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Lumbering the gauntlet: Cape Gannet fledglings
killed by African Penguins
a
b
c
c
Alist air M McInnes , Nicolas Suarez , Gavin M Rishwort h , David B Green , Pierre A
c
Pist orius & Lorien Pichegru
cd
a
DST/ NRF Cent re of Excellence at t he Percy Fit zPat rick Inst it ut e of Af rican Ornit hology,
Universit y of Cape Town, Cape Town, Sout h Af rica
b
Cent ro Nacional Pat agonico (CONICET), Puert o Madryn, Chubut , Argent ina
c
DST/ NRF Cent re of Excellence at t he Percy Fit zPat rick Inst it ut e of Af rican Ornit hology,
Depart ment of Zoology, Nelson Mandela Met ropolit an Universit y, Port Elizabet h, Sout h
Af rica
d
Seabird Division, BirdLif e Sout h Af rica, Randburg, Sout h Af rica
Published online: 12 Sep 2014.
To cite this article: Alist air M McInnes, Nicolas Suarez, Gavin M Rishwort h, David B Green, Pierre A Pist orius & Lorien
Pichegru (2014): Lumbering t he gaunt let : Cape Gannet f ledglings killed by Af rican Penguins, Ost rich: Journal of Af rican
Ornit hology, DOI: 10. 2989/ 00306525. 2014. 955143
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OSTRICH
ISSN 0030–6525 EISSN 1727-947X
http://dx.doi.org/10.2989/00306525.2014.955143
This is the final version of the article that is published
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Short Note
Lumbering the gauntlet: Cape Gannet fledglings killed by African Penguins
Alistair M McInnes1*, Nicolas Suarez2, Gavin M Rishworth3, David B Green3, Pierre A Pistorius3 and Lorien Pichegru3,4
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1
DST/NRF Centre of Excellence at the Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Cape
Town, South Africa
2 Centro Nacional Patagonico (CONICET), Puerto Madryn, Chubut, Argentina
3 DST/NRF Centre of Excellence at the Percy FitzPatrick Institute of African Ornithology, Department of Zoology, Nelson
Mandela Metropolitan University, Port Elizabeth, South Africa
4 Seabird Division, BirdLife South Africa, Randburg, South Africa
* Corresponding author, e-mail: amcinnes3@gmail.com
This account presents the first known observations of Cape Gannet Morus capensis fledgling mortalities as a result
of aggressive nest defence behaviour by African Penguins Spheniscus demersus. Observations were conducted
in 2013 on Bird Island, Algoa Bay, South Africa – the world’s largest breeding colony of Cape Gannets. Twentyeight attacks were witnessed between 21 March and 18 May 2013 of which 16 resulted in mortality. The absence of
previous observations of this phenomenon may be due to an unusually high proportion of African Penguin chicks
being present relatively early on in the breeding season, associated with a potential for increased nest defence at
this stage, and a larger number of gannet fledglings that year compared to previous years over the same period.
Keywords: interspecific aggression, nest defence behaviour, seabirds
African Penguins Spheniscus demersus and Cape Gannets
Morus capensis are breeding endemics to the Benguela
upwelling system off southern Africa’s south-west coast
where they breed on mostly island localities in close
proximity to their pelagic fish prey (Crawford 2005; Crawford
and Whittington 2005). These species share three island
breeding localities in South Africa: Bird Island (Lamberts
Bay) and Malgas Island off the west coast, and Bird
Island (Algoa Bay) off the south coast. Globally, populations of both species have declined over the last century,
although numbers of Cape Gannets have increased on
Bird Island, Algoa Bay, which is now the world’s largest
gannetry (Crawford et al. 2009). Recent decreases in
populations of these two species have been primarily attributed to changes in the distribution and abundance of their
principle prey species, anchovy Engraulis encrasicolus and
sardine Sardinops sagax; both of these seabird species are
currently listed as threatened (Crawford et al. 2007, 2011;
IUCN 2014). The peak breeding season of Cape Gannets
occurs during the austral spring and summer, whereas that
of the African Penguin is mostly during autumn and winter
(Crawford 2005; Crawford and Whittington 2005). Overlap
of sympatric colonies is thus limited to the early (laying and
incubation) and late (fledging) stages of breeding in African
Penguins and Cape Gannets, respectively.
Inexperience, particularly in finding food, of recently
fledged Cape Gannets results in high levels of mortality
(Oatley et al. 1992), as is the case for other seabirds (Hamer
et al. 2002 and references therein). Another significant
source of Cape Gannet fledgling mortality at some breeding
sites (particularly at Malgas Island) stems from predation by
Cape fur seals Arctocephalus pusillus pusillus where up to
83% of fledglings have been killed in one breeding season
(Makhado et al. 2006). In this study we document the first
known incidences of Cape Gannet fledgling mortalities
due to attacks by breeding African Penguins. In an effort
to explain these events we furthermore investigated interannual variation in the timing of breeding of African Penguins
and the fledging success of Cape Gannets for three successive breeding seasons (2011–2013) to see if any unusual
trends were evident for the 2013 breeding season.
The events described here all occurred on Bird Island
(33°50′ S, 26°17′ E), Algoa Bay, in the Eastern Cape of
South Africa between 21 March and 18 May 2013. The
island is home to the largest colony of Cape Gannets
globally with an estimated 80 000–90 000 breeding pairs
(Moseley et al. 2012; Oceans and Coasts unpublished data)
and is also the breeding ground of c. 3 000 breeding pairs
of African Penguins (Crawford et al. 2012). All observations
were fortuitously made by our research team while doing
routine nest shift observations of African Penguins every
3 h between 07:00 and 18:00 for the entire study period.
