Aberrant Colour Terminology
Most people have heard of albinism, but there are many other mechanisms that result in various aberrant plumages in birds. Recent general birding publications only mention a few of the possible aberrant conditions. The introduction in Sibley (2014) briefly touches on albinism, leucism, melanin reduction and carotenoid issues while warning readers that "It is generally impossible for an observer in the field to determine the underlying cause of the abnormality". Alderfer & Dunn (2014) contains a few words each on albinism, leucism and melanism. Reibler et al. (2014) also noted that this is an active area of research and that concepts are continually being refined and extended.
Terminology usage between references varies. The most complete and recent publication that I've come across was a 2013 paper in British Birds called What colour is that bird? The causes and recognition of common colour aberrations in birds by Hein van Grouw. Hein is a Bird Curator at the Natural History Museum in London, England, and is currently working with the British Trust for Ornithology on the occurence and causes of colour aberrations in blackbirds. His 2013 paper is cited in many recent papers on aberrant plumages in birds, therefore, it should be an appropriate reference.
Colour Basics
Natural colours in feathers arise from the deposition of pigments, the development of keratin nanostructures, or both. Other colouration is acquired through soiling, staining, sun bleaching or abrasion. (Davis 2007)
The most common pigments in birds are melanins. There are two forms of melanin: eumelanin and phaeomelanin. Eumelanin is responsible for black, grey and dark brown colours while phaeomelanin produces warm reddish-brown to pale buff. Carotenoids are the second most common pigment and vary in colour from pale yellow to scarlet red. (Grouw 2013)
There are also rarer pigments called psittacofulvins and porphyrins. Psittacofulvins produce red, orange and yellow strictly in birds of the order Psittaciforms (parrots, cockatoos, lories and lorikeets). Porphyrins are more widespread, being present in at least 13 orders of birds including owls and bustards (large, Old World, omnivorous bird). (Reidler 2014)
Attempts to age and sex the many Mountain Bluebirds (at least 4) that arrived in Nova Scotia during autumn 2015 led me to learn about structural colours. The blue colour seen in these westerners is a structural colour created by light scattered by keratin structures and air in the feathers along with melanin. Shawkey & Hill (2006) studied structural colours and showed that melanin's primary function is to absorb the incoherently scattered light, thereby enhancing colour purity of the light scattered by the keratin and air. The keratin structures not only help to produce blue feathers in birds, but also green and iridescent colours (Davis 2007).
Aberrant Colours
There are many genetic mutations that cause colour aberrations. According to van Grouw (2013), the six most common heritable colour aberrations are albinism, leucism, progressive graying, brown, dilution, ino and melanism. Progressive graying may or may not be heritable.
The table below is modified from van Grouw (2013), and summarizes the seven colour aberrations listed above.
Ghost Grackle
On Feb 3, 2016, I was visually sifting through a large flock of blackbirds at Pleasant Lake, Yarmouth Co., when something white caught my eye. It was the same size as the Common Grackles it was with, but I didn't really get a good look at it on that day. Two days later on Feb 5, the white bird showed up at my work feeder in Tusket (Fig. 1), about 3 km from Pleasant Lake. I got good looks of it this time and it was clearly an aberrant Common Grackle. I called Ervin Olsen who came quickly and was able to see the "Ghost Grackle".
Van Grouw (2013) admits that identifying the mutations causing aberrant colours in birds is difficult, but he is optimistic that many mutations can be named. Underlying mutations can be eliminated by examining this bird in detail and using the table above.
We can begin by eliminating albinism, leucism and progressive greying since the Ghost Grackle doesn't show all-white plumage or all-white feathers. Both the head and parts of the feathers that are protected by overlying feathers are a pale brown, not white. Figure 2 shows the bird with spread tail and wings which makes visible the parts of the flight feathers that are covered when the bird is perched. These normally covered parts of the feathers are less bleached than the exposed parts and show a more true representation of the colour of the feathers just after moult.
