Melanism and Leucism – Sunday 28th October 2012
While chatting recently to John Monks, the subject of melanism and leucism came up. We have all heard the expressions but how many of us understand the difference between them. I determined to undertake some research on the subject and hope you will enjoy my post this week.
Melanism, or melanosis, is a condition caused by a genetic mutation that gives a bird excess amounts of melanin in its feathers. This makes the feathers much darker than normal plumage, and many melanistic birds appear completely brown or black. There are two ways melanism can affect birds’ plumage:
- Normally dark markings are bolder and noticeably “overrun” their typical boundaries
- All the plumage is darkened and appears dark brown or black
Just like with leucism, melanism can vary for different birds and some individuals will show much darker plumage than normal, while other birds will have less noticeable changes in their coloration, particularly if they already have dark markings.
Dark Morphs and Melanism
While a true melanistic bird is rare, many bird species have regular colour morphs that show some degree of melanism. This creates a dark morph-variation of the bird’s typical plumage, and birders can learn to recognize the most common of these birds without difficulty. Two species with the most well-known dark morphs are red-tailed hawks and ferruginous hawks.
How to Identify Melanistic Birds
When a bird’s typical plumage can no longer be seen, identification can be more challenging. When looking at a melanistic bird, it is impossible to rely on colour alone to determine the species, since much of the colour will be overshadowed by the darker plumage. Instead, birders should pay particular attention to the bird’s size and shape, behaviour, feeding, range and song. If the bird is found in a flock, its associates can be strong clues about the species, even in mixed flocks. Carefully examining the bird’s legs, feet, eyes and bill is also useful, as these physical features are not affected by melanism.
Effects of Melanism on Birds
Whereas leucism can be dangerous for birds because it robs them of camouflage, melanism can actually be beneficial by helping conceal birds more fully. Melanistic birds in cold weather climates can also absorb solar radiation more efficiently, helping them regulate their body heat without expending as much energy. Studies of other melanistic animals, particularly felines, have indicated genetic links between melanism and stronger immune systems, which may give melanistic animals and birds better resistance to diseases. It is believed that these positive benefits may have helped give rise to the common dark colour morphs of different bird species.
Of course, a bird with melanism may still have difficulty attracting a mate because their coloration is not the expected breeding plumage. Too much melanin in feathers may also rob the birds of some feather flexibility, which could lead to brittle feathers that are subject to damage more easily.
Any abnormally colored bird can be a treat for birders to see. By understanding what bird melanism is and how it affects plumage, birders can easily identify melanistic birds and appreciate their uniqueness.
Melanism is the opposite of leucism meaning the subject has an overproduction of melanin resulting in a colour that is much darker than usual. Whereas subjects that are lighter than usual often have a much lower survival rate because they stand out in their environment, animals that are darker than usual may be at an advantage. One classic case of this is known as ‘Industrial Melanism’ as seen in the peppered moth.
Prior to the industrial revolution the peppered moth looked like this:
The light colouration made for a spot on match to lichen covered tree bark. Those moths that were too dark i.e. melanistic, were quickly picked off by predators. However once the industrial revolution began, pollution entered the air, killing off the sensitive lichens. This resulted in a reversal of roles; the light moths were now picked off the bare tree bark and the dark moths remained.
Melanism became an advantage in these situations. Other situations similar to these also occur, independent of human causes, for example melanistic nocturnal animals better suited to hide in the darkness of night.
To tell an albino from a leucistic animal as in these American alligators, take a look at the eye colour. Albinos have red eyes due to the lack of pigment, the red is due to blood vessels in the eyes. Animals that are leucistic will still have coloured eyes but will be lighter than usual and commonly blue.
Leucism is very similar to albinism, causing subjects to be white or at least lighter in colouration. This is due to a lack of melanin, the pigment mostly responsible for the colour. The difference between albinism and leucism is that in albinism, the body cannot produce melanin and in leucism, melanin can be produced but isn’t deposited normally on the skin, fur scales or feathers etc. Sometimes leucism occurs in patches resulting in what is referred to as pied or piebald colour morph.
Whether it is albinism, leucism or melanism, there are many ways in which an animal’s appearance may vary. Some may be a disadvantage, while others may provide an advantage.
A little fun fact for you:
Black feathers are stronger than others! This is why if you look at birds that spend a lot of time soaring such as gulls, white pelicans and white ibises, you will see many of them have wings with black tips. Regardless of the reason for their colours, as humans, we become fascinated with any deviations that we see in an animal’s colour. An instance when being different is truly celebrated.
Leucistic red kite and leucistic chaffinch photographs taken by myself at Gigrin and Loch of the Lowes respectively.
Information and a selection of remaining photographs provided by About.com and The Information Geek.
I hope you all remembered to turn back your clocks by 1 hour to daylight saving time!
The Dyfi Osprey Project and the Scottish wildlife Trust have kindly given their permission for us to post still and video images from their webcams. To visit their sites please click on the relevant link. Loch of the Lowes. Dyfi Osprey Project.