Kimon de Greef
4 June 2015
When Manie Louw took over a farm outside Paternoster last year his neighbours warned him about three things: stock theft, jackals, and pied crows. Louw, a 53 year-old sheep farmer from Calvinia, grew up raising livestock, and thought himself familiar with the risks of the trade. He brought 42 aging Dorper ewes to the coast with him, hoping to fatten them for slaughter on the farm’s Strandveld vegetation. “I didn’t know what people meant about the crows,” he said.
Since arriving in April he has been forced to slit the throats of 21 ewes and more than 30 lambs after discovering them with their eyeballs pecked out, a signature of the large black and white birds, which have been increasing in numbers in the Western Cape since the late 1980s.
He has watched crows descend moments after a birth, surgically jabbing at lambs that are still wet and unable to stand. Other times he has found adult sheep wandering drunkenly through the veld, bleeding from the face and unable to see. When he encounters a victim he takes out his knife and slaughters it on the spot, dumping the carcass if it isn’t ready for market. He estimates that this has cost him tens of thousands of rands to date.
“I hate killing my sheep like that,” he told me. “It’s the biggest problem I face here. But I have no choice.”
Louw is one of hundreds of local sheep farmers facing the threat of pied crow predation, which has escalated since the species (scientific name Corvus albus) expanded its range into the Western Cape. Pied crows occur throughout Sub-Saharan Africa but were relatively uncommon in the province until two decades ago, when they began shifting their distribution in response to climate change, which has warmed the south-western parts of the country while cooling regions in the north east. The birds have become ubiquitous in the Western Cape as a result — particularly in cities and shrubland areas, according to data from the Second Southern African Bird Atlas Project (SABAP2), a citizen science venture that maps the abundance and distribution of birds across Southern Africa.
Their arrival in large numbers has potential biodiversity implications in addition to posing challenges for farmers, although researchers have not yet conducted the studies necessary to detect negative ecological impacts.
Pied crows are aggressive, opportunistic feeders and will eat anything from small animals, eggs, fruit and seeds to roadkill and human scraps. They are considered highly intelligent — one ‘exclusive avian products’ dealer from Georgia, USA, has trained a pair of South African pied crows to retrieve golf balls and peck at the keys of a piano — and can readily adapt to new environments, tailoring their diet and behaviour to suit different conditions. This versatility enables pied crows to colonize a wide range of habitats but can have negative consequences for native species, which risk being preyed upon or outcompeted by the encroaching birds.
Whether or not this represents a genuine ecological threat has become a controversial topic in local birding and conservation circles, with disagreement about how best to assess and respond to the crows’ dominion and spread.
John Fincham, a retired Bellville veterinarian and regular SABAP2 contributor, is among those who believe that pied crows should be treated as a ‘problem species’ due to their adverse effects on Cape wildlife. In 2013 Fincham and a Ceres farmer named Nollie Lambrechts documented the killing of at least 315 small tortoises by a single breeding pair of crows over the course of one year, publishing their findings in a University of Cape Town publication, Ornithological Observations. “This rate of predation is almost certainly not sustainable,” they wrote. “Any defenseless species will be equally at risk, including the eggs, nestlings and fledglings of many birds. These, as well as dwarf chameleons, geckos, skinks and other small prey, may be digested entirely, leaving no trace.”
Their work followed a 2011 report in Africa Birds & Birding about pied crows mobbing raptors for their prey, adding to fears that vulnerable species were being placed under pressure by the advancing flock — a ‘murder’, to use the correct collective noun for crows.
A statement released by conservation body BirdLife Africa in 2012 acknowledged “potential threats” to indigenous species but warned against control or poisoning “in any manner whatsoever” until the completion of further research. Responding to this statement, Fincham and Lambrechts wrote that the “hard evidence of heavy predation on tortoises” they had collected was “unlikely to be unique”. “If it is confirmed that similar situations are widespread there is a responsibility to do whatever is practical,” they argued. “That (pied crows) are intelligent and adaptable does not mean they should be allowed to proliferate to the extent that they contribute to declines.”
But according to Arjun Amar, an avian conservation biologist at the Percy FitzPatrick Institute of African Ornithology, housed at the University of Cape Town, there is an important distinction between observing individual mortalities and inferring population-level impacts. “If you repeatedly see pied crows hunting tortoises or eating the eggs of other birds then it’s natural to believe that this will have wider consequences,” he told me. “The reality — that increased predation doesn’t always make a difference — can be more difficult to grasp.”
