(1) Light temperature is ever-changing during the winter solstice.  It never properly stabilises.
(2) The purity of daylight is already much better come mid-morning in mid-summer than it is at any time during a winter's day, simply because the sun never gets high enough above the horizon in winter.
(3) Colour temperature is very stable for long periods of the day in mid-summer.  While this may seem like an advantage, a high, bright sun brings it's own challenges both for bird observation and photography.

Accurate white balance calibration of images using a grey card is the final step required to bring the colours captured into line with a standard colour pallet.  Ambient light is ever changing in response to the height of the sun in the sky, the presence of cloud and other atmospheric conditions.  This can only be accurately corrected for by using a grey card.  Even then there are some other factors to be considered including the presence of other illuminant sources in the environment such as bright foliage, flowers and other reflective surfaces which can cast colours onto the subject and give a false representation of colour to the camera and observer.  For more details see HERE.  For the purpose of investigating the nuances of white balance I created a special test rig.  HERE are some further experiments on white balance and the use of the grey card. 

So, in summary, accurate colour capture is far from easy and without the right approach to camera calibration, the results can be way off target.

Colour Profiling (CP)

In some cases it may be nice to try and compare the colours of a number of birds of the same species or closely related taxa using a standard method.  For example, here in Western Europe we have the ongoing challenges in assigning vagrant Chiffchaff's to one or other species or taxa.  When we are dealing with subtle, low saturation colours the camera calibration techniques, birder's colour pallet and colour sampling method described above may not be good enough to allow for a meaningful comparison of colours.  So, I came up with a colour profiling method intended to allow for a meaningful comparison to be made.  While this tool may not answer all of the questions I hope it will at least go part of the way, and who knows, it might even shed some new light into this very difficult area.
 
There may be other complex species groupings that could benefit from a similar approach including for example Lesser Whitethroats and Reed Warblers and similarly complex groupings in North America.   For more see HERE.

The composite above was created using the CP method.  For more see HERE.

The Importance of Colour for Bird Identification

It could be argued that colour is not all that critical for bird identification, certainly here in the Western Palearctic at least.  Evidence for this is in the success of The Advanced Bird Guide by Nils van Duivendijk - a field guide designed intentionally without any plates.  That rule may not hold for the avifauna for other continents.

I hadn't originally intended expending this much time on colour theory and colour management in this blog, but I have certainly learnt a lot in the process and I would encourage anyone who sees colour as very important in terms of bird identification to research this whole area in fine detail.