An Ethiopian mystery is solved!

I am away this week, so I am taking the opportunity to let colleagues tell you about some of our International Research.  In today's guest blog, Paul Donald talks about the mystery of the charismatic Ethiopian bush-crow...

Today was a great day – an email from the international Journal of Ornithology confirming that a scientific article by my colleagues and I has been accepted for publication. Not a massively rare event for an RSPB researcher as our department produces many such papers each year, but this one is a bit special for me because it answers one of the great mysteries of African ornithology, one that has intrigued me for years.

The conundrum is this: why does the Ethiopian bush-crow have such a restricted distribution?  Or, put another way, why should a smart and adaptable bird that eats almost anything, that can survive happily in even the most heavily degraded habitats, that likes hanging around villages, have a world range so small it would fit comfortably inside Norfolk?  Surely it should be common and widespread, like so many others in the crow family?  In 1946, the formidable ornithologist Constantine Benson concluded “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”.  He was stumped, and since then others have looked and failed to find an answer.  But the reason they failed, we now believe, is that they were looking for a barrier invisible to the human eye, like a glass wall.

Ethiopian bush-crow family (Paul Donald)

Increasingly desk-bound researchers like me love getting their boots dusty, and you don’t get dustier than by hacking through the bush in southern Ethiopia, recording the exact location of bush-crows and their nests.  But it was only back in the lab that we spotted the big pattern that others had missed: the bush-crow’s range exactly follows the edge a unique bubble of cool, dry climate.  Inside the bubble, where the average temperature is less than 20°C, the bush-crow is almost everywhere.  Outside, where the average temperature hits 20°C or more, there are no bush-crows at all.  A cool bird, that appears to like staying that way.  

Bush-crow habitat (Paul Donald)

Why is this species so completely trapped inside its little bubble?  We don’t yet know, but it seems likely that it is physically limited by temperature – either the adults, or more likely its chicks, simply cannot survive outside the bubble, even though there are thousands of square miles of identical habitat all around.  Whatever the reason, alarm bells are now ringing loudly, as the storm of climate change threatens to swamp the bush-crow’s little climatic lifeboat.  And once gone, it's gone for good.

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