I’m not sure where you are in the world as you read this, but in my home province of Ontario the scenery has changed drastically in the last 24 hours. Our first snowfall of the season was a banger, with nearly a foot of the stuff accumulating over the course of a day. I am already over it.
Winter weather does make me think of winter birding, however, and when you conjure winter birds you likely imagine some white ones. Snow Buntings, Snowy Owls, Snow Geese, ptarmigan, gulls and others are all perfectly at home in the snow, and their whiteness certainly helps them blend with their surroundings.
But these are all birds that are meant to be white. They’ve evolved to be that way and their colour serves a purpose. That’s all well and good, but if you’re an avid backyard birder or birdfeeder-watcher (or Facebook-birder) there’s another kind of white bird that may come to your attention. Consider, for instance, this American Crow:
Now I will do you the great credit of assuming that you recognize something is amiss about this particular bird. I will do you the further credit of imagining you’ve deduced that it has something to do with colour. If you guessed “uh, it’s white” you are correct! Now on to the tricky business of guessing why.
There are two similar conditions which can cause whiteness in a bird that shouldn’t be white: the one everybody thinks of, and the one everybody doesn’t. We’ll start with the seemingly obvious.
The inevitable suggestion when someone sees a white bird – or, in fact, any white animal – is that it must be albino. I’m not sure why we default to this diagnosis, but it may be because it occurs in humans and thereby interests us more than the alternative.
To understand albinism, we have to take a shallow dive into the pool of cell biology. Our bodies – and those of all living creatures – are made up of countless tiny building blocks called cells. These cells come in different varieties and perform different tasks, but the ones we’re interested in deal with colour.
In our skin there are cells called melanocytes, and in these melanocytes is a pigment called melanin. Melanin is dark (usually black or brown) in colour, and the amount of it that your body produces determines how light or dark your skin is. Melanin is responsible for the dark colours in many, if not all, other animals too.
Albinism, then, occurs when the body doesn’t produce melanin. Most often this is because of a genetic condition that causes a problem with an enzyme called tyrosinase, which is needed for melanin production. No tyrosinase means no melanin means no dark colours.
The kookaburra above is a great example of an albino bird, and it shows a few characteristics that are helpful in determining the condition. Firstly it is entirely affected – there is no such thing as “partially albino”. If you see patches of normal coloration alongside the white bits, that should be a red flag. Secondly albinism often (though not always) affects the eyes, and here we see the pale, pinkish eyes we would expect.
It should be noted that there is some debate over the correct definition of albinism, and while an absence of melanin is perhaps the most common one, it does create some weirdness. Melanin is the primary pigment in mammals and birds, but some other animals add a twist. Consider this creature:
This Corn Snake has no melanin, and therefore no dark colours. It even has pink eyes, and so we could reasonably call it albino. And yet, it does have red pigment, so it is not white as we imagine albinos should be. Tricky business. It is, in fact, not uncommon for all kinds of albino animals to show at least a little colour, and many appear pinkish or yellowish instead of truly white.
Albino animals are, in most cases, not common in the wild. This is mostly because the condition causes some associated health problems, typically involving the eyes, and these health problems make it difficult for albino animals to survive to adulthood. While it is possible to see an albino bird at your birdfeeder, it is far more likely the white bird you see is white for another reason.
Leucism is albinism’s lesser-known and more-common cousin. The term can refer to several different conditions, but most commonly it is used to describe a problem with pigment cell differentiation. What’s that? Here comes cell biology lecture number two, and this one is a little more complicated:
Remember the melanocytes from before? Well back when you were just a little ball of cells in a uterus somewhere, you had something called a neural crest. In that neural crest, many different types of cells developed, and then migrated to their final homes in other parts of your body. Among them were the melanocytes (and other types of pigment cells, in other animals who have them).
In leucistic animals, the melanocytes and other pigment cells either don’t fully develop or don’t properly migrate to where they’re supposed to be. No pigment cells means no colour.
Why is it important to know the difference between albinism and leucism? Well in your day to day life, it’s probably not. But if you want to know which condition is affecting the white bird at your feeder, understanding the underlying biology is helpful.
Because the pigment cells for the eyes come from a different place than the rest, leucism does not affect the eyes. Dark eyes, then, are a hint (though not a guarantee) that you may be looking at a leucistic bird.
Also, because albinism is a total inability to produce melanin, it always affects the entire body. In leucism, sometimes it’s only some cells which fail to develop or migrate. This means that a leucistic bird may have patches of white and patches of normal colour. Remember this crow?
While many would call this bird albino, his dark bill, eye and face suggest otherwise. This bird shows an extreme (almost entire) case of leucism, but it’s not always like that. The American Robin below wears it with a little more subtlety:
Seemingly because leucism does not affect the eyes, and thus does not cause associated ocular health problems, it is a much more common condition to see in adult wild birds than albinism. Many seem to survive quite well in spite of their unusual colorations, and some leucistic birds are seen at feeders and other known locations for many years.
If you’re a birder, feeder-watcher, or nature-lover, knowing a bit more about these interesting anomalies can make for exciting discoveries, often close to home. And if you’re a cell biologist, I’ve been hacked and I don’t know who wrote this.
American Crow – By Dominic Sherony – Leucistic American Crow (Corvus brachyrhynchos)Uploaded by Magnus Manske, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=21240538
Kookaburra – Public Domain, https://commons.wikimedia.org/w/index.php?curid=4058292
Corn Snake – By Lietuvos zoologijos sodas – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=42420266
American Robin – By maw2741 – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49366376