pied pirates Crow threat to raptors?

There has been much debate about the effects on biodiversity of increasing Pied Crow and other corvid populations in Africa, but little has been quantified. It is evident from the latest bird atlas data that there are now more Pied Crows in southern Africa than there were a decade ago and that the species has spread into areas of the Karoo where it did not occur previously. But what effect is this having on other bird species?

Text by Rob Simmons & Phoebe Barnard

albert froneman arious reports indicate that one a Rock Kestrel and another a Southern crows are impacting negatively Pale Chanting Goshawk, all of which were on other creatures in their en- carrying mice in their talons. Prey-carrying vironment. Farmers complain raptors are usually provisioning dependent aboutV increasing predation by ‘crows’ on females or nestlings, so piracy affects more lambing ewes, birders suggest that more at- than simply the bird with the food. tacks are occurring on passerine birds, and The following three incidents serve to conservationists and members of the pub- highlight the modus operandi of the crows. lic report greater numbers of crows killing We then investigated other parts of Africa small tortoises. However, almost nothing to determine if piracy targeting raptors is has been quantified as to the level of such limited to western South Africa as a zone predation, the magnitude of the upsurge of Pied Crow range expansion or is a more in crow interactions and the effect on the widespread strategy. species being attacked. We do not yet have In the West Coast National Park in such data, but we do take a first look at 2008, our attention was drawn to a Black- what may be an increasing and somewhat shouldered Kite that was calling from a surprising addition to the negative effects height of over 150 metres and circling up- of crows – those on birds of prey. wards while being pursued by a single Pied In the course of 10 years’ field work on Crow. As the kites were breeding in trees Previous spread Feeling outnumbered… harriers and kestrels along South Africa’s on the east side of Langebaan Lagoon, A trio of White-necked Ravens vies for West Coast we have observed a slow increase we presumed that this one’s prey was in- airspace with a Verreaux’s Eagle in the in the Pied Crow population in areas such tended for its dependants. After about two mountainous terrain they both inhabit. as the West Coast National Park and along minutes of circling and swooping by the As the largest member of the crow tribe, the stretch of road between Steinkopf and first crow, a second Pied Crow appeared, ravens make formidable and persistent Port Nolloth in the Northern Cape. During flapping vigorously, calling noisily and aerial opponents. this time we observed at least five incidents circling to gain height. The approach of JAN COETZEE of Pied Crows in pursuit of raptors carry- the second crow caused the kite to drop its ...we take a first look at what may be an increasing Below A Pied Crow and a Yellow-billed ing prey, calling as they closed in and thus prey. One of the crows caught the kill in birds were calling. After about a minute of Kite vie for possession of a mouse carried by attracting additional crows. Three of these mid-air and all the birds descended with- pursuit, during which the crow was slowly and somewhat surprising addition to the negative an out-manoeuvred Black-shouldered Kite. incidents involved Black-shouldered Kites, out further interaction. The two crows gaining on and swooping at the kite, a sec- effects of crows – landed in large trees about 500 metres into ond Pied Crow approached and entered those on birds of prey the marsh, where we presumed they had a the fray. The kite avoided its pursuers nest. The kite made no attempt to retrieve with relative ease until another raptor, a its prey. Yellow-billed Kite, joined the attack. Under It is only in the last three of the 10 years Stealth bomber takes on the B52! Cape In the second incident, in September the combined attention of the three scav- we have spent on the West Coast that we’ve Vultures (adult shown) and Bearded 2009, a pair of calling Pied Crows attract- engers, the Black-shouldered Kite released witnessed such kleptoparasitic attacks by Vultures are often the subject of attacks by ed our attention. They were in pursuit of its prey, which appeared to be grabbed Pied Crows on raptors. These four incidents ravens and this is particularly prevalent at a Rock Kestrel about 40 metres above the by one of the crows as the Yellow-billed in three regions spaced more than 100 kilo­ vulture restaurants where both gather to ground in coastal fynbos in the Rondeberg Kite flew away unharassed. The Black- metres apart indicate that such piratical feed. The ravens appear to target flight and Nature Reserve. The kestrel, which was shouldered Kite then descended rapidly activity is not a learned behaviour localised tail feathers in their harassment. breeding nearby, was clutching a mouse over our vehicle, screaming loudly, and to a few individual crows. And it may be on in its talons. Both crows dived at the kes- headed towards a tree on the roadside, the increase, judging from the apparent re- trel, which initially evaded them easily but where it struck another, previously un­ cent rise in frequency, perhaps in concert soon tired under their combined attack. observed Black-shouldered Kite in the with the growing crow population. After another two minutes of pursuit, the back, causing the perched bird to squeal kestrel relinquished its prey. The mouse and fall about two metres. Despite this ied Crows along South Africa’s was deftly retrieved by one of the two act of misplaced aggression, the first kite West Coast have thus far limited crows, which was then promptly chased made no attempt to retrieve its prey. their piratical attacks to the small- by the other. The kestrel flew away, mak- Subsequent to these observations, in er raptors. However, in our study ing no attempt to recover its kill. It was in October 2010 natural history filmmaker Pareas, even though the crows have had all likelihood breeding in one of the nearby Claudio Velásquez Rojas witnessed a Pied ample chance to see and pursue other prey- nest-boxes, but this could not be verified. Crow in the West Coast National Park fly carrying raptors, notably Black Harriers, In the third incident, recorded at mid- down to an active Black-shouldered Kite African Marsh-Harriers and African Fish- day on 9 September 2010 in farmland nest, remove a ‘pale object’ and fly off Eagles, we have seen few interactions be- south of Elands Bay, a prey-carrying Black- with it, hotly pursued by a parent bird. tween crows and harriers. Is this the norm? shouldered Kite was observed circling We assumed it was a young nestling, and To answer this we solicited information about 80 metres above the ground and if this is the case it would be the first evi- through the African raptor specialists’ list- being chased by a single Pied Crow. Both dence we have of direct predation. server [email protected]. roB simmons

