One of the delights of birding is discovery of the unusual, whether a rare species, a behavior never seen before, or a quick glimpse of a legband on a migrating songbird. Some of my more exciting finds have been birds affected by an abnormal genetic condition called leucism (pronounced LUKE-ism). This rare mutation results in a total or partial reduction of color in a bird’s plumage due to inadequate deposition or fixation of pigments, namely melanin, in the feathers.
Leucism expressed in the primary and covert feathers of a male Red-winged Blackbird.
Leucism is distinctly different and relatively less rare than albinism. The albino mutation affects the melanin pigments of the entire bird, giving albino birds their characteristic ‘white’ (colorless) plumage with pink eyes and skin (though, importantly, albinism does not affect carotenoid pigments). Leucistic birds, however, have normal coloration of the eyes, bill, legs, and bare parts.
Birds with leucism show a variable amount of white or pale in the plumage, including:
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Bold white patches where the bird shouldn’t have any
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Paler overall plumage that looks faint, diluted, or bleached
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Overall white plumage with little or no color visible
Leucistic American Redstart — “Bold white patches where the bird shouldn’t have any”
Leucistic (left) and normal Black-billed Magpies — “Paler overall plumage that looks faint, diluted, or bleached”
Leucistic Red-tailed Hawk — “Overall white plumage with little or no color visible”
My resident leucistic Song Sparrow, “White Cheeks”
Exactly how common is leucism in birds? It’s difficult to find robust stats, but ProjectFeederWatch, an online database of bird species observed and reported by citizen scientists, has recorded only about 1,600 leucistic birds of any species out of 5.5 million distinct birds reported. In my 40+ years of birding, I have only seen a handful of leucistic birds, including Black-billed Magpie, American Crow, Red-winged Blackbird, Black-capped Chickadee, American Robin, Canada Goose, Dark-eyed Junco, and Song Sparrow (my beloved, longtime feeder friend ‘White Cheeks’).
Leucistic Dark-eyed Junco is like a little beacon for avian predators.
Exciting as it is to encounter leucistic rarities, birds with this mutation face special challenges in the wild. Coloration is a vital component of individual survival and reproduction, and leucism may exact significant costs to an individual bird’s fitness. Lighter or “flashy” plumage may reduce a bird’s protective camouflage, and thereby increase detection by predators. In species where plumage colors play a key role in courtship rituals, leucistic birds may be unable to find mates. Melanin is an important structural component of feathers; birds with extensive leucism have weaker feathers, which can compromise flight capability and insulation against harsh weather. White feathers also reflect heat more efficiently, which is concerning for birds needing solar radiation to stay warm in cold climates.
The genetic origins of bird leucism are well-studied; however, external, non-heritable factors of leucism remain poorly understood. Illness or food deficiency is thought to be a mechanism. In such cases, birds are unable to absorb sufficient quantities of the amino acid tyrosine from their food, resulting in poor melanin synthesis. Another mechanism, “progressive greying,” involves the continuous loss of pigment cells with age. By this mechanism, the number of white feathers would increase after every moult, similarly to what happens in maturing humans or other mammals.
As a result of leucism, birds will not have classic plumage colors represented in field guides, making bird identification a bit of a head-scratcher. Many leucistic birds still show a faint wash of color in recognizable patterns on their feathers, though color may be muted. Pure white birds can be a bit more challenging to identify, but key in on the bird’s size, shape, feeding habits, behavior, vocalizations, and associated birds as indicators of its species. Look also at the color of the feet, bill, and gape, which will be typical for the species.
Leucistic Northern Cardinal — the carotenoid pigments shine through.
Canada Goose showing salt ‘n pepper “progressive greying” — eat your heart out George Clooney.
Have you ever seen a leucistic or albino bird? If so, what species?
The floor is open...please share your latest bird news!
Happy Thanksgiving to all!
