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Ostrich 2004, 75(4): 325–326
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OSTRICH
ISSN 0030–6525
Short Note
Pygmy Falcon predation of nestlings of their obligate host,
the Sociable Weaver
Rita Covas
1,2
*, Onno Huyser
1,3
and Claire Doutrelant
1,4
1
DST Centre of Excellence in Birds as Keys to Biodiversity Conservation at the Percy FitzPatrick Institute, University of Cape Town,
Rondebosch 7701, South Africa
2
Present address: Institute of Cell, Animal and Population Biology, University of Edinburgh, King’s Buildings, West Mains Rd, EH8 9RB,
Edinburgh, Scotland, United Kingdom
3
Present address: 2 River Close, Noordhoek, Cape Town, South Africa
4
Present address: CEFE–CNRS 1919 Rte de Mende 34293, Montpellier cedex 5, France
* Corresponding author, e-mail: Rita.Covas@ed.ac.uk
The Pygmy Falcon Polihierax semitorquatus is a small,
shrike-like raptor which does not build its own nest. In south-
ern Africa it is dependent on the communal nest masses of
Sociable Weavers Philetairus socius (or occasionally other
species) to breed and roost (Maclean 1973, Steyn 1982).
This dependence is reflected in the falcon’s distribution and
habitat affinities, which largely coincide with those of the
Sociable Weaver (Brown 1989). This falcon appears to feed
mostly on small reptiles, large insects and only occasionally
on small birds and rodents (Maclean 1970, Brown et al.
1982, Steyn 1982). This would account for its passive co-
existence with the Sociable Weaver, a relatively small
passerine (27–29g — Maclean 1973, Covas et al. 2002).
Still, there are reports of Pygmy Falcons preying on Sociable
Weaver nestlings (Maclean 1970) and fully fledged birds (De
Swardt 1990), but this is believed to only happen occasion-
ally (Maclean 1970). Richard Liversidge photographed a
Pygmy Falcon with an adult Sociable Weaver in its talons
(Holmes 1987: 17).
Here, we report on Pygmy Falcons preying on Sociable
Weaver nestlings on different occasions and displacing most
of the inhabitants of one colony at a site near Kimberley, in
the Northern Cape Province, South Africa. We also describe
the weavers’ animosity to the presence of falcons as
opposed to the indifference that has been reported else-
where.
The events reported here took place during a four-year
study (1998–2001) of Sociable Weavers at Benfontein
Game Farm (Covas 2002, Covas et al. 2002). The farm is
situated about 6km south-east of Kimberley (approximately
28°53’S, 24°89’E). The study site contains c. 25 Sociable
Weaver colonies, 21 of which were monitored almost on a
daily basis during the breeding season as part of an ongo-
ing study on the weavers’ breeding biology (Covas 2002).
Pygmy Falcons were absent from the area during the first
two years of the study, although many colonies showed
signs of previous occupation by the falcons as evidenced by
an accumulation of pinkish faeces around the entrance of
some chambers.
From early March 2000 until the end of the weavers’
breeding season in late April, a pair of Pygmy Falcons was
often seen perching on electricity pylons between two
Sociable Weaver colonies. The falcons were seen prospect-
ing at one of the colonies that contained nestlings, but were
chased by one of the observers to avoid interference with an
ongoing experiment. On another occasion, a Pygmy Falcon
(presumably one of the pair observed) was seen at another
colony which contained two chambers with weaver
nestlings. Both chambers previously contained two chicks,
but on inspection one chamber only had one chick remain-
ing. We watched the falcon flying into the chamber that con-
tained the single chick and flying out with the chick in its
talons. It perched on a nearby branch and consumed the
head first, then the wings, leaving the rest of the body virtu-
ally untouched. After the falcon’s departure we inspected the
remains of the nestling and found the body of the Sociable
Weaver chick that was missing from the same chamber. This
latter chick had been eaten in the same manner. The chicks
were nine and 10 days old. One of the chicks from an adja-
cent chamber in the same colony also disappeared, but we
could not ascertain whether it had also been taken by the
Pygmy Falcon. The following season, in October 2000,
another Pygmy Falcon (female) was seen taking two
nestlings from two different chambers. One of the nestlings
was fully feathered (more than 15 days old) and the other
was still naked (less than five days old). The fully-feathered
nestling was eaten in the same way as described above (we
could not see how the smaller chick was eaten).
Whenever we saw Pygmy Falcons at a Sociable Weaver
colony the weavers became agitated and started alarm-call-
ing in a manner similar to their reaction to snakes, their main
nest predator. This is different from their reaction to preda-
tors of adults, such as Gabar Goshawks Micronisus gabar.
In reaction to the latter, the weavers normally perched quiet-
ly under cover, preferably near or in the nest (R Covas pers.
obs.).
At the end of the 2000–2001 Sociable Weaver breeding
season, a pair of Pygmy Falcons moved into one of the
Covas, Huyser and Doutrelant326
colonies, still in the same area, and proceeded to lay two
eggs. When the falcons moved in, this colony had four
chambers with Sociable Weaver eggs or chicks and most of
the other nest chambers were used by non-breeding
weavers for roosting purposes. The contents of the nests
progressively disappeared and none of the chicks fledged,
although we never saw the falcons taking eggs or chicks
from the chambers. The weavers also stopped roosting in c.
