Back in May 2015, in one of my first blog posts, I did some research on melanism in raptors for a write-up of a Lanner Falcon that had made a meal of an unfortunate melanistic Gabar Goshawk. You can read the whole story and see Aden Gower’s pictures here: Grey is the new Black. I hope you’ll forgive me for plagiarizing myself and repeating some of that research below.
Polymorphism (multiple forms) is widespread in birds, occurring in at least 3.5% of all bird species. It is especially prevalent in raptors of the family Accipitridae, in which 22% of species occur in more than one morph. Just think of all the variations of e.g. Tawny Eagle, Wahlberg’s Eagle, Ayres’s Hawk Eagle, Booted Eagle, European Honey Buzzard and Common (Steppe) Buzzard. It is not common in Falconidae, but even in that family there are a few notable examples, such as the rare dark morph of Eleonora’s Falcon. The phenomenon is by no means restricted to birds. Melanistic jaguars, canids, tamarins, antelopes, camels, seals, squirrels, rodents, owls, penguins, vipers, lizards…the list goes on.
What is melanism? It is a condition with an abnormally high expression of eumelanin pigment, which gives the bird a much darker appearance. Melanin increases feathers’ resistance against wear and bacterial degradation, which is why most birds of prey have black on their wing tips, and why the much-stressed flight feathers are dark in most bird species. However, an overabundance of melanin can also be disadvantageous as increased resistance to UV radiation can lead to problems synthesizing Vitamin D, which may lead to a variety of auto-immune disorders. Melanism can also reduce reproductive success if colour is an important mate selection criterion.
In addition, it has been empirically demonstrated that melanistic morphs (or other colour ‘anomalies’ such as albinistic, leucistic or erythristic individuals) may be subject to higher predation rates. The most frequently cited example of this, which you would have probably learned about in high school biology, is the case of Britain’s famous peppered moths. The nocturnal insects occur in two colour morphs: a dark one, and a mottled one. When they roost by day against tree trunks the mottled form is better camouflaged, and less prone to predation by birds. As our friend Charles’ theory predicts, over time this selection pressure lead to a skew in the proportion of morphs, with the dark one becoming much the rarer. However, as pollution levels in England increased, making the trees more sooty and killing off the lichens, the dark morph became better camouflaged and thus more common.
Thanks for the science lesson Faansie, but can we get back to identifying Ovambo Sparrowhawks please?