Structural yellow

[Recio]

Hi everybody,

Let's analyse a primary flying feather from an Ara araurana (not sure 100% but the feather was besides an aviary with Ara arauranas in the Loro Parque - Canary Islands - Spain). See the pics bellow and you will notice that the underpart of this feather is not grey but yellow. I have some reasons to think that this is a structural yellow, that is that the yellow we see is not due to the presence of psittacins (yellow pigments) but to the reflexion of the yellow wavelengths due to a highly ordered surface cortex acting as a "special mirror" selectively for this wavelength. Before continue and for better understanding you should read about structural colours here:

http://en.wikipedia.org/wiki/Structural_coloration

Let's analyse this feather.

The external part is blue and is not iridescent. This is due to constructive interference in the spongy zone by a mechanism named "deformed matrix" allowing to produce non iridescent blues, since the nanochannels in the spongy-like keratin are randomly oriented and there is not an ordered structure allowing to change the perceived wavelength depending on the view angle.





The inner part is yellow-golden and it is iridescent, that is its apparent colour changes depending on the view angle. Iridescence is a common phenomenon in structural colours due to a highly ordered nanostructure on the surface where light incides, so that the apparent colour depends on the angle of incidence. Probably the specific structural mechanism for this yellow colour is diffraction grating.




... and for those thinking that these pics do not belong to the same feather:





We all know the effect of destroying feather structure with a hammer : it destroys the spongy zone and constructive interference disappears. Then the blue colour becomes grey-black due to the underlying melanin. But ... what happens with the yellow colour of the underside? Have a look at these pics after "hammering" the feather at the marked areas:

Blue side:





Yellow side:




As you can see the blue side becomes grey-black but the yellow side appears even more yellow. What does it means? To me it means that this yellow colour is not due to constructive interference, which would depend on the spongy zone, but to a surface phenomenon related to the cortex structure.

Other facts to think that this is not a pigment dependent yellow:

1. Psittacins are very expensive to produce and if you look at any primary flying feather containing psittacins you will notice that they are not present in the whole feather but only in the part directly exposed to the ligth (external part). Structural yellow depends on feather structure and is present in the totality of the feather as you can see in the pic.

2. Lutinos (only showing psittacin pigment) are not iridescent, and most of structural colours are iridescent, like the yellow in this feather.

3. If it was a pigment dependent yellow the presence of structural blue and yellow psittacin would have produced an apparent green colour ... and you can see that it is not the case. The yellow wavelength is reflected in the surface and can not mix with the blue wavelength produced on the other side of the feather.

Let me know your comments about this possible structural yellow before trying to understand Emeralds.

Terry said before closing the psittacin genetics forum that, till now, there was not any evidence of the existence of structural yellows in parrots ... but this does not mean that they do not exist.

Regards

Recio

[prodigy]

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[Recio]

Very interesting thinking Peter,

Could it be that the redness we see in your birds head is related to feather structure? I am quite sure that yes but we will see it later, when examining the red psittacins in structural mutations. Maybe Lee could tell us about the expression or red psittacins in Misty birds which is suppossed to be a structural mutation, probably at the cortex level.

Regards

Recio

[prodigy]

or a break down in structure

[Recio]

Feather structure could change the way psittacins are deposited in the cortex and thus how they appear to us (similar to what happens with melanins depending where they are deposited: foreground and background melanin are different percieved).

Recio

[Recio]

Let's go on.

Here you can see the normal accepted standard feather structure ...




... and the accepted mechanism of colour production:





Now imagine a mutation on the cortex outer surface able to produce a reflexion of the incident yellow wavelengths (they are "inside" the whole spectrum incident ligth) and try to sort out how green series birds (with psittacin in the cortex) and blue series birds (without psittacin in the cortex) would look like.

Regards

Recio

PS: I have used orange colour for psittacins because yellow over white is hard to see.

[Johan S]

As you can see the blue side becomes grey-black but the yellow side appears even more yellow. What does it means?

Recio, isn't this an indication of pigment deposited in the cortex, though? I have never done this test, but if you take a green feather and hit it to destroy the nanostructure of the spongy zone, does it become a brighter green, or yellow? By my interpretation of your thoughts, it should become yellow.

[prodigy]

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[prodigy]

Recio,

would sun light also not damage this sponge zone therefore limiting the light on the top of the feather VS the bottom of the feather ?

[Recio]

As you can see the blue side becomes grey-black but the yellow side appears even more yellow. What does it means?

Recio, isn't this an indication of pigment deposited in the cortex, though? I have never done this test, but if you take a green feather and hit it to destroy the nanostructure of the spongy zone, does it become a brighter green, or yellow? By my interpretation of your thoughts, it should become yellow.

Hi Johan,

If you destroy the spongy zone there is not ligth interference and what we see is the grey black colour of the underground melanin in the medula absorbing all the wavelengths. If you take a green feather and destroy the spongy zone you will get the equivalent of a grey green colour mutation. The psittacin yellow is deposited in the cortex and not affected by destroying the spongy zone.

Regards

Recio

[Recio]

Recio red in Misty will limit me to the neckring of the green misty cock bird. Lets see what i can do here. i have green in hens and young violet green misty. I have cocks in blue and violet blue.

Do you know of anybody owing a green series adult misty cock? I have not found any pic but only a description of the ring for the SF Misty in Deon's book, but not for the DF green Misty. Maybe Madas?

Regards

Recio

[Recio]

Hi Peter,

First let's correct for the colours coding:




When you destroy the spongy zone the ligth "passing" through (the non reflected blue wavelengths) is absorbed by melanin in the medula. So it is useless to increase redness.

