The restricted range of the Ethiopian Bush-crow Zavattariornis stresemanni is a consequence of high reliance on modified habitats within narrow climatic limits

Abstract

We attempt to describe and explain the pecu- liarly restricted distribution of the globally threatened Ethiopian Bush-crow Zavattariornis stresemanni .A t 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 pre- dicted the known distribution almost perfectly, and sug- gested 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 17.5-20C. 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-km 2 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 indi- vidual 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 km 2 , of which perhaps 4,500 km 2 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 tempera- ture 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.