Crows feeling the heat – and loving it

With their noisy banter and untidy nests, crows can be inconvenient for people. But people are convenient for crows, a scientific study has found.

Across much of South Africa the Pied Crow benefits from modern infrastructure – in particular electricity pylons – and warming temperatures caused by climate change, according to a study by the Percy FitzPatrick Institute of African Ornithology at the University of Cape Town.

Electricity pylons have provided useful nesting sites in South Africa's steadily warming western scrub-land where traditional tree nest sites are a scarce commodity – much to the crow's satisfaction. The net result is an increase in the regional Pied Crow population, says the study published this week in the international journal Diversity and Distributions.

"Pied Crow numbers have increased in response to climate warming, with their spread facilitated by electrical infrastructure in south-western South Africa," the study says.

Significantly, it is the combined effect of climate change and electrical infrastructure that has fuelled the increase, the study shows, and not any single factor.

Findings were based on an analysis comparing two bird atlas surveys, conducted 20 years apart, which were then matched with geographic and climate data. The method produced some intriguing results:

• While there has been an increase in Pied Crow populations in the warmer south west, there has been a decrease in the cooler north east. There is a strong relationship between temperature warming in the period between the two surveys and crow population changes.

• By contrast there is no relationship between changes to crow populations and current levels of urbanisation or powerline density. However in the south west where numbers have increased there is "a significant relationship between increases in Pied Crow reporting rates and the density of powerlines".

Study co-lead author Dr Susie Cunningham says results show how human infrastructure has allowed crows to track a "preferred climate bubble' into the south west as this area of the country has warmed under climate change.

"They've been enabled to move into the tree-less Karoo by our provision of power pylons to nest on. As climate change progresses, we expect to see more and more synergies of this kind allowing species to change their distributions in the landscape," Cunningham says.

The range expansion is partly due to the crow's high intelligence and adaptability – qualities that sometimes irk mankind, according to co-lead author Chrissie Madden: "Being generalists they are not constrained by a specific diet or nesting requirements. With powerlines being suitable nest sites, and road-kill a constant supply of food, we are providing perfect crow habitats in the southwest," Madden says.

The new study also provides valuable insight into how species respond to global change patterns: one anthropogenic (human driven) factor like climate change might have a different impact on a particular species when combined with another anthropogenic factor, or several other factors.

In the case of the Pied Crow, although the impact of climate change alone explains some of the increase in Pied Crow populations between the two surveys, these increases can be better explained when looking at the combined effect of climate change and power-line density.

"In light of this, we caution that other studies exploring climate-related distribution changes should take into account observed patterns of climate change within the study region, as well as explicitly investigating potential non-climatic drivers," the report says.

The study provides "a clear example of compound influence of multiple global change drivers promoting a significant change in a species range and reporting rate."

Cunningham says the study is important since it links crow population changes with anthropogenic landscape transformation (pylons) and climate change. "We are altering the environment, and naturally there will be some losers and some winners. Pied Crows are currently winning" Cunningham says.

The authors believe their findings may also contribute to the current debate around crow control or culling by drawing attention to the impact of larger processes like climate change and infrastructure. "How we approach this issue is potentially controversial if crude methods of culling are implemented" says Madden.

Although several crow species are increasing worldwide, most climate change research tends to focused on declining species. Researchers believe that the new South African study could help understand how to manage both the Pied Crow and other members of the 'corvid' family, which are widely considered to be predatory pests.

Chrissie Madden acknowledges that "like many other crow species around the world, Pied Crows have a reputation as nuisance species, which are known to mob, harass, and compete with raptors for nesting sites, and are notorious nest predators".

In recent years, public concern has been heightened by the perceptions that species of conservation concern are under threat from their increases. However, despite this concern and anecdotal reports of high predation levels by certain pairs, no scientific studies have yet quantified whether these Pied Crow increases are likely to lead to any biodiversity impacts.

However, Dr Arjun Amar supervising author of the current research stated "we can't be sure what impact these increases are likely to have on other biodiversity, until we conduct further research. Although our recent global review suggested that in most cases corvids actually have very little impact on their potential prey populations".

The work was funded by DST-NRF Centre of Excellence funding to the Percy FitzPatrick Institute, University of Cape Town and the paper can be found at onlinelibrary.wiley.com/doi/10.1111/ddi.12381/full .

More information:
Cunningham, S. J., Madden, C. F., Barnard, P., Amar, A. (2015), Electric crows: powerlines, climate change and the emergence of a native invader. Diversity and Distributions. doi: 10.1111/ddi.12381

For more information please contact Dr Arjun Amar, Percy FitzPatrick Institute of African Ornithology, NRF-DST Centre of Excellence, Biological Sciences, University of Cape Town, email: arjun.amar@uct.ac.uk, tel: +27 (0)21 6503304 or +27 (0)795855603.