African natural history’s best kept secret?
by Sam Jones
In line with Terry Townshends excellent piece some time ago on Jankowski’s Bunting (a very rare bird indeed), I thought it pertinent to carry on this theme, bringing some limelight onto undoubtedly one of the worlds most remarkable avian subjects and a bird I have had the pure privilege of a fascinating and compelling journey in conducting some of the first ecological study on during 2013; the Ethiopian Bush-crow Zavattariornis stresemanni.
“..enigmatic and baffling… one the most remarkable African discoveries of the 21st century”
Fry et al. (2000)
The Bush-crow was discovered (in the eyes of science at least) in 1937, when a specimen was collected during an expedition to southern Ethiopia led by Edoardo Zavattari from the Zoological Institute of the Royal Institute in Rome, leading to its formal description in 1938. Its genus name followed that of the finder, while the species name comes in honour of Erwin Stresemann, the influential german ornithologist. This name is often carried into its other widely used vernacular, Stresemann’s Bush-crow.
This relatively recent discovery is interesting, but undoubtedly the most intriguing aspect of its existence in this region is its bizarre range-restriction to one tiny area of nondescript rangeland and thornscrub in the arid Borana zone of southern Ethiopia. Adding complexity, these rangelands have been subject to anthropogenic influences by the pastoralist livelihoods of the native Borana for centuries, with vast expanses of seemingly identical habitat existing directly adjacent to its occupied range.
An early ornithologist studying the region, Constantine Walker Benson (1946), remarked-
‘The reason for this remarkably restricted distribution is not at all apparent to me. There seems to be nothing at all unique or distinctive about its environment.’
This bizarre range-restriction continued to perplex ornithologists while concern also grew over its population trends and pressures on habitat within the range. Until recently, however, the reasons underpinning its peculiar occupancy remained unsolved. This mystery was finally unearthed, when research by Donald et al. (2012) discovered its area of occupancy to be described, with remarkable precision, by a climatic envelope of some 6,000km2, harbouring a cooler, drier and more seasonal local climate than its surrounding areas. This compelling finding brought to light one of African ornithology’s best kept secrets and what is, on current knowledge, probably a unique case within the avian kingdom.
To add complexity to the puzzle, the species appears unspecialised in its diet choice and (in relation to its reliance on human modified habitats) its broader habitat preferences, residing in good numbers around villages and heavily modified grazing land. Aspects of its behaviour and known ecology are also peculiar, complex social interactions, co-operative breeding and its enormous and conspicuous nests to name but a few. It is also widely recognised by the local people and fondly named the Kaka, a term synonymous with its common vocalisations.
All this leads to the natural burning question therein, to further understand what is driving this range-restriction from the biological perspective of the bird and crucially for its conservation, what impacts these might have in the face of its long and short term threats. The Bush-crow is currently listed at ‘Endangered’ under the IUCN RedList although trends in its putative population decline remain unclear.
A body of observational evidence has indicated the likely sensitivity of the Ethiopian Bush-crow to high temperature and considering the critical recent discovery of the climatic limits that describe the bush-crows range restriction, this has been identified as one area of key ecological research to be addressed. This was precisely the aim for inquiry, to investigate the behavioural impacts of high temperatures on this most unique subject.
Early findings (although some needing further clarification) appear to support previously mooted ideas that it is a vulnerability of immature birds to high ambient temperatures that might be a key determinant in the range-restriction dynamics of the species. Furthermore, considering the seasonal polarity of dry/wet seasons within the climatic envelope and the co-operative breeding strategies of the species, it appears likely that the Bush-crow has developed a specific set of behavioural and ecological traits that allow for careful energy allocation in relation to high temperatures experienced during the post-breeding season, while tending temperature affected young.
Further research will aim to clarify these relationships and provide a greater body of baseline information for use in predicting the future impacts on the Bush-crow as a product of global warming.
Understanding and interpreting the puzzle of the Ethiopian Bush-crow is still in its earliest stages and future work will no doubt shed more light on this captivating and romantic evolutionary tale. In my eyes at least, there is also no finer setting for this story than in the beautiful Borana zone of southern Ethiopia, a region as fascinating and beautiful as its endemic birdlife.
“Whatever the reason this bird is confined to a bubble, alarm bells are now ringing loudly. The storm of climate change threatens to swamp the bush-crow’s little climatic lifeboat – and once it’s gone, it’s gone for good.”
Prof. Nigel Collar- BirdLife International
Call for observations
The Ethiopian Bush-crow still remains poorly understood biologically and much of its basic natural history is unknown. A large number of observations alongside more systematic work were made during 2013 field campaigns that have added to our knowledge pool of the species. If any readers have birded the region and spent time watching Bush-crows, any field observations of note could well add to our existing understanding of the species. Currently, these notes are in preparation for publication and if you feel you may have something to add- please don’t hesitate to get in touch!
Post-script; White-tailed Swallow Hirundo megaensis
Another southern Ethiopian endemic, the White-tailed Swallow Hirundo megaensis, a species I have declined to mention thus far, also shares almost exactly the same range as the Ethiopian Bush-crow. Perplexingly, its sister taxa, the Ethiopian Swallow Hirundo aethiopica of which it is visibly similar, is widespread and common throughout east, west and central Africa. What the ecological drivers behind this range-restriction are, from the case of the Swallow, are entirely unknown and arguably even more baffling than the Bush-crow.
All photographs ©Sam Jones.
Benson, C.W.(1946) Notes on the birds of southern Abyssinia. Ibis. 88:180-205.
Donald, P. F., Gedeon, K., Collar, N. J., Spottiswoode, C. N., Wondafrash, M., & Buchanan, G. M. (2012). The restricted range of the Ethiopian Bush-crow Zavattariornis stresemanni is a consequence of high reliance on modified habitats within narrow climatic limits. Journal of Ornithology, 153(4):1031-1044.
Fry, C.H., Keith, S. & Urban, E.K.(2000) The birds of Africa, vol VI. Academic, London