Pied Crow. Picture Peter Ryan Pied Crow. Picture Peter Ryan
Environment Writer
The pied crow population is increasing in the Western Cape – with global climate change and power lines being the two main reasons that these noisy birds are migrating south-west.
Researchers Susie Cunningham and Chrissie Madden, from UCT’s Percy FitzPatrick Institute of African Ornithology, compared data on crow distribution from two bird atlas surveys conducted 20 years apart, and matched that with climate data.
They found that while there had been an increase in pied crows in the warmer south-western area of the country, crow numbers had decreased in the cooler north-east.
Contrary to assumptions made in many climate change studies, temperatures had become cooler in the northern latitudes of South Africa, which had also tended to become drier. In contrast, South Africa had warmed “most dramatically” in the arid midwest.
They then looked at urbanisation and the density of power lines in South Africa, and found that in the south-west, there was a significant relationship between this increase and the density of power lines.
Crows use power lines to nest and also as convenient places to watch for roadkill.
The reason power lines were important for crows in the south-west is that there were not a lot of trees in the Karoo for them to use as nest sites. Cunningham said the study showed that power lines had allowed crows to follow a “preferred climate bubble” into the warmer Western Cape.
“They have been enabled to move into the treeless Karoo by our provision of power line pylons to nest on. As climate change progresses, we expect to see more and more synergies of this kind, allowing species to change their distribution in the landscape,” Cunningham said.
Madden said crows were able to move more freely as they were “generalists” and not restricted by a specific diet or nesting site.
The researchers regard their study as important because it links the changes in crow populations to changes caused by humans: climate change and the building of power lines.
“We are altering the environment and there will be some losers and some winners. Pied crows are currently winning,” Cunningham said.
The other reason the study was important was because it highlighted that when looking at changes in bird populations as a result of climate change, researchers also needed to look at other possible causes not related to climate change.
The researchers said that recently, the public had become worried about the impact of crows on other birds, as they were known to “mob and harass” birds of prey, compete with their nest sites and prey on their nests.
However, there was no scientific study that had established whether the increase in crows was having a negative effect on biodiversity.