The Great Scottish Crow Mystery - 2

The hooded crow, above, is being pushed into the northwest corner of Scotland by the advance of its cousin, the carrion crow. Tom Bryson has suggested several reasons why this might be happening.

The hoodie's retreat might be connected to climate change, though he thinks this is unlikely as the hoodie seems as well-adapted to a wide variety of environments as the carrion crow - he's seen them in Moscow rummaging through dustbins in both the depths of bitter winter and in 35C summer heat.

Another possibility is that the move is a response to changing land use, and particularly the management of forestry and the work of gamekeepers protecting birds like grouse, but he can't see why this should advantage the carrion crow.

Tom thinks that the most likely reason is connected to intensive livestock and arable agriculture. In both Denmark and Scotland, the areas which have seen huge increases in intensive farming are in the south and east. In both countries, carrion crows populations in these areas have exploded, but they haven't been able to expand to the south and east - the North Sea blocks eastward expansion and there are more carrion crows to the south - so they've moved north and west, pushing the hoodies before them

This, as Tom points out, isn't really an answer as it doesn't explain why the carrion crow thrives in areas where modern farming techniques are used, while the hoodie doesn't - that remains a mystery.

Photo of carrion crow thanks to Erik Paterson of East Kilbride on Flickr, here.