Dilution's effect on a black bird such as a Common Grackle would produce a grayish body. The Ghost Grackle is more pale brownish and doesn't appear to show any gray at all. Both types of ino require at least a pink bill and feet, which is not the case for this bird. It is clearly not melanistic, so we are left with the mutation known as brown. After progressive graying, brown in the most common colour aberration in birds. (van Grouw 2013)
More technically, brown is is the case where the number of eumelanin pigments remains unchanged, but the appearance of the pigment is altered. The alteration is specifically that the eumelanin sythesis is incomplete as the eumelanin is not fully oxidised, changing what is normally black to brown. In this mutation, the pigment phaeomelanin is unaffected (van Grouw believes that there is no phaeomelanin in Common Grackles). This partial oxidation results in plumage that is very sensitive to sunlight and will bleach quickly. This susceptibility to bleaching is why checking the less exposed parts of the plumage is required. (van Grouw 2013)
How is the iridescence that characterizes the Common Grackle effected by the reduced oxidation of the eumelanin? Maia et al. (2009) found that the role of melanin in the production of iridescence in Blue-black Grassquits was to provide additional refraction interfaces and to absorb incoherently reflected light, which are instrumental in the production of iridescence. Therefore, it is safe to assume the eumelanin in the Common Grackle is essential to the creation of iridescence. Shawkey et al. (2006) do look at a few grackle species, but not the Common Grackle, and only mention melanin's role in creating refraction interfaces and do not touch on its role in absorption of incoherently reflected light (possibly an oversight).
One obvious feature of this bird remains unscrutinized. The head and neck are much darker than the body, even if it was exposed to sunlight as much as the exposed body plumage that has quickly bleached towards white. This is due to the relatively high concentration of eumelanin in the head and neck. A higher concentration of pigment bleaches less quick than a lower concentration, which in this case, results in a darker head and neck. (van Grouw pers. comm.)
While the head and neck of a typical Common Gracle is clearly iridescent, the body appears to show less idirescence, perhaps tending somewhat towards glossy. Maia et al. (2011) explain that iridescent feathers requires a continuous layer of melanin beneath the surface keratin where as gloss is produced by a less organized, quasi-ordered melanin layer. Matte black feathers show even less melanin order and continuity compared to those showing gloss. This suggests a relatively higher concentration of melanin in the iridescent feathers compared to that of the less iridescent feathers (i.e. glossy or matte feathers).
While it is possible that a higher concentration of melanin in the head and neck are due to the pigment's role in iridescence, there might also be another, possibly related, reason. Van Grouw (pers. comm.) shared his experience of non-iridescent black birds birds like Carrion Crow and Black Oystercatcher. He explains that these birds also show a higher melanin concentration in the head, which is not related to visible iridescence. Shawkey et al. (2006) describe how a small number of morphological steps are required for matte feathers to evolve into iridescent ones. Perhaps the high concentration of melanin in Carrion Crow and Black Oystercatcher hint at past iridescence or possible future iridescence in these species. This evolution idea is simply speculation, it would be interesting to see research on this topic.
It seems that the mutation brown in this Common Grackle has both changed the blacks to browns and entirely removed any perceptible gloss or iridescence. The reduced oxidation of the eumelanin has also disrupted the mechanisms that create gloss and iridescence, but still provides differing intensities of pale matte brown colouration to the feathers.
There are many very technical papers available for free download from the internet on the subject of feather colouration and aberrant plumags. All of the journal references below are freely downloadable; if you've got an interest in this topic, and want to lean more - dive in!
Acknowledgements:
I wish to thank Hein van Grouw for his e-mail comments regarding the photos of the Ghost Grackle that I had sent to him. Maxine Quinton and Kathleen MacAuley provided feedback to draft versions of this post which were extremely helpful.
References:
Alderfer, J., J.L. Dunn. 2014. (Ed). Complete Birds of North America, 2nd Edition. National Geographic Society. Washington DC, USA.
Davis, J. N. 2007. Color abnormalities in birds: A proposed nomenclature for birders. Birding 39:36–46.
Elphick, C., J.B. Dunning Jr., & D.A. Sibley. 2009. The Sibley Guide to Bird Life & Behavior. Alfred A. Knopf, New York.
Grouw, Hein van (2013) What colour is that bird? The causes and recognition of common colour aberrations in birds. British Birds 106: 17-29.
Maia, R., J. V. O. Caetano, S. N. Báo, and R. H. Macedo. 2009. Iridescent structural colour production in male blue-black grassquit feather barbules: the role of keratin and melanin. Journal of the Royal Society Interface 6:S203-11.
