Sunday 7 January 2018

Carrion crow a long way from home

This morning after I fed the hens I noticed two crows among the hens, a carrion crow and a hooded crow. We just don't get carrion crows here as a rule. Because they tend to be south of a line drawn from Glasgow to Bettyhill on the North coast of Scotland.

Hooded crow
The distribution of both of these species wasn't always like this. A hundred years ago the two populations were separated by a line running from Dumfries to Dundee on the East coast. Along that line the two species interbred to produce hybrids which are fairly easily identified by their plumage.Changes in the carrion crow/ hooded crow hybrid zone in Scotland . Cook 1974

Over the last hundred years a number of population studies have documented the change in distribution of the two species in Scotland, Italy and Denmark. In the Ligurian Alps the hoodies have moved further up the mountains and along the Danish / German border here has been a similar shift.

Carrion crow
Why has this happened. Immediately one thinks...... must be climate change!...... but hooded crows can cope with much higher temperatures and seasonal range of temperatures  than we experience here in Scotland from Central Russia to Spain. There mus be some other agency at work.

Modern intensive agriculture could to be the most likely factor. Carrion crows thrive in lowland Britain's industrialised agriculture and perhaps the hooded crow can't compete so readily in this environment. Similar changes in the intensity of farm production at lower levels in the Italian Alps and Germany could have driven the change there.

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