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!