20 chambers in the vicinity of the Pygmy Falcons’ nest, as
evidenced from spiderwebs accumulating at the entrance of
some chambers and general lack of nest maintenance. This
colony used to have over 40 birds. It appeared that about
half of these had left the colony within less than two weeks
of the falcons’ occupation. When we caught the Sociable
Weavers at this nest at the beginning of the following sea-
son (September 2001), the Pygmy Falcons were still occu-
pying the same chambers and there were only 12 Sociable
Weavers remaining. Three Sociable Weavers from this
colony were subsequently caught at another colony, indicat-
ing that some individuals had permanently emigrated from
the colony where the falcons had settled.
Hence, our observations at Benfontein contrast with
what has been reported elsewhere about the interactions
between Pygmy Falcons and their host, the Sociable
Weaver. In other areas in the southern Kalahari, it appears
that Pygmy Falcons do not displace their hosts and often
breed in nest masses with breeding weavers (R Covas pers.
obs., Maclean 1973). Additionally, depredation of Sociable
Weavers by Pygmy Falcons has been reported as occa-
sional (Maclean 1973, De Swardt 1990). An eight-year study
of Pygmy Falcon pellets from the Kalahari showed that birds
constituted less than 4% of the falcons’ diet, although this
could be an underestimate since Pygmy Falcons seem to
digest bones very efficiently and hence ingestion of naked
nestlings would be difficult to detect (CW Sapsford pers.
comm.). Even though we only witnessed predation at three
nests, it is worth emphasising that apparently there was only
one pair of Pygmy Falcons in the area and our observations
took place only during a relatively short period of our study.
Furthermore, predation caused by snakes in Sociable
Weaver nests is very high (an average 70% of nests get
depredated in our area; Covas 2002) and only intensive
observation of nests or video recording would allow us to
accurately assess the impact of the falcons in this popula-
tion.
The events reported here raise two questions: (1) how
common is predation and displacement of Sociable
Weavers by Pygmy Falcons? (2) what are the costs and
benefits of this co-habitation for both parties? Further
detailed studies in other areas of these species’ distribution
range would allow answering the first question. Our study
area approaches the edge of the Sociable Weaver distribu-
tion range and one could speculate that for some reason this
type of habitat might be less favourable for the falcons, mak-
ing them behave atypically. If, however, this behaviour is
shown to be widespread, obtaining good estimates of costs
and benefits involved should allow us important insights into
the evolution of the close association between species.
Maclean (1970) suggested that Pygmy Falcons might be
able to deter the snakes that commonly raid Sociable
Weaver nests. This is not totally in agreement with our
observations at Benfontein and CW Sapsford’s (pers.
comm.) observations in the Kgalagadi Transfrontier Park of
Boomslang Dyspholidus typus and Cape Cobras Naja nivea
prospecting at Sociable Weaver colonies occupied by
Pygmy Falcons. Still, it would be worth investigating whether
the possible negative impact caused by Pygmy Falcons is
overcome by a significant drop in snake predation. A similar
situation has been described for Rock Pigeons Columba
oenas nesting in association with Hobby Falcons Falco sub-
buteo: even though the falcons occasionally prey on
pigeons, the pigeon colonies with resident falcons experi-
ence higher reproductive success since the falcons keep
away other predators (Bogliani et al. 1999). More studies are
needed to understand the relationship between the closely-
associated Pygmy Falcons and Sociable Weavers.
Acknowledgements — Mark Anderson kindly invited us to submit
this manuscript to the Richard Liversidge Memorial Issue. We are
also very grateful to Mark, who initiated the Sociable Weaver study
at Benfontein, for sharing with us his experience with the weavers
and for help with numerous other issues. We thank all our volunteer
field assistants for their help, and Colin Sapsford, Claire
Spottiswoode and Martim Melo for discussion and comments on the
manuscript. RC was funded by a doctoral grant from Program
Praxis XXI, FCT, Portugal. CD was funded by a post-doctoral grant
from the National Research Foundation, South Africa.
References
Bogliani G, Sergio F and Tavecchia G 1999. Woodpigeons nest-
ing in association with Hobby Falcons: advantages and choice
rules. Animal Behaviour 57: 125–131
Brown CJ 1989. Pygmy Falcon population in the central Namib
Desert, Namibia. Gabar 4: 10–13
Brown LH, Urban EK, Newman K 1982. The Birds of Africa, Vol. 1.
Academic Press, London
Covas R 2002. Life-history Evolution and Co-operative Breeding in
the Sociable Weaver. PhD thesis, University of Cape Town, Cape
Town
Covas R, Brown CR, Anderson MD and Brown MB 2002.
Stabilizing selection on body mass in the Sociable Weaver
Philetairus socius. Proceedings of the Royal Society, London B
269: 1905–1909
De Swardt DH 1990. Pygmy Falcon catches Sociable Weaver in
flight. Gabar 5: 27
Holmes M 1987. Sociable Weavers — nature’s engineers. African
Wildlife 35: 15–17
Maclean GL 1970. The Pygmy Falcon Polihierax semitorquatus.
Koedoe 13: 1–21
Maclean GL 1973. The Sociable Weaver, parts 1–5. Ostrich 44:
176–261
Steyn P 1982. Birds of Prey of Southern Africa. David Philip, Cape
Town
Received January 2004, accepted May 2004
Editor: MD Anderson
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