Cape Town - There are several collective nouns for crows but the best-known is probably “a murder of crows” – and observations by local birders show just why it’s so apposite.
In 2012, veteran Ceres Karoo farmer Nollie Lambrechts made the surprising, and disconcerting, discovery that a pair of Pied Crows nesting in a windmill on his farm Fonteinskop had fed that season’s chicks at least 160 small tortoises before they fledged.
His observation, reported online and in the Cape Argus, created quite a stir, particularly because it followed growing concern among birders that the local population of this indigenous crow species – an extremely intelligent and effective predator – was growing rapidly and having an increasingly negative impact on other indigenous birds, reptiles, amphibians and small mammals.
But their concerns were essentially based on anecdotal evidence. While taken seriously by BirdLife SA, this premier bird conservation group said it needed to have the threat properly quantified and that it would not condone any cull in the meantime.
“(We) do not support the control or poisoning of indigenous Corvid (crow) species in any manner whatsoever. (We) reserve action on this issue until adequate scientific evidence demonstrates the need for appropriate action for threatened species and a full and balanced appraisal of this perceived threat has been completed,” it said in an August 2012 statement.
BirdLife was supported by the Cape Bird Club, whose conservation committee said they were resisting requests to take action against these “nuisance birds” in the city.
Now Lambrechts and local birder John Fincham have provided more evidence that may help tip the scales against the species.
In a paper in the recent edition of online journal Ornithological Observations, they report Lambrechts’s findings of the crows’ 2013 breeding season on his farm that make for even more disconcerting reading.
“Alerted by the slaughter of the previous year, a thorough collection of carapaces established that at least 315 small tortoises had been killed to feed the chicks and parents,” they write.
“This rate of predation is almost certainly not sustainable and any defenceless 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 of predation.”
One concern is the endangered Geometric Tortoise, already extinct in parts of its original range.
The authors say observations such as theirs and from several other sources “are a source of powerful ecological information that must be recognised”. They cite the Calvinia district, where Pied Crows often converge in loose flocks of 50 to 100 and whose numbers have increased to such an extent that they now outnumber the other two indigenous crow species – the Cape Crow and the White-necked Raven – by a ratio of 30:1.
“The hard evidence of heavy predation on tortoises in order to feed successive broods of four chicks each is unlikely to be unique. A comprehensive survey to establish the extent to which this degree of damage may be replicated needs to be undertaken urgently.
“If it’s confirmed that similar situations are widespread, there is a responsibility to do whatever is practical, at least via pilot projects. That corvids are intelligent and adaptable does not mean they should be allowed to proliferate to the extent that they contribute to declines of some reptiles and other birds,” they argue. - Cape Argus