“Look at that beautiful whooping crane! What’s with the gray feathers on it?”
A partly white Sandhill crane is an overwintering resident here on the Refuge this year. Louie*, as affectionately called by the Refuge volunteers, has caused a great amount of confusion amongst visitors. “Is that a whooping crane?” “Is this an albino bird?” Luckily for Louie, it is totally unaware of its celebratory status and lives a normal life with its mate.
The confusion is understandable. Coloration in bird feathers and skin may result from one of several pigments or their combinations. Thus, a mutation in their production can result in aberrations of color. Correctly classifying and identifying the color mutations requires knowing what changes cause differences from the original pigmentation, then understanding which pigments produce the typical colors of that bird.
The most common pigment in all animals is melanin, of which there are two types: eumelanin and phaeomelanin. The former imparts black, gray and dark brown feathers. Brick-red and light brown colors are conferred by phaeomelanin. All melanin pigments are granules synthesized in special cells called melanocytes. These granules are deposited in feathers, skin, scales of the feet, and the covering of the bill. A difference in the melanistic colors depends on the amount of pigment deposition as well as the size of the granules. Additionally, combinations of melanin and other pigments, such as those produced from carotenoids, may influence shades of colors.
Both forms of melanin are synthesized from the amino acid, tyrosine, by the enzyme tyrosinase. A bird with a genetic mutation that influences the amount of tyrosine or production of the enzyme, will affect melanin synthesis. Such a bird may exhibit a complete absence of coloration associated with melanin (complete, or 100% albino) or a significant reduction in melanin pigments (an albinoid). Luecism, an incomplete loss of coloration, is more complicated.
In addition to melanin, another important pigment involved in bird coloration is produced from carotenoids. These compounds are usually derived from foods that birds eat, which are then transformed into color pigments by enzymes and deposited in feathers. Associated colors are pale yellows to scarlet reds. Although the molecular genetics associated with carotenoid coloration are not well known, variations of carotenoid bird coloration are apparently linked with sensitivity to environmental conditions, such as food availability. Additionally, not all red-based coloration is a result of carotenoid pigments. Depending on the bird species, it can be linked with one or more of the melanins, carotenoids, or a combination of both!
Although discrepancies are prevalent, most of the scientific literature defines bird albinism as “a total lack of both melanins in feathers, eyes and skin as a result of an inherited absence of tyrosinase” (van Grouw). The abnormality is caused by mutations in the gene(s) responsible for synthesis of the pigment melanin throughout the animal’s entire body. Carotenoid pigmentation is usually unaffected. Therefore, an albino bird of a species that typically produces colors from carotenoids may be mostly white but still exhibit yellow or red feathers.
On the other hand, leucism (or leukism) is defined as a partial or total lack of the melanins, eumelanin and/or phaeomelanin, in the feathers as a result of a disorder in the deposition of these pigments in the feathers. Both disorders, albinism and leucism, are inherited. However, the enzyme tyrosinase is normally present in leucism, but transformation of color into feathers is disrupted. For instance, all cells or patchy groups of cells that produce or transfer pigments may fail to develop properly. Leucism is the most common color aberration in birds, but frequently confused with albinism.
Degrees of leucism** can range from totally white individuals (100%) to only a few white feathers (less than 25%). In some leucistic forms, skin and scaly parts may also lack color or reduced normal coloration. Regardless, leucistic birds have colored eyes, whereas albino birds exhibit pink eyes.
Unlike an albino, our sandhill crane Louie has some patchy normal gray colored feathers amongst the mostly white body. It also has the typical red cap on its forehead, normal orange eye color and a slightly lighter shade of dark-colored legs. The presence of the red skin cap, patches of gray feathers, orange color in the eyes, and color on the legs indicates that the condition in this bird is related to abnormal production of melanin in specific groups of cells on its body. It is a leucistic bird.
Keep in mind that albinism is associated only with the production of a single pigment, melanin. If Louie bird was an albino, it would likely have all white feathers, pink eyes, and pinkish legs. In other words, it would be almost all white with some pinkish flesh and eyes. “What about the red cap?” you may ask. Well, it depends on what pigments impart the red skin patch on its forehead. Melanin? Carotenoids? Or is it a combination? No one knows!
