By Bart Wursten
The Green-headed Oriole is a bird with a strangely fragmented distribution, occurring only on various isolated mountains. This as such is not unusual. With ancient climactic shifts, vast areas of moist rainforests were fragmented and the Green-headed Oriole only survived on the slopes of isolated mountains.
Some of the flora and fauna relying on these special circumstances would also become fragmented and be left only on these mountains. Because this happened a long time ago, evolution changed and adapted these species, and they may now be different from each other. To a certain extent this is true for the Green-headed Oriole, as the one on Mount Gorongosa is considered a separate race, recognized by the white panel on the wings. What makes the distribution of the Green-headed Oriole so unusual is the fact that it occurs on some mountains and yet not on others that are equally suitable—it is on Mount Gorongosa but never was in the Eastern Highlands of Zimbabwe. They have the same rainfall and the same altitude and latitude and they are obviously connected in many other ways as other flora and fauna clearly show. Some plants first thought to be Eastern Highland endemics are also found on Mount Gorongosa, as our own surveys have shown. In Malawi the Green-headed Oriole occurs on the Thiolo Escarpment but not on the nearby Mt Mulanje nor on Mt Namuli across the border in Mozambique. But then it shows up on two different ranges in Tanzania while absent on others. I don't think there are any clear explanations for this.
The Great Rift Valley often explains such strange features. Especially in Kenya, where this crack is much more pronounced, it has split ancient species that have evolved into different species. The west side of the rift now has a different species of Hornbill than the east side of the rift. Unfortunately this does not explain the Green-headed Oriole’s unusual range because most of the places where it is, as well as the ones where it’s absent, are all on the same side of the rift. The climate change which fragmented its distribution happened much more recently.
The Green-headed Oriole’s elusive reputation has more to with the inaccessibility of its habitat. Because of the wars, Mount Gorongosa has been out of bounds for a long time, and it remains a difficult trek to get there. However, once you reach the forest it is not a rare bird. It is not easy to see, however, as forest birds never are. The trees in the forest are up to 40+ meters tall, and the bird hides in the canopy or flies around, seen against the light from one tree to the next. Too much forest birding and you end up with a neck brace!