It’s a common knee-jerk reaction to call any bird with abnormal white feathers an albino. This is understandable given that albinism is a recessive genetic condition that causes afflicted vertebrates to appear entirely white or pink, while animals with patches of white are often called partial albinos. However, by breaking down the three more common conditions that cause abnormal white feathers in birds: total amelanism or albinism, partial amelanism or leucism, and absence of carotenoids (also termed carotenism), this blog post will help you learn the difference between these conditions and the terminology surrounding them.
A true or complete albino is an individual that lacks the ability to produce melanin, and is therefore, totally amelanistic. This inability is due to a mutation in the section of DNA that encodes for tyrosinase, which is an enzyme that catalyzes melanin production. The complete lack of melanin is why albinos have very light skin and reddish eyes (the reddish eye color seen is caused by the densely packed blood vessels of the retina behind the iris). Despite the common use of the word albino, the condition is rare to see in the wild. Albino birds are rare because most don’t survive to adulthood due to poor vision, inability to camouflage, and the lack of protection from UV radiation.
|This Laysan Albatross chick is a good example of a true albino with all white feathers. Notice the red eye and pink skin clearly showing no melanin production in this bird. Photo by Dan Maxwell.|
A partially amelanistic bird is similar to a true albino bird in that some or all feathers can lack melanin giving it a white appearance. There are cases of partially amelanistic birds with only one white feather and others showing all white feathers. As defined by Jeff Davis, a partially amelanistic bird has an abnormal absence of all melanin from parts of the plumage, skin, eyes, or all three. The difference between a true albino and partially amelanistic bird is that partially amelanistic birds can produce melanin, but the melanin is not being deposited or produced at certain locations. This can manifest as white feathers or pink skin. Unlike the red eyes of a true albino, a partially amelanistic bird will most likely have normal eye coloration due to normal melanin deposition in the eye. This is why eye color is a crucial field mark to notice when looking at an abnormally white bird in the wild.
Partially amelanistic birds with patches of feathers containing melanin are sometimes referred to as piebald or pied. The term leucism is a more specific term that usually describes a bird that has lost the ability to deposit melanin in some or all feathers, but will always have normal skin and eye coloration.
Here is an example of a partially amelanistic Western Wood-Pewee with only a few white feathers. This bird was caught in 1969 at PRBO’s Palomarin field station. Photo credit R. Stewart.
Recently on SEFI, we saw a Black-throated Blue Warbler that was not only rare to be seen on the west coast, but also a genetic rarity. This male had white feathers scattered between blue and black feathers on his cheeks, neck, flanks, and back giving it a frosted, patchy look. This is a good example of a partially amelanistic individual, where some feathers never received melanin while developing, leaving them white.
|Partially amelanistic Black-throated Blue Warbler. Photo credit Kyle Marsh.|
The strange aspect of this bird is due to the lack of blue coloration on his head and back. The absence of blue color is not directly caused by the absence of melanin, because blue coloration in a bird’s plumage is caused by feather structure, not pigment.
To understand why the blue coloration is missing we have to dive down into the structure of the feather on a microscopic scale. Within the feathers there are keratinized cells that contain gas-filled vacuoles. These vacuoles are smaller than a wavelength of visible light, so when light hits these vacuoles, the blue light waves, which are almost the shortest light waves of the visible spectrum, are scattered and reflected back out, while the longer light waves pass through. The scattering of light in many directions by particles or gas filled vacuoles is called the Tyndall effect. Behind the keratinized cell structure, there is a dark, melanistic layer that absorbs the backscattered light and prevents these longer waves from being seen. The absorption of all other colors enriches the blue light being scattered. The structure which absorbs the backscattered light needs melanin to function properly. The white feathers on our Black-throated Blue Warbler were completely lacking melanin throughout the feather, including the structure that absorbs the backscattered light. This causes all the light waves to be reflected back out of the feather giving it white coloration.
Photo of a normally pigmented Black-throated Blue Warbler. Notice the black on the flank. Photo credit Dan Maxwell.
|Notice the black that is missing from the flanks, throat and cheek. Also you can see the lack of blue coloration on the head and back. Photo credit Kyle Marsh|
To make things a bit more confusing many birds use carotenoids for added coloration in their feathers. Carotenoids are pigments created by plants, bacteria, or algae. These pigments can give feathers shades of red, orange and yellow depending on which carotenoids are absorbed and deposited within the feathers. A bird that is unable to absorb, transport, or deposit carotenoids will show white coloration where there would normally be color. The white coloration from lack of carotenoids would not be considered partial amelanism or albinism, so the term carotenism was recently coined by Jeff Davis to describe abnormalities in pigmentation due to carotenoids.
|Varied Thrush without carotenoid pigments compared to a normally pigmented varied thrush. Photo credit PRBO.|
This Varied Thrush was called an albino at the time, but clearly has melanin within the majority of its feathers. This bird doesn’t have the ability to use carotenoids to color its feathers leaving them white instead of the striking orange we normally see in this species. This could have been caused by a poor diet with no carotenoids (not likely), or this individual had an inability to absorb, transport or deposit these pigments to the feathers leaving them white.
Now that you know the basics of common aberrations to feather pigmentation you can be even more excited when you see one of these rarities fly by in the wild. For more information on feather coloration visit the sites below.
Posted by Kyle Marsh-Fall Intern