Journal of Ornithology

, Volume 153, Issue 4, pp 1031–1044 | Cite as

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

  • Paul F. Donald
  • Kai Gedeon
  • Nigel J. Collar
  • Claire N. Spottiswoode
  • Mengistu Wondafrash
  • Graeme M. Buchanan
Original Article

Abstract

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.

Keywords

African birds Climate change Environmental niche models Biogeography Habitat selection 

Zusammenfassung

Das beschränkte Areal des Akazienhähers Zavattariornis stresemanni ist eine Folge hoher Abhängigkeit von nutzungsveränderten Lebensräumen innerhalb enger klimatischer Grenzen

Wir versuchen, die eigentümlich beschränkte Verbreitung des global gefährdeten Akazienhähers zu beschreiben und zu erklären. Auf regionaler Ebene legen Modelle, die nur die Bodenbedeckung berücksichtigen, eine wesentlich weitere Verbreitung geeigneter Habitate in Nordost-Afrika nahe als das tatsächlich besetzte Gebiet. Modelle hingegen, die nur Klimavariablen berücksichtigen, bilden die tatsächliche Verbreitung nahezu vollkommen ab. Sie lassen darauf schließen, dass das Verbreitungsgebiet durch ein Klimaareal begrenzt wird, das kühler, trockener und saisonaler als das der umliegenden Regionen ist. Außerhalb einer sehr engen Spanne der mittleren Jahrestemperatur von 17.5 bis 20°C ist die prognostizierte Vorkommenswahrscheinlichkeit gering. Innerhalb des als klimatisch am besten geeigneten vorhergesagten Gebietes konzentrierten sich die Beobachtungen von Akazienhähern in 1 km-Quadranten mit geringfügig, aber signifikant niedrigerem „Normalised Difference Vegetation Index“ (NDVI), was auf eine Bevorzugung von Bereichen mit niedriger Photosyntheseaktivität hinweist. Auf kleinerer räumlicher Ebene innerhalb eines 10 km2 großen Untersuchungsgebietes im Kernareal der Art lagen die Nester in 30 m-Quadranten mit höherem NDVI, aber stets unweit von Bereichen mit niedrigem NDVI. Bei diesen Gebieten handelt es sich um offenes Grasland, dass gemäß standardisierter Beobachtungen an individuellen Vögeln das Hauptnahrungshabitat darstellt. Dennoch ist eine höhere Vegetation für Schlafplätze und die Anlage der Nester wichtig, die durchschnittlich fast 5 Meter über dem Boden errichtet wurden. Daher erscheint das Artareal primär durch ein einzigartiges Klimaareal bestimmt zu sein, innerhalb dessen die Vögel parkähnliches, offenes Graslands mit eingestreuter höherer Vegetation bevorzugen, das weitgehend durch Rodung und Beweidung entstanden ist. Die Nahrung der Vögel erscheint unspezifisch, die Methoden des Nahrungserwerbs sind vielfältig. Die Modelle schätzen das klimatisch optimale Artareal auf etwa 6.000 km2, von denen ca. 4.500 km2 eine geeignete Bodenbedeckung aufweisen. Nach unserer vorsichtigen Schätzung beträgt die weltweite Population mindestens 9.000 Brutpaare, mit einer möglicherweise größeren Zahl zusätzlicher nichtbrütender Helfer. Das kleine, klimatisch beschränkte Areal der Art lässt vermuten, dass der globale Klimawandel das langfristige Überleben der Art ernsthaft gefährdet.

Supplementary material

10336_2012_832_MOESM1_ESM.docx (392 kb)
Supplementary material 1 (DOCX 392 kb)

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Copyright information

© Dt. Ornithologen-Gesellschaft e.V. 2012

Authors and Affiliations

  • Paul F. Donald
    • 1
  • Kai Gedeon
    • 2
  • Nigel J. Collar
    • 3
  • Claire N. Spottiswoode
    • 4
  • Mengistu Wondafrash
    • 5
  • Graeme M. Buchanan
    • 1
  1. 1.Conservation Science DepartmentRSPBBedfordshireUK
  2. 2.Saxon Ornithologists’ SocietyHohenstein-ErnstthalGermany
  3. 3.BirdLife InternationalCambridgeUK
  4. 4.Department of ZoologyUniversity of CambridgeCambridgeUK
  5. 5.Ethiopian Wildlife and Natural History SocietyAddis AbabaEthiopia

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