As for mistaking Louie for a whooping crane, the patches of gray feathers, shorter height, and the red crown on top of its head distinguishes it from the tall white-bodied whooping crane. Whooping cranes have a red malar on the underside of their chin in addition to a red crown on top of their head. Louie does not.
Conversely, hunters should not mistakenly assume that a whooping crane is an albino sandhill crane. Complete albinism in cranes is rare, and the probabilities of finding an albino sandhill crane is extremely low. Although hunting sandhill cranes is legal in several states, shooting a whooping crane is a federal offense everywhere; they are a protected species under the Migratory Bird Treaty Act. One hunter in Wisconsin discovered the hard way that not knowing how to tell the difference between sandhill and whooping cranes, especially suspected albino or leucistic crane, could be very costly.
Addedum: The motivation for writing this post was the prevalent confusion and misinformation about albinism and leucism. This is not unreasonable. It is a complex biological topic, where even definitions in the literature disagree. However, the presence of a leucistic celebrated bird here at the Refuge is fostering mistaken identities of many bird species here and elsewhere. For instance, two visitors asked me about the ‘leucistic’ snow geese. After described and pointed out in person, some people mistake the dark-morph snow goose, commonly referred to as a ‘blue goose’, as a ‘leucistic’ snow goose.
On the other hand, a ‘leucistic’ Canada goose was also spotted on the Refuge during Festival week. Two independent photographs do suggest that the individual bird is a ‘leucistic’ Canada goose: the body feathers of the bird were all white, but the neck and head were completely typical Canada goose coloration and shape. Compared to the normal Canada geese surrounding it, this goose was the same size, hence much larger than a snow goose. Phenotyptically, and as seen in the photographs, these observations strongly suggest a leucistic individual. However, hybridization with another species or subspecies cannot be discounted without closer examination of the individual bird.
Interspecies hybridization is more common in birds than in mammals, in which sterility is more often the result when it does occur. So always keep that in mind when seeing the odd-ball bird that does not fit the classic descriptions! In fact, the red-tailed hawk as a species not only has several color morphs, but individuals have been documented as hybrids between a Swainson’s hawk, and a Sharp-shinned hawk.
Animal hybridization is of special interest to me as a biologist; it often challenges even biologists as well as wildlife lovers. A good example in mammals is the mule x horse, which, although extremely rare, can result in a viable offspring. Then there’s my obsession: the controversial case of the red wolf, currently classified as Canis rufus, but whose genetic analyses suggest it might be more coyote than wolf! So, don’t be so quick to discount an observed bird just because it does not exactly ‘fit’ the ‘mold’ of traditional identification traits or historical habitats.
While molecular technology increases our knowledge and answers many questions, it also tends to raise more questions. 🙂 I think that’s why I love it and am thankful for my work in the molecular biology field the last 10 years.
Photograph will be included soon!! I promise Photo inserted! Thanks to fellow volunteer and photographer here at Bosque del Apache NWR. Thanks, John!
** For those that are interested in more details about leucism, there are many types. They all are associated with different genes and inheritance. An absence of only one type of melanin is called schizochroism. Non-eumelanin schizochroism is defined as a complete reduction of eumelanin, which results in only reddish-brown coloration in the feathers. Non-phaeomelanin schizochroism is defined as the complete reduction of phaeomelanin. In this mutation, feathers contain only black/gray and brown coloration. These single mutations are rare.
References for further reading:
Nesbitt, SA, and Schwikert, ST. 1998. Maturation and Variation of Head Characteristics in Sandhill Cranes. Wilson Bull., 110(2): 285-288.
Price, TD. 2006. Phenotypic plasticity, sexual selection and the evolution of colour patterns. J Exp Biol, 209(Pt 12):2368-76.
Roulin, A, and Ducres AL. 2013. Genetics of colouration in birds. Semin Cell Dev Biol., Jun-Jul (6-7): 594-608.
Sibley, David. “Abnormal coloration in birds: Melanin reduction.” http://www.sibleyguides.com/2011/08/abnormal-coloration-in-birds-melanin-reduction/; Web Retrieved 11.27.2014.
US Fish and Wildlife Service. “Foster a Land Ethic That Would Make Aldo Leopold Proud”. http://www.fws.gov/midwest/news/730.html; Web Retrieved 11.27.2014.
van Grouw, H. 2006. Not every white bird is an albino: sense and nonsense about colour aberrations in birds. Dutch Birding, 28: 79-89.