To explain, Amar invoked the concept of the ‘doomed surplus’, a term for the common evolutionary strategy of spawning more offspring than can feasibly survive to adulthood. (Existence, for humans and all other living things, is at base a vehicle for passing on DNA to future generations, bounded by statistical frequency distributions of death and other forms of reproductive incapacitation; parenthood can be crudely reduced to a tradeoff between wasting energy on too many progeny and not having enough to justify the effort at all.) A proportion of every generation, he explained, no matter the species, is quite literally born to die — meaning that if a chick isn’t plucked from its shell by the long black beak of a crow it is likely to starve, or be devoured by some other predator, while its parents forage during the winter months.
“It’s counter-intuitive,” Amar said, “but pied crows might simply be doing things that would have happened anyway. We don’t have enough information at this stage to tell.”
Research in the northern hemisphere, where a number of crow species are considered pests — in some cases for mutilating lambs, as in South Africa — has revealed that removing perceived problem birds from an ecosystem can have little to no effect on the survival rates of affected species. A recent review paper by Amar, French ecologist Beatriz Arroyo and former University of Cape Town postgraduate student Chrissie Madden found that Corvids — crows, ravens and magpies — had a “small overall” impact on other bird species, with just 10% of studies reporting significant effects of predation on the abundance of prey.
“The situation may well be different here, but on the basis of evidence from the Northern Hemisphere it doesn’t look like pied crows will necessarily have an impact,” Amar said. An obstacle to evaluating the birds’ effects locally, he explained, was the fact that “almost nothing” was known about their basic biology and behaviour. “Research here tends to focus on threatened species, or on ecosystems that can’t be studied elsewhere. Crows are common yet we understand very little about them at all — their breeding behaviour, survival rates, juvenile dispersal, territoriality, home ranges and other really fundamental stuff. This makes designing experiments difficult.”
That pied crows are technically native to South Africa, and not introduced from some distant country, further complicates questions about how to deal with their spread. The National Environmental Management: Biodiversity Act (NEMBA), published by the Department of Environmental Affairs, lists procedures for preventing or mitigating the costs of invasive species, but is not applicable to indigenous populations that have expanded as a consequence of human activity. Amar and his colleagues have shown that shifts in pied crow distribution can be explained by changing temperature and rainfall patterns — both direct manifestations of climate change — and, secondarily, by the construction of electricity pylons in the treeless environment of the Karoo, which the birds have appropriated as nesting sites. In terms of the NEMBA legislation these findings are insufficient to justify expensive eradication programs — such as those set up for the Indian House Crow (Corvus splendens), a close pied crow relative and notorious global invasive, resident in Cape Town since the 1980s, that has been aggressively targeted by the city’s Invasive Species Unit for the last six years.
“It’s probably too late to substantially reduce the pied crow population,” Amar told me. “But with a systematic program of culling you could bring their numbers down.”
While conservationists debate the most appropriate approach to dealing with pied crows, many sheep farmers have taken matters into their own hands. Shooting pied crows is permissible — the species is not protected by environmental legislation — but poisoning them is strictly forbidden. “If farmers go that route it’d be a real worry,” Amar told me. “Endangered species like foxes, jackals and vultures would be at risk.”
According to Thys Delport from the Western Cape Predator Management Forum (PMF), a platform for curbing predation in the livestock and game ranching sectors, pied crows are not yet considered a major threat in the province. “Jackals and lynx are our main problems here,” he said, “although we do receive reports of crows attacking lambs in certain areas.” He was not aware of any incidents where farmers had used poison to control the birds but emphasized that the PMF was opposed to poisoning “in all forms”.
For Manie Louw, stationed on his farm six kilometres outside Paternoster, there is little solace knowing that crows are considered less of a threat than conventional predators, which have given him little trouble over the last year. The crows circle high above the veld or perch on the telephone cables or descend into the bluegum thicket behind the farmhouse, crying out to one another in maddening tones. Sometimes they disappear for weeks only to return suddenly: Louw has no idea where they go. He watches them and continues farming his sheep, undefended against the skies.
“There’s a little hokkie outside the house,” he told me. “We used to put the baby lambs in there, let the children feed them with a bottle. The crows got in and attacked those lambs, too.”
A lamb’s eye weighs less than five grams but is rich in fats and protein. The crows cannot eat anything more substantial because their beaks are not sharp enough to puncture flesh. That they just take the eyeballs, leaving his animals blinded for so little, is what troubles Louw the most.
“I like nature; I like everything that’s on this farm,” he said. “But eventually I had to do something about the crows.”
Kimon de Greef is a freelance journalist and researcher from Cape Town.