52 pied crow AFRICA – BIRDS & BIRDING october/november 2011 pied crow 53 this article; the hawk was injured and required veterinary treatment before it could be released.

ut do crows influence raptor breeding success? In one of the few published accounts of crow impacts on raptors (Animal Con- servationB 2002(5): 21–28), Arjun Amar and Steve Redpath found that crows do indeed occasionally attempt piracy of Hen Har- riers in Scotland (one success in three at- tempts). The researchers then tested to see if Carrion Crow (a sorry grey version of southern Africa’s Pied Crow) numbers were affecting harrier breeding. They first dem- onstrated that Carrion Crow numbers had increased by 80 per cent in 16 years, and then removed crows from certain Hen Har- rier territories. By placing artificial eggs in ground nests they showed that fewer eggs

peter usher were lost in territories where crows had been removed and that crows were thus Augur Buzzards are one of the main At a feeding site in Dundee, also in significant egg thieves. However, the same victims of harassment by Pied Crows in East KwaZulu-Natal, White-backed Vultures was not true of crows on real Hen Harrier Africa, but piracy of prey has not yet been perch in the surrounding trees but do not nests; the removal of crows had no effect recorded. feed because the crows chase them as soon on nest success, presumably because Hen as they attempt to land, feed or fly past. Harriers effectively prevented crows from We received six responses from experienced Culling ‘a few’ crows has made very little stealing eggs or nestlings. This is one of the raptor-watchers: three each from East and difference there, but the harassment varies few experimental tests of crow numbers on southern Africa. seasonally and disappears in spring. breeding success and the results indicate Daudi Petersen indicated that in 2006 East African raptor guru Simon Thom- that we must not jump to conclusions that Pied Crows killed a fledgling Verreaux’s sett reported that a month-old Martial crows are having a negative impact just be- Eagle-Owl on the nest at Olasiti, Arusha, Eagle nestling was killed on its nest in cause of their opportunistic behaviour. Tanzania. Although the owl pair have re- Kenya by Pied Crows that pecked its eyes In summary, it appears that while crow mained on the territory, they no longer out, after a local herder kept the parent behaviour is not limited to piracy and is breed. The crows also regularly harass and birds away. Pied Crows also successfully not restricted to small raptors, it does have steal prey (such as hedgehogs) from the chased and pirated Lanner Falcons and a direct effect on food acquisition by scav- owls, and harass Augur Buzzards. Petersen Tawny Eagles that were preying upon engers and in some instances can lead di- ascribed this to crow numbers reaching un- starlings at a colony. Thomsett further rectly to the death of nestlings. However, naturally high population levels. notes that House Crows profitably target most of these observations are anecdotal In two other responses, Peter Usher other raptors in coastal Kenya and that and we do need a more concerted study reported and photographed Pied Crows flocks of Pied Crows try to wrest prey from of crows, preferably with removal experi- pestering several East African birds of hawks or falcons sitting on recent kills in ments, to determine to what degree they prey such as Augur Buzzards and kites, Kenyan farmland. are influencing raptor populations. It is but no piracy was involved. Similarly, in A fifth incident reported from South also important that we assess the effect on South Africa, vulture researchers Sonja Africa in September 2009 by Ann Koeslag other vulnerable species, such as tortoises, Krueger and Louis Phipps reported that involved a pair of Pied Crows harassing small mammals and passerine birds. Pied Crows and White-necked Ravens in- a breeding Black Sparrowhawk carrying creasingly hassle Bearded and Cape vul- prey. The sparrowhawk was returning to tures at vulture restaurants and have been its nest but the persistent mobbing of the ACKNOWLEDGEMENTS seen pulling their tail feathers both on crows forced it to drop its prey. Koeslag We thank all correspondents, particu- the ground and in the air. Persecution is says this is unlikely to be the only such larly Simon Thomsett, Daudi Petersen, well known at the Giant’s Castle vulture incident and she may have missed others Peter Usher, Louis Phipps, Sonja Krueger, restaurant in the KwaZulua-Natal Drakens­ during her 10 years of monitoring Black Ann Koeslag, Sharon Yodaiken, Rita berg, where both Cape and Bearded vul- Sparrowhawk nests. This is borne out by Hofmeyer, Yvonne Githiora, Arjun Amar tures are bullied by crows and ravens to the grounding of an adult Black Sparrow­ and Claudio Velásquez Rojas, for their the extent that visitors now complain hawk by a mob of crows at the Univer- observations. about the perceived ‘crow problem’. sity of Cape Town during the writing of

54 pied crow AFRICA – BIRDS & BIRDING