Light Sensitivity of Pigeon Feathers
It has long been known and generally accepted that brown pigmented pigeons tend to fade when exposed to sunlight. This gradual lightening of the color is readily noticeable when looking at an extended wing. The areas of the flights that are exposed in a folded wing, the tips, are lighter than the “shadow” portion of the feather that has the flight above it shielding that part of the feather from the sun.
There was interest in using this fading tendency to see if it would disclose whether similar looking lemon pigeons were brown or blue. Subjective attempts at fading lemon feathers in natural sunlight were inconclusive; they found that all lemon feathers readily faded.
The industries that work with colored products (paints, plastics fabrics, printing, etc.) have developed ways of objectively determining color that allow them to consistently match the target color lot after lot. This minimizes the use of subjective evaluations of color by humans and the variability that comes with each person seeing color a little differently, even if they are not “color blind,” and color perception changing based on lighting conditions, viewing angle, etc. I work in the printing world wherein our customers require specific colors on the product labels and consistent color between lots sent. I have access to a color spectrophotometer which shines the different wave lengths of light within the visible spectrum on a sample and based on the amounts of the light reflected at those wave lengths calculates a point within three dimensional color space that corresponds to the color of the sample. Knowing where the color target is in color space allows one to measure the next lot and determine how close it comes to the target and make adjustments accordingly to bring the new lot within acceptable tolerance from the ideal. This way the sleeves match the body of a shirt when cut from different bolts of cloth, the dashboard matches the side door panels, etc.
The spectrophotometer used and the software accompanying it uses the CIELAB model of color space, visually depicted below, wherein a sample’s intensity is rated on the z axis and given a L* value. The sample’s green to red value is it’s a* reading and its blue to yellow value the b* reading.
Depiction of 3 dimensional CIELAB color space
These pigment industries also use expensive fadeometers to accelerate the aging of samples under simulated sunlight to determine the light fastness, or fade resistance, of the color formulations, finding out in days or weeks what the samples might look like in weeks or months of natural sunlight exposure.
This study brings the objectivity
of the spectrophotometer to the evaluation of the feathers and the
reproducibility of a “fadeometer” to the light
exposure of the samples. An X-Rite model SP-62
spectrophotometer was used for the color evaluation of the feathers and
an inexpensive, about $150, fading chamber was fabricated mounting a UV
fixture inside a reflective metal box. This report also
includes data obtained when feather samples were naturally faded in
sunlight and then periodically evaluated spectrophotometricl
After developing the methodology, using the results from samples of known blue lemon, brown lemon and ash red lemons, the author was able to successfully classify blind sampled lemon feathers as to underlying basic color affected by the lemon mutation, which is the topic of another separate report. Also, it was demonstrated that not only brown pigmented pigeon feathers are affected by exposure to UV light.
Fading for the most part is the loss of intensity of color. This is expressed by the DL* component, change in light/dark value, of the colorimeter readings. There are some subtle changes in the red/green and blue/yellow axes of color space, but those will not be discussed here.
When samples of various colored pigeon feathers were examined by colorimeter readings it was found that white feathers have an L* value of about 93 out of the possible 100. Black feathers are about 23, quite a bit lighter than total blackness. The bar area of blue bar wings and tail approximate the L* readings of black selfs.
The UV fading chamber that was constructed utilized 24 inch fluorescent bulbs, one being UVA and the other being UVB emitting, to simulate the ultraviolet element of natural sunlight. The intensity of the UV in the chamber was calculated, and measured, to be about one eighth of average sun exposure. The amount of natural sunlight intensity and its amount of UV bombardment is quite variable. It varies with time of the year and location on the earth. The UV fading caused in Florida is much more intense than it is in Minnesota. It is more in July than it is in December. Putting feather samples out in the sun will fade them, but not in a repeatable, controllable manner. The UV chamber used in this study brought consistency to the UV exposure of the samples and was demonstrated to obtain similar results on repeat samples processed in different batches during the study. Samples were placed 6 inches beneath the bulbs and exposed 24/7 between the times of periodic evaluation with the spectrophotometer. Four sets of samples, of assorted colors, were evaluated during this study.
To ensure that the same areas of the feathers were read on the spectrophotometer the feathers were mounted inside folders made of 20 point clay coated chipboard with a hole punched through the top page of the folder. This hole was sized to be only slightly larger than the sample port of the spectrophotometer. To enhance reproducibility, orientation of the feather barbs was noted to ensure subsequent readings were taken at the same alignment, even though intentional rotation of the samples did not appreciably change the readings of the samples due to the spherical design of the X-Rite spectrophotometer.