Maia, R., L. D'Alba and M.D. Shawkey. 2011. What makes a feather shine? A nanostructural basis for glossy black colours in feathers. Proc Biol Sci 278(1714):1973-1980.
Reidler R., C. Pemse, J. Druzik, M. Gleeson and E. Pearlstein. 2014. A Review of Color-Producing Mechanisms in Feathers and Their Inflence on Preventative Conservation Strategies. Journal of the American Institute for Conservation Vol. 53 No. 1, 44-65.
Shawkey, M.D. and G.E. Hill. 2006. Significance of a basal melanin layer to production of non-iridescent structural plumage colour: evidence from an amelanotic Stellers' Jay (Cyanocitta stelleri). Journal of Experimental Biology 209:1245-1250.
Shawkey, M.D., M.E. Hauber, L.K. Estep & G.E. Hill. 2006. Evolutionary transitions and mechanisms of matte and iridescent plumage coloration in grackles and allies (Icteridae). Journal of the Royal Society Interface, 3, 777-786.
Sibley, D.A. 2014. The Sibley Guide to Birds 2nd Ed. Alfred A. Knopf, New York, N.Y.
Most people have heard of albinism, but there are many other mechanisms that result in various aberrant plumages in birds. Recent general birding publications only mention a few of the possible aberrant conditions. The introduction in Sibley (2014) briefly touches on albinism, leucism, melanin reduction and carotenoid issues while warning readers that "It is generally impossible for an observer in the field to determine the underlying cause of the abnormality". Alderfer & Dunn (2014) contains a few words each on albinism, leucism and melanism. Reibler et al. (2014) also noted that this is an active area of research and that concepts are continually being refined and extended.
Terminology usage between references varies. The most complete and recent publication that I've come across was a 2013 paper in British Birds called What colour is that bird? The causes and recognition of common colour aberrations in birds by Hein van Grouw. Hein is a Bird Curator at the Natural History Museum in London, England, and is currently working with the British Trust for Ornithology on the occurence and causes of colour aberrations in blackbirds. His 2013 paper is cited in many recent papers on aberrant plumages in birds, therefore, it should be an appropriate reference.
Colour Basics
Natural colours in feathers arise from the deposition of pigments, the development of keratin nanostructures, or both. Other colouration is acquired through soiling, staining, sun bleaching or abrasion. (Davis 2007)
The most common pigments in birds are melanins. There are two forms of melanin: eumelanin and phaeomelanin. Eumelanin is responsible for black, grey and dark brown colours while phaeomelanin produces warm reddish-brown to pale buff. Carotenoids are the second most common pigment and vary in colour from pale yellow to scarlet red. (Grouw 2013)
There are also rarer pigments called psittacofulvins and porphyrins. Psittacofulvins produce red, orange and yellow strictly in birds of the order Psittaciforms (parrots, cockatoos, lories and lorikeets). Porphyrins are more widespread, being present in at least 13 orders of birds including owls and bustards (large, Old World, omnivorous bird). (Reidler 2014)
Attempts to age and sex the many Mountain Bluebirds (at least 4) that arrived in Nova Scotia during autumn 2015 led me to learn about structural colours. The blue colour seen in these westerners is a structural colour created by light scattered by keratin structures and air in the feathers along with melanin. Shawkey & Hill (2006) studied structural colours and showed that melanin's primary function is to absorb the incoherently scattered light, thereby enhancing colour purity of the light scattered by the keratin and air. The keratin structures not only help to produce blue feathers in birds, but also green and iridescent colours (Davis 2007).
Aberrant Colours
There are many genetic mutations that cause colour aberrations. According to van Grouw (2013), the six most common heritable colour aberrations are albinism, leucism, progressive graying, brown, dilution, ino and melanism. Progressive graying may or may not be heritable.
