Rarities in Avian Coloration: Leucistic Birds

Grab a bird field guide and flip through its pages quickly – so fast that you can feel a breeze against your face. As the pages flicker before your eyes like a cartoon, the flashes of colors will tell stories of evolution. These usually have two basic storylines. The color of a bird’s feathers either tells us something about the nature of the bird’s environment or about the sexual preferences of picky females. Occasionally, however, there are individuals whose coloration does not fit into either of these schemes, and those are the ones I’d like to talk about today.

Several mechanisms contribute to an animal’s coloration, the most common of which involve pigments. Mammals, with their relatively dull tones, rely on melanins and melanin derivatives in their skin. Birds, known for their dazzling colors, add carotenoids, lipochromes, and porphyrins to the mix. In addition, structural colors often interact with pigments, further expanding their visual palette.

Melanin, however, remains the basic pigment for both mammals and birds. It has two forms, pheomelanin (red-yellow tones) and eumelanin (brown-black tones), produced by specialized cells called melanocytes. But, genetic mutations can result in the complete or partial absence of this pigment in the skin (and, more rarely, in the retina), which is called albinism or leucism respectively.

Albino birds are completely white, and because they lack melanin in the retina as well, their eyes typically appear pink. Less commonly, some birds have a strikingly pale or patchy appearance, with eye color remaining normal, slightly lighter, or even blue, and this is called leucistic coloration.

You may have noticed strangely-colored birds in your area, which at first glance looked like the ones you see every day. It is in cities that leucistic birds can be seen more often than in the wild. There may be several reasons for this.

Leucism is often caused by recessive alleles of genes. For the trait to manifest, two such alleles must meet in the same body. It’s like rolling two dice and waiting for two sixes to come up. In small urban populations where there is no shortage of inbreeding, these rarer alleles are more likely to meet and cause a color change in the carrier.

Another reason that leucistic birds are more common in cities is the lower predation pressure, particularly from avian predators like hawks. Urban areas can serve as relatively safe havens for birds whose conspicuous plumage would otherwise make them easy targets in the wild.

Unusual plumage coloration can pose another major problem for an individual. Although it may not always affect their survival chances, it can have a dramatic impact on their reproductive success. Females of many birds are notoriously picky and obsessive about plumage pattern and color. A leucistic male can be a bad joke to them. Fortunately, there are tolerant Samaritans who will give these weirdos a chance, as some studies have shown.

Melanin in feathers also serves purposes beyond mere aesthetics. It acts as a structural reinforcement within the keratin matrix of feathers. Notice how even otherwise pure white gulls have black wingtips? Without melanin, feathers wear more quickly, potentially compromising flight efficiency, increasing energy expenditure, and once again increasing the risk of predation.

Then there’s the phenomenon first noticed by German zoologist Constantin Wilhelm Lambert Gloger in 1833. He observed that mammals and birds living in humid regions tend to be more heavily pigmented than their counterparts in drier, cooler climates. In addition to melanin’s critical role in UV protection, it turns out that melanin-rich feathers and fur are more resistant to bacterial degradation – something particularly useful in equatorial regions where microbes thrive.

In a previous article, I discussed structural colors, one of the relatively common sources of bird vibrancy. This time I’ve focused on the opposite phenomenon: genetic mutations that result in a complete or partial loss of coloration. Both albinism and leucism are relatively rare in nature, making any sighting of such an animal a special event. If you have ever been lucky enough to capture a photograph of a wild leucistic or even albino bird, I would love for you to share it with me. I’d be happy to include your image in an update of this article.