This mounting of the samples within opaque folders not only ensured that the samples were evaluated in the same area but when the folders were opened one was able to readily see the fading contrast between the exposed portion of the sample and non-exposed portion. See photo #1 below for a visual of the sample folders. The lighter dots in the terminal bars are the UV exposed portion of the feathers and correspond to the holes punched in the top ply of the folders. Although silver and brown are similar in initial color, the rate of fading is considerably different. But silver feathers do fade.
Photo #1 brown (left) and silver (right) tail feathers after 132 days exposure to UV in chamber.
For this presentation, results from the evaluation of terminal bar areas of tail feathers were used. The feathers sampled were chosen to be internal retrices. They were either the third or fourth feathers in from the edge of the tail to ensure that they were “shadow” feathers in the closed tail and thereby had received minimal amount of natural sunlight exposure. When spread factor feathers were evaluated the portion tested was in the area that would have had a terminal bar, if the bird had not been spread. Tail feathers were used as they provided a convenient source of non-UV exposed samples. Some work was done with wing bar, wing shield and flight feathers, but that is not included herein.
Chart 1 is a summary of the fading found in the year long study of many dozen samples. Fading here is defined as the percent of the potential fading of the sample. This was determined by the difference of the CIELAB L* value at the period of evaluation from its initial L* value (before UV exposure) divided by the difference of the sample initial L* value subtracted from 93 (the L* value of the average white feather). The legend distinguishes the phenotype of the class depicted and the number of samples which resulted in the plots. The fractional number after the color description documents in the numerator the number of samples averaged through the 33 day period of evaluation. The denominator is the number of samples represented in the evaluations done beyond 33 days.
It is readily seen that both blue lemons and brown lemons fade quite easily as do khaki feathers. They quickly loose most of their color intensity. Blue feathers change very little. Brown and silver feathers fade at a much slower rate than the lemon and khaki feathers with brown fading about three times the rate of silver (dilute blue). At the termination of the study neither brown nor silver had achieved the total expected fading. The lines of their percentage fading had not leveled out as seen with the lemon and khaki samples. The dip in the 99 day khaki value is attributable to degeneration of the feather rather than it getting darker than it was at the 88 day evaluation.
The divergence of the plots of the blue lemons verses the brown lemons has been used by the author to develop a fading test that has been able to differentiate between blue and brown lemons in the lab which have been confirmed by subsequent breeding results.
A second study was done
similar to the above, but in this instance the natural Minnesota
sunlight during mid-July through September was used as the UV
source. A smaller number of samples was used, two per color,
to verify the results of the controlled UV exposures. Samples
were mounted similarly to those above, but this time the folders were
made of opaque white vinyl with an aluminum foil sheet inserted as an
added opacifier to the top ply of the folder. The vinyl was
used as it is more inert to the wind and rain of natural exposure than
cardboard and the foil inserted as a safety factor to ensure opacity
with the higher level of UV bombardment. See
Photo #2. The folder
was attached on the sloped portion of a south facing flypen resulting
in close to normal sun angle at solar noon. Colors evaluated
were blue and brown along with their respective dilute versions, silver
and khaki. Feathers were evaluated spectrophotometricl
Natural sun UV exposed blue, silver, brown and khaki tail feathers after 60 days
Both the retrice terminal bar and a portion of the non-bar feather vane were UV exposed. Exposure to wind and rain accelerated the degradation of the feather samples. Often they had to be dried and manually “preened” to reconnect the barbs in the exposed areas to get an intact enough sample for spectrophotometric evaluation. The khaki samples degraded more readily than the others and their evaluation had to be terminated prior to the rest.
Chart 2 depicts the results of the terminal bar fading found in this study. As with the controlled UV study the khaki and brown feathers readily fade. And again silver feathers fade at about a third the rate of brown. Under these much more severe conditions there is eventually even some slight fading of the blue feathers in the terminal bar area after 30 days.
Chart 3 plots the fading of the non-terminal bar vane portion of the tail feathers. The khaki feathers very quickly faded. In these lighter areas of the feather the fading of brown and silver feathers is almost the same. The brown fading rate is only slightly higher than the silver. An interesting find was that the blue area of the vane darkened during the UV exposure. This can be seen as the dark dots below the terminal bars of the blue feathers in Photo #2.
The color of pigeon feathers is affected by exposure to UV energy. The X-Rite spectrophotometer is able to objectively determine the amount of this UV effect more discernibly than subjective evaluations by the human eye. The fading induced during these studies by both artificial and natural UV found that not only brown, and even more so its dilute khaki, fade; but also silver, the dilute of blue, fades appreciably. The tendency to fade is probably not only a matter of whether the feather is brown or blue pigment, but also a matter of pigment density. Khaki feathers faded much more readily than brown, silver more than blue. The lemon feathers, which approximate khaki feathers in color and intensity faded quite similar to khaki feathers. There is a divergence of fading found between brown lemons and blue lemons which can be used to diagnose probable base color of a lemon sample.
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