Lanner in the road by Aden Gower

On Sunday 19 April 2015, Aden Gower was traveling between Vryburg and Rustenburg in South Africa’s North West province when he noticed an odd shape in the road. As he slowed down the amorphous shape morphed into a raptor with its wings outstretched and drooped in the protective ‘mantling’ posture, indicating that it was sitting on a newly acquired prey item. Aden quickly figured out that the predator was a juvenile Lanner Falcon Falcon biarmicus, but what was the prey species? He managed to signal to oncoming traffic to slow down, as the Lanner picked up its prey and flew heavily to a small raised earthen bank on the side of the road. Aden grabbed his camera and managed to take a few shots…which he subsequently emailed to me to help identify the Lanner’s victim.

Aden described the prey bird as having ‘black feathers up to the knee joint, with yellow legs that have a black stripe running down the middle and onto each claw, while the tail was barred black and white.’ What else can we make out from the images? Firstly, the prey is quite large: perhaps about 60% the size of the Lanner, judging by the length of the tail and primaries. The legs, and particularly the tibiae, are very long. Likewise the toes seem long, but strong. Plumage-wise it appears entirely black, but with odd white patches showing through here and there. The legs are yellow, with black areas on the anterion surface and black claws. There is distinct white barring on the remiges and rectrices. What does this leave us with?

Lanner with prey by Aden Gower
ABOVE: Juvenile Lanner Falcon with unknown prey. The falcon seems to have been feeding on its prey for a few minutes, as the large and easily accessible pectoral muscles are already mostly devoured. (Photos by Aden Gower)
Lanner with prey by Aden Gower
ABOVE: Why is this is a juvenile Lanner, instead of say a young Peregrine? There are some differences in wing and tail shape and proportions, but other clues visible in these images are: 1) the narrow ‘moustache’ which would be much broader in Peregrine; 2) the blotchy breast markings, which are finer and more extensive in young Peregrine; 3) the pale creamy-cinnamon crown (dark in Peregrine); 4) and the unmarked ‘trousers’ which are typically spotted or streaked in Peregrine. (Photos by Aden Gower)

The combination of the large size, long tibiae, strong toes and hints of white underdown suggests to me that this is an Accipiter, and the colour combinations suggest a Gabar Goshawk Micronisus gabar. The kicker however, is that this is an example of the uncommon melanistic (black) morph, instead of the normal grey morph. 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.

The kicker, however, is that this is an example of the uncommon melanistic (black) morph, instead of the normal grey morph

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.

There has been a whole barrage of arguments, defenses and counter-arguments in regards to the peppered moth hypothesis. Countless papers, many books and a few professional lifetimes of academics have been dedicated to peppered moths (some fascinating further reading here: Moths and Melanism). Suffice to say that melanism and other colour variations is a widespread, much-researched but still relatively poorly understood phenomenon.

Melanistic Gabar Goshawk by Kevin Ravno
The incidence of the melanistic form (‘black morph’) of Gabar Goshawk varies regionally, but can be as high as a quarter of the total population in some areas. (Photo by Kevin Ravno;

Black morphs are relatively common in Gabar Goshawks, and account for a significant proportion of the population in many areas. 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. Interestingly, both normal grey chicks and black chicks are often raised in the same nest, suggesting that it is controlled by a recessive gene (perhaps the melanocortin 1 receptor or MC1R gene, which has been shown to code for melanin pigmentation in various vertebrates). Ovambo Sparrowhawks also have a melanistic morph, but it is apparently much rarer than in Gabar Goshawks (I’ve never seen a black Ovambo, but I’ve seen about 30 or so black Gabars). Melanistic morphs also occur in Western Marsh Harrier and Montagu’s Harriers, whereas birds like Black Sparrowhawks and Bat Hawks vary tremendously in the amount of black vs. white in their plumage.

What’s the story in this case? I have two theories. Firstly, and perhaps most likely, it may be that a third party was involved. Namely, a car. Being mostly ambush predators of birds, Gabar Goshawks often fly low over the ground, weaving in between trees and crossing gaps to flush small birds. They frequently flash across roads, and it is quite possible that it was struck by a car, and then discovered by the Lanner (Lanners do occasionally feed on carrion, and Gabars are frequent victims of vehicle strikes). Other sources of Gabar mortality include flying into fences or drowing in farm reservoirs, and they have been recorded as prey item of Walhberg’s Eagle. Interestingly, there is also a record of a family of Southern Pied Babblers killing a Gabar after it attacked one of their own (not bad for a bunch of oversized warblers hey?).

My second theory is that perhaps the Gabar was so distracted with its own catch that it presented an easy opportunity for the Lanner to surprise and overpower. Incidentally, on the same day as Aden saw this drama unfold, I also happened upon a Gabar on a road verge, about 150 km away. When I drove past the Gabar suddenly took flight from the ground, releasing his hold on a dove which fluttered off in the opposite direction. I got the impression that the Gabar got the fright of its life when my car roared past, and that it simply wasn’t minding its surroundings.

In summary, I do not think a 135-195 gram Gabar Goshawk is a particularly large prey item for a Lanner, which regularly takes birds up to 1.5 kg including pigeons and gamebirds. Nevertheless, it is an interesting observation and makes one wonder if the Gabar would have been more fortunate if it had been of the normal grey variety instead of the conspicuous melanistic morph. Thanks to Aden for sharing his notes and photos!