Ethiopian Bush-crow and thermal adaptation

Ethiopian Bush-crow: Naked skin area could serve thermoregulation

The Ethiopian Bush-crow Zavattariornis stresemanni is an extremely range-restricted Ethiopian endemic that is confined to dry and relatively cool south Ethiopian acacia savannah habitats. It has been assumed that the specific temperature conditions in these areas might reflect a temperature optimum to which these birds have adapted. With non-tropical magpies Pica and ground-jays Podoces being among their closest relatives, bush-crows might have developed particular adaptive traits to persist under the tropical conditions of the south Ethiopian plains. However, this thermal adaptation hypothesis has not been tested so far. Using an infrared thermography system we investigated if bush-crows are able to regulate their body temperature by emitting heat.

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Climate and the restricted range of the Ethiopian Bush-crow

Distribution of the 4,455 1-km cells predicted to contain suitable land cover (dark blue) within 

the envelope of 6.001 1-km cells with optimal climate suitalility (light and dark blue) in southern Ethiopia. 

In a study, published in 2012 in the Journal of Ornithology*, we attempt to describe and explain the peculiarly restricted distribution of the globally threatened Ethiopian Bush-crow Zavattariornis stresemanni. At a regional scale, models containing only correlates of land cover suggested a far wider distribution of suitable habitat in north-east Africa than the area actually occupied. However, models including only climate variables predicted the known distribution almost perfectly, and suggested that the species’ area of occupancy is delimited by a pocket of climate that is cooler, dryer and more seasonal than surrounding areas. The predicted probability of occurrence was low outside a narrow range of mean annual temperatures of 17.5–20°C. Within the area predicted to be climatically most suitable, records of Bush-crows were concentrated in 1-km cells of marginally but significantly lower normalised difference vegetation index (NDVI), indicating a preference for areas of lower photosynthetic activity.  At a finer spatial scale within a 10-km2 intensive study site in the core of the species’ range, nests were located in 30-m cells of higher NDVI but always close to areas of lower NDVI. These areas of lower NDVI comprise open grassland, which standardised observations of individual birds showed to be the main foraging habitat. However, taller vegetation is also necessary for nesting and roosting; the average height of nests from the ground was nearly 5 m. Therefore, the species’ range appears to be defined primarily by a unique climate pocket within which it shows a preference for park-like habitats of grassland interspersed with taller vegetation, largely the result of clearance of vegetation by people and their associated grazers. The diet appeared unspecialised and a wide range of feeding methods was observed. Models estimate the species’ optimal climatic range to cover around 6,000 km2, of which perhaps 4,500 km2 has suitable land cover. We tentatively estimate the global population to be at least 9,000 breeding pairs, with a potentially larger additional population of non-breeding birds, particularly nest-helpers. Several climate models predict increases in both temperature and  precipitation in the southern highlands of Ethiopia. The species’ narrow climatic range suggests that global climate change may therefore pose a serious threat to its long-term survival.

* 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: 1031-1044.

Construction and thermal insulation of Ethiopian Bush-crow nests

The main function of bird nests is to provide shelter for eggs and nestlings in order to support optimal offspring development. While this basic condition applies to all nests, an enormous constructional variety in nests has evolved. Regarding its thermal properties, the avian nest construction generally represents a compromise between heat conservation,heat dissipation, and protection from external heat.

The Ethiopian Bush-crow is an endemic Ethiopian bird species which has a very small and climatically distinct range that is cooler, dryer and more seasonal than the surrounding areas. Bush-crows build large domed nests made of thorny twigs with comparatively thick walls and a soil-lined bottom. In a preliminary study* we studied the thermal properties of a bush-crow nest with the aid of IR thermography. Our results show that there was no heat emission in the early morning and that nest surface temperatures clearly corresponded to the temperature changes during the day. Therefore we hypothesise that the nest construction provides substantial thermal insulation during the night time and presumably also delays temperature increase in the internal nesting chamber during the day time. We suggest that the bush-crow’s specific nest construction may have contributed to its persistence in the climatically unique south Ethiopian conditions.

* Töpfer, T. & K. Gedeon 2012: The construction and thermal insulation of Ethiopian Bush-crow (Zavattariornis stresemanni) nests: a prelimary study. Avian Biology Research 5 (4): 198-202.

Thermographic image of a bush-crow nest just before sunrise (a) and at noon (b).

Why share Ethiopian Bush-crow and White-tailed Swallow an identical range in spite of a very different history of colonization?

Distribution map of the three Hirundo species

There are two endemic bird species in southern Ethiopia which live in almost an identical range: Ethiopian Bush-crow and White-tailed Swallow. But apparently their respective ancestors come from very different parts of the world. The closet relatives of Zavattariornis - ground jays of the Genus Podoces - live in Central Asia, whereas the relatives of Hirundo megaensis are all from Africa. Dor et al (2010) describe the relationship of the swallows as follow:

"...the ‘Pearl-breasted Swallow’ clade, is comprised of the Pearl-breasted (Hirundo dimidiata), White-tailed (H. megaensis) and Pied-winged swallows (H. leucosoma). ... Phylogenetic relationships of the White-tailed Swallow, a species endemic to a small region in southern Ethiopia, have not been previously examined, but based on plumage traits it was expected to be close to the Pearl-breasted Swallow  (Sibley and Monroe, 1990; Turner and Rose, 1989), and our analysis confirmed this affinity. Indeed, the genetic divergence between H. megaensis and H. dimidiata (0.7%) is by far the smallest existing among species in the genus (otherwise, range = 2.0–11.6%) and is equivalent to the divergence among populations of the polytypic H. rustica (range 0.25–1.6%). H. megaensis and H. dimidiata inhabit similar habitats, but are separated by a range disjunction of about 1,500 km..."

It remains one interesting question: Why share Zavattariornis stresemanni and Hirundo megaensis a nearly identical tiny range in spite of a very different history of colonization?

Avifauna of Ethiopia: A new approach in conservation

Harenna Forest (Bale Mountains, Oromia Regional State)

In Ethiopia, there are currently seven national parks, wildlife sanctuaries as well as several other protected areas. The conservation status is unsatisfactory in most of these sites. There are several reasons, but one of the challenges is without question the tremendous population pressure. For example, large parts of the Harenna Forest (Bale National Park) is used today by settlers. The decline in dense forests 1973-2000 was 15% between 1973 and 2000. About 4% of the forest disappeared completely (Pinard et al, no date).  After restrictive protection measures and bans have failed in recent years, one focuses on a more collaborative approach that incorporates the interests and knowledge of the local population.

This corresponds in principle to the efforts of IUCN to establish so-called "Indigenous and community Conserved Areas" (ICCA) - a so far unconventional reserve category (Kothari 2009). It remains to be seen whether and to what extent this concept is successful. For the Borana territory in southern Ethiopia Bassi and Tache (2011) have demonstrated the possibilities, but also the limitations of such an approach. Practical obstacles existed, especially in the massive immigration of settlers from other areas, political marginalization of pastoralists, and a de facto privatization of formerly communal land.