The table below is modified from van Grouw (2013), and summarizes the seven colour aberrations listed above.
| Mutation | Gene Action | Effect |
|---|---|---|
| Albinism | Total lack of both melanins in feathers, eyes and skin. | All white plumage, red eyes, and pink feet and bill. |
| Leucism | Partial or total lack of both melanins in feathers and skin. | All-white plumage or all-white feathers mixed in with normal-coloured ones. Pink or normal bill and feet and always normal-coloured eyes. Is typically bilaterally symetrical. |
| Progressive Graying | Partial or total lack of both melanins that progresses with age. | All-white plumage or all-white feathers mixed in with normal-coloured ones. Pink or normal bill and feet and always normal-coloured eyes. |
| Brown | Qualitative reduction of eumelanin. | Originally black is brown, originally reddish/yellow/brown is unaffected. |
| Dilution | Quantitative reduction in both melanins (1) or only eumelanin (2). | (1) Originally black is silvery gray, originally reddish-/yellow-brown is buff/cream. (2) Originally black is silvery gray, originally reddish-/yellow-brown is unaffected. |
| Ino | Qualitative reduction both melanins, strong (1) or weak (2). | (1) Originally black is very pale brown/cream, originally reddish-/yellow-brown hardly visible. Eyes pinkish, pink feet and bill. (2) Originally black is light brown, originally reddish-/yellow-brown is buff/cream. Pink feet and bill. |
| Melanism | Abnormal deposit of melanin. | Increase of black and/or reddish-brown. |
Ghost Grackle
On Feb 3, 2016, I was visually sifting through a large flock of blackbirds at Pleasant Lake, Yarmouth Co., when something white caught my eye. It was the same size as the Common Grackles it was with, but I didn't really get a good look at it on that day. Two days later on Feb 5, the white bird showed up at my work feeder in Tusket (Fig. 1), about 3 km from Pleasant Lake. I got good looks of it this time and it was clearly an aberrant Common Grackle. I called Ervin Olsen who came quickly and was able to see the "Ghost Grackle".
 |
| Figure 1. Aberrant Common Grackle in Tusket, Feb 5, 2016. Photo by Alix d'Entremont. |
Van Grouw (2013) admits that identifying the mutations causing aberrant colours in birds is difficult, but he is optimistic that many mutations can be named. Underlying mutations can be eliminated by examining this bird in detail and using the table above.
We can begin by eliminating albinism, leucism and progressive greying since the Ghost Grackle doesn't show all-white plumage or all-white feathers. Both the head and parts of the feathers that are protected by overlying feathers are a pale brown, not white. Figure 2 shows the bird with spread tail and wings which makes visible the parts of the flight feathers that are covered when the bird is perched. These normally covered parts of the feathers are less bleached than the exposed parts and show a more true representation of the colour of the feathers just after moult.
 |
| Figure 2. Aberrant Common Grackle in Tusket, Feb 5, 2016. Photo by Alix d'Entremont. |
Dilution's effect on a black bird such as a Common Grackle would produce a grayish body. The Ghost Grackle is more pale brownish and doesn't appear to show any gray at all. Both types of ino require at least a pink bill and feet, which is not the case for this bird. It is clearly not melanistic, so we are left with the mutation known as brown. After progressive graying, brown in the most common colour aberration in birds. (van Grouw 2013)
More technically, brown is is the case where the number of eumelanin pigments remains unchanged, but the appearance of the pigment is altered. The alteration is specifically that the eumelanin sythesis is incomplete as the eumelanin is not fully oxidised, changing what is normally black to brown. In this mutation, the pigment phaeomelanin is unaffected (van Grouw believes that there is no phaeomelanin in Common Grackles). This partial oxidation results in plumage that is very sensitive to sunlight and will bleach quickly. This susceptibility to bleaching is why checking the less exposed parts of the plumage is required. (van Grouw 2013)
How is the iridescence that characterizes the Common Grackle effected by the reduced oxidation of the eumelanin? Maia et al. (2009) found that the role of melanin in the production of iridescence in Blue-black Grassquits was to provide additional refraction interfaces and to absorb incoherently reflected light, which are instrumental in the production of iridescence. Therefore, it is safe to assume the eumelanin in the Common Grackle is essential to the creation of iridescence. Shawkey et al. (2006) do look at a few grackle species, but not the Common Grackle, and only mention melanin's role in creating refraction interfaces and do not touch on its role in absorption of incoherently reflected light (possibly an oversight).