Why yellowness/redness apparently increases after hamering the feather in both pigmental yellow and structural yellow? I think that this is related to the effect of platening an ovoid structure. The exposed cortex surface to our eyes is bigger. It is like crashing an egg on the table ... the area of the visible white shell appears bigger than before crashing the egg.
Something else : in the case of the structural yellow, after hamering the feather, the more intense yellow area appears less iridescence further confirming that the cortex has lost its curved line.

Regards

Recio

[Recio]

Now imagine a mutation on the cortex outer surface able to produce a reflexion of the incident yellow wavelengths (they are "inside" the whole spectrum incident ligth) and try to sort out how green series birds (with psittacin in the cortex) and blue series birds (without psittacin in the cortex) would look like.

Come on people ... I know you can answer it ... or may be the idea is not well expressed?

Recio

[Johan S]

So surely less spongy zone in the middle allows for more light to pass?

The reduction in the spongy zone will better expose the eumelanin in the medula. As reported by Inte Osman, the width of the spongy zone for a blue bird is about 9 micrometer. How do we reduce the width of the spongy zone? Add one dark factor, and it reduces to 6 micrometer, and DF dark reduces it even further to about 3 micrometers. And of course, you know the result when looking at the phenotype of blue, cobalt and mauve. The bird becomes darker, and not brighter as one might expect from "more light".

If we look at experiments we did in the past with my red birds feathers, the feather became more red once the feather structure was destroyed.

before hammer


after hammer


Showing us a few things:

1)The red is a pigment and not and effect of feather structure
2)We saw more red in the feathers when the "spongy zone" was broken down

The pictures are interesting, but I don't see the increase in red that you are reporting in these pics. To me they look the same, just one feather more damaged. Do you have some pictures that shows this better?

now for the next question?????

what existing mutations do we know of in IRN's that cause the breakdown of "spongy zone" thus allowing us to see a greater part of the color spectrum ??

The grey mutation. And it won't allow us to see a greater part of the colour spectrum. Just the opposite, it will allow for better absorption of the colour spectrum. Also the reason why Recio and Ben are interested in the result of the deep mutation combined with grey.

or have i missed the plot complacently ?

Complacently. Damn autocorrect.

[Johan S]

If you destroy the spongy zone there is not ligth interference and what we see is the grey black colour of the underground melanin in the medula absorbing all the wavelengths. If you take a green feather and destroy the spongy zone you will get the equivalent of a grey green colour mutation. The psittacin yellow is deposited in the cortex and not affected by destroying the spongy zone.

Thought about and discussed this with my wife this morning over coffee. And we came to the same conclusion, so it makes sense.

Now imagine a mutation on the cortex outer surface able to produce a reflexion of the incident yellow wavelengths (they are "inside" the whole spectrum incident ligth) and try to sort out how green series birds (with psittacin in the cortex) and blue series birds (without psittacin in the cortex) would look like.

Come on people ... I know you can answer it ... or may be the idea is not well expressed?

Recio

Green series birds with such a mutation would then have a double mechanism for reflecting yellow wavelenghts, via psittacin and via the cortex structure modification. So it is really hard to say how it will differ from a normal green bird without quantifying the ratio of these mechanisms compared to the wildtype. What I mean, how is the barbule surface affected? Does the structure modification take up the space of some of the psittacin granules? Or is the granule surface maintained and the new structures fit in between these granules, resulting in a larger surface area to reflect yellow? The answer is tricky, but I suspect will depend on how the barbule surface ratio has been affected. If there is a larger surface that can reflect yellow light, the bird will appear more yellow because there is more yellow light reflected in comparison to blue (than in the wildtype), and there is less of the spongy zone exposed to create blue light, i.e. a double mechanism of more yellow AND less blue. But it all really should depend on the microstructure and the surfaces.

Blue series birds should again look green, but not fluorescent. Once again, it will however depend on the microstructure that causes the ratio of the blue and yellow light we see.

How does this align with your thoughts?

[Recio]

Hi Johan,

It aligns very well ... but let's make it easier: imagine that every yellow wavelength is reflected by the mutated external surface of the cortex so that no yellow wavelength "hits" on the psittacin granules. In this situation both birds (green and blue series) would dysplay exactly the same phenotype: green... but with an iridescent shade and an apparent colour changing with ligthing/angle conditions (not exactly like a wild green because the structural yellow is differently perceived by our eyes than the "pigmental" yellow).

This is a specific situation and, of course, there are more variables to consider, as you have just developped, but it allows us to have in mind a model to explain similar phenotypes in green and blue series birds when a structural mutation is present.

... and as you say (you have stollen me the second part of my reasoning ... despite similar phenotype under normal lighting, the use of uv would allow us to detect the green series birds (fluorescent because the presence of psittacins) and the blue series ones (non fluorescent because lacking psittacins).

I think that these ideas fit quite well with the described results for Emeralds ... but to be sure we should check under normal ligthing and under uv the offspring of an Emerald paired to a wild green bird.

Regards

Recio

[Johan S]

imagine that every yellow wavelength is reflected by the mutated external surface of the cortex so that no yellow wavelength "hits" on the psittacin granules.

Recio, I'll play along with the assumption above, but I really doubt that would be the case. Then I agree with your interpretation of the 'imagined' scenario.

... and as you say (you have stollen me the second part of my reasoning ... despite similar phenotype under normal lighting, the use of uv would allow us to detect the green series birds (fluorescent because the presence of psittacins) and the blue series ones (non fluorescent because lacking psittacins).

Reading minds in a different hemisphere is just one of the things I'm good at.