In mid April 2017, Lieben Swanevelder found himself staring at a rather odd raptor. The bird was cooling off in a small puddle in the road in Etosha National Park, Namibia. Realizing that this might be something worth documenting, Lieben grabbed his camera and fired off a couple of shots (which came out beautifully, I might add). But he was left in the dark regarding this dark raptor. Some Facebook arguments, confused emails and off-the-record phone calls between various experts later, the pics found their way onto my site and we think we have come up with an ID. Turns out his photos are indeed something special – one of the very few times that a melanistic morph Ovambo Sparrowhawk, Accipiter ovampensis, has been photographed! Thanks for sharing Lieben!
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?
ABOVE: Ovambo Sparrowhawks have vinaceous red eyes, which typically look dark brown or blackish from afar. Note the small head and fairly obvious orange-yellow orbital ring around the eye. This individual is not pitch-black, but rather a dark sooty charcoal colour. In a normal Ovambo, the underparts would be barred right up tot he chin. Photo by Lieben Swanevelder.
It turns out melanistic Ovambo Sparrowhawks only account for 1-2% of the population. The only other photo that I could locate online was from Per Holmen, and guess what? Also from Etosha, in February 2012. Could it be that there is a localised population there in which melanism is more prevalent? n=2 is probably too small a sample size to come to such a conclusion, but it is definitely worth looking out for melanistic Ovambos up there. But black Gabar Goshawks are far more common. Data from southern Africa suggests dark morphs represent approximately 10% in Namibia, 15% in Zimbabwe, 11% in the South African bushveld, 7% in Kruger National Park and up to 25% in Swaziland.
So how do we know this is an Ovambo and not a Gabar? In a word, shape. Experienced birders know that shape, structure and proportions are often far more helpful than colouration, and if the bird lacks the normal colouration, shape becomes essential. There are also some bare part colour differences. Some pointers below, and in the image captions.
- The proportionately small head (“snaky head” in one book) is immediately obvious. Compare it to the big-headed Gabars below.
- Secondly, Ovambo Sparrowhawks have characteristically ‘swollen’ cere, which looks like a bump above the nostrils. Gabars and other Accipiters also have a fleshy cere, but not as pronounced as in Ovambo.
- Eye colour is similar in Gabar and Ovambo, but the former generally has a more reddish or orange-red bill. In Ovambos the cere/gape tends to be paler orange. That being said, melanistic birds may show varying degrees of black on their cere (and legs).
- Ovambos usually show a fairly obvious orange orbital ring around the eye. A few Gabars are similar, but most have a dark greyish orbital ring which does not contrast much. Indeed, in normal grey Gabars there is often a dark mask around the eye and towards the bill, giving them an angry look. Ovambos have more ‘open’ and paler faces. This Etosha bird definitely has an obvious orbital ring.
ABOVE: Melanistic Ovambo Sparrowhawk. Again note the proportionately small head. The legs and toes are long and very thin in this species, and tend to be paler orange than Gabars. Also, dark morph Gabars usually have black scales on the front surface of their tarsi. The legs are plain orange in dark Ovambos. Note that the diagnostic white rump/uppertail coverts of grey morph Gabar Goshawk is absent in dark morph birds, so this cannot be used to separate them from dark Ovambos. Photo by Lieben Swanevelder.
ABOVE: Melanistic Ovambo Sparrowhawk. Do I need to say it again? Tiny head! It almost looks comically small from this angle. This bird seems to be mostly in fresh plumage, but some inner primaries and all but the inner pair of tail feathers are worn – note the definite contrast between the dark innermost tail feathers and the outer ones. While we’re looking at the tail, you can clearly see the white shaft-streaks that are characteristic of Ovambo Sparrowhawk. Photo by Lieben Swanevelder.
ABOVE: Melanistic Ovambo Sparrowhawk. This is the only other photo of a melanistic Ovambo that I could quickly locate. Taken by Per Holmen on 26 February 2012, also in Etosha. Let’s look at the features again: small head, orange orbital ring, orange cere with bump, very long and slender orange legs and toes and no black scales on front of legs. Could this even be the same bird that Lieben photographed 5 years later? Who knows. It is interesting that Per’s bird is also not pitch black like most Gabar Goshawks, but is a dark charcoal instead. Photo by Per Holmen.
The one obvious question is, shouldn’t this bird be black instead of dark grey? A good question. It is certainly not nearly as black as typical melanistic Gabar Goshawks. Indeed, on some of the more heavily exposed photos it looks almost “normal” grey. Nevertheless, it still lacks any barring below. I suspect that things are not always so black-and-white when it comes to colour aberrations. Dark plumage in birds is controlled by the melanocortin-1 receptor gene (MC1R); this little section of the genome codes for a specific amino acid, which results in the expression of melanocytes; thus increased activation of MC1R results in the synthesis of more dark-coloured eumelanin. But many birds show a range of colour morphs with different degrees of eumelanin and yellowish or reddish phaeomelanin. Think Tawny Eagle, Wahlberg’s Eagle and Steppe Buzzard for example, or jaegers and skuas for that matter. Perhaps this is a melanistic bird, but with a lower degree of melanism.
Then again, how much do we really know about this rare mutation in Ovambo Sparrowhawks? Perhaps this charcoal colour is the rule rather than the exception? If you have any thoughts or comments, I would love to hear them!