One obvious feature of this bird remains unscrutinized. The head and neck are much darker than the body, even if it was exposed to sunlight as much as the exposed body plumage that has quickly bleached towards white. This is due to the relatively high concentration of eumelanin in the head and neck. A higher concentration of pigment bleaches less quick than a lower concentration, which in this case, results in a darker head and neck. (van Grouw pers. comm.)
While the head and neck of a typical Common Gracle is clearly iridescent, the body appears to show less idirescence, perhaps tending somewhat towards glossy. Maia et al. (2011) explain that iridescent feathers requires a continuous layer of melanin beneath the surface keratin where as gloss is produced by a less organized, quasi-ordered melanin layer. Matte black feathers show even less melanin order and continuity compared to those showing gloss. This suggests a relatively higher concentration of melanin in the iridescent feathers compared to that of the less iridescent feathers (i.e. glossy or matte feathers).
While it is possible that a higher concentration of melanin in the head and neck are due to the pigment's role in iridescence, there might also be another, possibly related, reason. Van Grouw (pers. comm.) shared his experience of non-iridescent black birds birds like Carrion Crow and Black Oystercatcher. He explains that these birds also show a higher melanin concentration in the head, which is not related to visible iridescence. Shawkey et al. (2006) describe how a small number of morphological steps are required for matte feathers to evolve into iridescent ones. Perhaps the high concentration of melanin in Carrion Crow and Black Oystercatcher hint at past iridescence or possible future iridescence in these species. This evolution idea is simply speculation, it would be interesting to see research on this topic.
It seems that the mutation brown in this Common Grackle has both changed the blacks to browns and entirely removed any perceptible gloss or iridescence. The reduced oxidation of the eumelanin has also disrupted the mechanisms that create gloss and iridescence, but still provides differing intensities of pale matte brown colouration to the feathers.
There are many very technical papers available for free download from the internet on the subject of feather colouration and aberrant plumags. All of the journal references below are freely downloadable; if you've got an interest in this topic, and want to lean more - dive in!
Acknowledgements:
I wish to thank Hein van Grouw for his e-mail comments regarding the photos of the Ghost Grackle that I had sent to him. Maxine Quinton and Kathleen MacAuley provided feedback to draft versions of this post which were extremely helpful.
References:
Alderfer, J., J.L. Dunn. 2014. (Ed). Complete Birds of North America, 2nd Edition. National Geographic Society. Washington DC, USA.
Davis, J. N. 2007. Color abnormalities in birds: A proposed nomenclature for birders. Birding 39:36–46.
Elphick, C., J.B. Dunning Jr., & D.A. Sibley. 2009. The Sibley Guide to Bird Life & Behavior. Alfred A. Knopf, New York.
Grouw, Hein van (2013) What colour is that bird? The causes and recognition of common colour aberrations in birds. British Birds 106: 17-29.
Maia, R., J. V. O. Caetano, S. N. Báo, and R. H. Macedo. 2009. Iridescent structural colour production in male blue-black grassquit feather barbules: the role of keratin and melanin. Journal of the Royal Society Interface 6:S203-11.
Maia, R., L. D'Alba and M.D. Shawkey. 2011. What makes a feather shine? A nanostructural basis for glossy black colours in feathers. Proc Biol Sci 278(1714):1973-1980.
Reidler R., C. Pemse, J. Druzik, M. Gleeson and E. Pearlstein. 2014. A Review of Color-Producing Mechanisms in Feathers and Their Inflence on Preventative Conservation Strategies. Journal of the American Institute for Conservation Vol. 53 No. 1, 44-65.
Shawkey, M.D. and G.E. Hill. 2006. Significance of a basal melanin layer to production of non-iridescent structural plumage colour: evidence from an amelanotic Stellers' Jay (Cyanocitta stelleri). Journal of Experimental Biology 209:1245-1250.
Shawkey, M.D., M.E. Hauber, L.K. Estep & G.E. Hill. 2006. Evolutionary transitions and mechanisms of matte and iridescent plumage coloration in grackles and allies (Icteridae). Journal of the Royal Society Interface, 3, 777-786.
Sibley, D.A. 2014. The Sibley Guide to Birds 2nd Ed. Alfred A. Knopf, New York, N.Y.
Very well written, Alix.
ReplyDelete