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Ornithological Observations
An electronic journal published by the Animal Demography Unit at the University of Cape Town and BirdLife South Africa
Ornithological Observations accepts papers containing faunistic information about birds. This includes descriptions of distribution, behaviour,
breeding, foraging, food, movement, measurements, habitat and plumage. It will also consider for publication a variety of other
oth interesting or
relevant ornithological material: reports of projects and conferences, annotated checklists for a site or region, specialist bibliographies, and any
other interesting or relevant material.
Editor: Arnold van der Westhuizen
HOW MANY TORTOISES DO
O A PAIR OF PIED CROWS
CRO
CORVUS ALBA NEED TO KILL TO FEED THEIR CHICKS?
C
John E Fincham and Nollie Lambrechts
Recommended citation format:
Fincham JE, Lambrechts N 2014. How many tortoises do a pair of Pied Crows Corvus alba need to kill to feed their chicks?, Vol 5: 135-138
URL: http://oo.adu.org.za/content.php?id=129
Published online: 25 May 2014
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Ornithological Observations, Vol 5: 135-138
135
HOW MANY TORTOISES DO
O A PAIR OF PIED CROWS
CRO
CORVUS ALBA NEED TO KILL TO FEED THEIR
CHICKS?
John E Fincham1* and Nollie Lambrechts2
1
SABAP2 Atlaser 11587
2
Farmer, Ceres-Karoo
*Corresponding author: [email protected]
In a position statement BirdLife South Africa (BLSA) has highlighted
the potential impact of the increasing abundance of Pied Crows
Corvus albus on South African biodiversity (BLSA 2012). This
potentially negative impact was further underlined in an editorial by
the Chief Executive Office (CEO) of BLSA where he emphasised the
need for research into the increasing abundance of Pied Crows
Crow and
the possible impacts of this increase before anyy action is taken
(Anderson 2013). The CEO wrote: "It needs to be demonstrated that
crows are causing significant damage to livestock and pose a threat
to biodiversity, including raptors and tortoises." The Chairperson
Chair
of
the Cape Bird Club's Conservation Committee has supported the
BLSA position. He has described how the Committee has resisted
requests to take action against these "nuisance birds" in the city. He
has further emphasised the need for "solid evidence" and speculated
that this may be a long time
me in coming (Whitelaw 2013). The
recommendations made by BLSA were rational based on available
information when they were tabled. This paper presents proof of
heavy predation on tortoises by a pair of Pied Crows at a single nest
site in order to rear successive broods of chicks.. The data collected
co
by the observers should be added to
o other data on this same
phenomenon.
Fig 1 - Four Pied Crow chicks in the nest on the windmill in the Ceres Karoo
©Nollie Lambrechts
Pied Crow predation on tortoises in the Karoo
During 2012 a pair of Pied Crows nested on a windmill in the Ceres
Karoo (S 33°04.607' E 19°53.197') and reared four chicks (Fig 1). An
accumulation of carapaces of small tortoises was noticed beneath
the nest – eventually 160 carapaces or parts thereof were counted.
These
ese results have been reported online (Lambrechts 2012).
The following year four more chicks fledged from the same nest to
join the rapidly increasing crow population. Alerted by the slaughter
of the previous year,, a thorough collection of carapaces established
estab
that at least 315 small tortoises had been killed to feed the chicks
and parents (Figs 2 and 3).
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Ornithological Observations, Vol 5: 135-138
136
Psammobates geometricus is locally extinct in parts of its original
range and the remaining population is endangered
endangere (McLachlan 1978;
Baard 1993). Heavy predation on tortoises by the White-necked
White
Raven Corvus albicollis (formerly
ormerly called Cape Raven) has also been
described (Uys 1966).
Fig 2 - The carapaces of 315 small tortoises killed by crows to feed four
chicks in 2013 ©Nollie Lambrechts
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
d
chameleons, geckos, skinks and other small prey may be digested
entirely leaving no trace of predation. The Geometric Tortoise
Ecology of Pied Crows
The ecological role of Pied Crows has been adapting over time to a
range of environmental changes that relate mainly to human
population pressure, which is often associated
ed with degradation of
habitat and loss of biodiversity,, such as replacement of species –
rich renosterveld by monocultures of wheat, potatoes or grapes, and
human settlements. Using a windmill in the Karoo as a safe haven
for nesting and a vantage point is an example of opportunistic
adaptation in an ongoing natural experiment. Both the first and
second southern African bird atlas projects (SABAP1 and SABAP2)
SABAP2
have monitored this adaptive process in South Africa, Lesotho and
Swaziland. Pied Crows have been observed in 1 307 quarter degree
grid cells (QDGC) with four or more checklists in both atlas projects.
The number of QDGCs with increased reporting rates in SABAP2 is
close to double those with decreases (LG Underhill, in litt). A parallel
study reported a significant increase in Pied Crow abundance across
South Africa that is greatest in urban areas and shrubland (Madden
2013).
Observations such as the examples that follow are a source of
powerful ecological information that must be recognised. In the
Calvinia district where the human population density is low, Pied
Crows often converge in loose flocks of 50 to 100. The three
indigenous corvid species occur in the area but Pied Crows have
increased to outnumber the other two species by 30:1 or more
m
(F van
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Ornithological Observations, Vol 5: 135-138
137
damage may be replicated needs to be undertaken urgently.
If it is confirmed that similar situations are widespread there is a
responsibility to do whatever is practical, at least via pilot projects.
projects
Key components of research and some inevitable obstacles have
been summarised (Anderson 2013).. Clarity on action,
acti
responsibilities
and accountability is needed.
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.
- oo0oo References
Fig 3 - The range in size of tortoises killed by Pied Crows to feed their
chicks ©Nollie Lambrechts
). Rob Martin and Jessie Walton have compiled a
der Merwe, in litt).
bird list for the 100 000 ha Kogelberg Biosphere Reserve (pers.
comm.). It has been updated over several years. The corvid
population originally consisted predominantly of Cape (Black) Crows
but the ratio has been reversed. Experienced birders frequently
observe excessive harassment of raptors by Pied Crows (Anon
2013). Persistent reports that Pied Crows mutilate ewes and lambs
need to be investigated thoroughly.
Discussion
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
Anderson MD 2013. Nothing to Crow About. Editorial. African
Birdlife 2(1): 2.
Anon 2013. Looking at Pied Crows. Promerops 294: 11-13.
11
Baard EHW 1993. Distribution and status of the geometric tortoise
Psammobates geometricus in South Africa. Biological
iological Conservation,
63: 235-239.
Bird Life South Africa 2012. Position statement on the potential
impact of an increased abundance of Pied Crows Corvus albus on
South
African
biodiversity,
25
August
2012.
http://www.birdlife.org.za/about-us/our-organisation/position
organisation/positionstatements
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Ornithological Observations, Vol 5: 135-138
138
Lambrechts N 2012.
http://www.westerncapebirding.co.za/news/1139/it_sometimes_takes
_160_tortoises_to_raise_four_crow_chicks
Madden CF 2013. The impacts of corvids on biodiversity. Unpubl.
MSc thesis, Percy FitzPatrick Institute of African Ornithology,
Department of Biological Sciences, University of Cape Town.
McLachlan GR 1978. South African Red Data Book – Reptiles and
Amphibians. SANSP Report 23.
38
Uys CJ 1966. At the nest of the Cape Raven. Bokmakierie 18: 3841.
Whitelaw D 2013. Letter. African Birdlife 2(2): 6.
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