Video footage of one incident was recorded and uploaded
to the following link: http://youtu.be/PC5I_oQVZGo.
Nests of African Penguins were monitored between the
months of February and May for three years commencing
in 2011. Nest checks were performed which involved
recording nest contents at weekly/biweekly intervals noting
number of adults present, number of eggs and/or chicks
for a set of 266 marked nests dispersed throughout the
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2
McInnes, Suarez, Rishworth, Green, Pistorius and Pichegru
breeding colony of this species on Bird Island. No nest
checks were conducted during the first half of April 2012
and the entire month of May 2012. This data was quantified
to examine the proportion (%) of chicks in active nests
(i.e. nests with either eggs or chicks) between all time
periods. Further on this, Cape Gannet chick survival (from
1–3 weeks post-hatching until fledging) was estimated for
three breeding seasons: 2010/11 (n 17 nests), 2011/12
(n 35 nests) and 2012/13 (n 30 nests). Only nests with
recently hatched chicks were selected and were visited on
at least three occasions between December and March.
The frequency of penguin chicks in active nests was
compared between all years for the period that corresponds
with the Cape Gannet fledging period, here 15 February to
31 March (Oatley et al. 1992), in order to assess whether
2013 was an anomalous year in either regard.
Twenty-eight gannet fledglings were observed being
attacked by breeding African Penguins: five occurred during
the last week of March, 19 during April and four during
May. A large proportion of these incidences (20 of 28, 71%)
involved gannets walking through the penguin colony, the
remainder were of birds landing directly within the penguin
colonies. Attacks typically occurred from within reach of the
penguins’ nesting territory and included deliberate pecking,
mostly to the hind-neck and the back of the skull, and flipper
slapping to the fledglings’ flanks, head and wings (Figure 1).
These attacks resulted in 16 observed mortalities, with most
attacks being initiated by two or more penguins. Several
additional Cape Gannet fledglings were observed dead
within close proximity to African Penguin nests every week
during the study period, but it is unknown whether these
fatalities were due to African Penguin attacks.
There was a relatively high proportion of African Penguin
nests with chicks earlier in the 2013 breeding season
compared to the previous two years (Figure 2). There was
also a significant difference in the observed frequency of
African Penguin nests with chicks, during the Cape Gannet
fledgling period, in 2013 compared to 2011 and 2012
(2 47.7, df 2, P 0.0001) with 30% of active nests
during this period in 2013 having chicks compared to 13%
(a)
and 16% for 2011 and 2012, respectively. Chick survival
for Cape Gannets was also highest during the 2012/13
breeding season with 96.7% of chicks from monitored nests
fledging compared to 76.5% and 42.9% for the 2010/11 and
2011/12 breeding seasons, respectively (Figure 3).
Nest defence behaviour in colonial nesting seabird species
is usually directed at conspecifics at the onset of the breeding
season for territory or nest defence (e.g. Duffy 1983; Williams
1995; Somers et al. 2007) or directed at potential predators
at varying degrees of intensity throughout the breeding
season, e.g. terns (Cullen 1960) and skuas (Andersson
1976). Intraspecific aggression in Spheniscus penguins is
well documented (e.g. Seddon and van Heezik 1993; Stokes
and Dee Boersma 2000) and African Penguins have been
recorded killing a Kelp Gull Larus dominicanus (Ward 1998),
a known predator of their chicks and eggs (Pichegru 2013).
As far as we are aware, the events observed in the current
study represents the only other case of African Penguins
killing another seabird species, a rare occurrence for a
seabird species that does not prey on other birds.
A possible explanation for this unusual occurrence may
be related to the relatively high number of active penguin
nests already at the chick stage during the period when
Cape Gannets were fledging in 2013, in conjunction with a
large number of gannet fledglings resulting from a particularly high breeding success. The Cape Gannet population
on Bird Island almost doubled between 1986 and 2005
with signs of a slight decrease in 2008 before increasing
again to about 90 000 breeding pairs in 2013 (Crawford
et al. 2009; Oceans and Coasts unpublished data). It is
therefore possible that encounters between breeding African
Penguins and Cape Gannet fledglings are likely to have
increased as a density dependent function of this trend
(sensu Ashbrooke et al. 2010).
Although we cannot rule out the possibility of this phenomenon having occurred during previous breeding seasons, we
expect this to have been unlikely during 2011 and 2012. Our
team did regular nest checks during both the 2011 and 2012
African Penguin breeding seasons and the highly conspicuous nature of this interspecific aggression is unlikely to
(b)
Figure 1: Cape Gannet fledgling attacks. (a) Fledgling being pecked at the back of its head by adult attending large chicks. (b) Fledgling
being pecked and flipper slapped
60
50
3
2012
90
2013
40
30
20
10
1–14 15–28 1–15 16–31 1–15 16–30 1–15 16–31
Feb
Feb
Mar
Mar
Apr
Apr
May May
NEST CHECK PERIOD
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N = 30
100
2011
FLEDGING (%)
NESTS WITH CHICKS (%)
Ostrich 2014: 1–4
Figure 2: Proportion of active African Penguin nests with chicks for
two-week periods between 2011 and 2013. No data was recorded
for the first half of April 2012 and during May of the same year. The
dashed rectangle denotes the Cape Gannet fledging period
have gone unnoticed, at least during the times when these
authors were present in the colony. In addition, during 2011
and 2012, a large proportion of the African Penguin colony
to the north of the island was frequented several times a day
and, given that the majority of birds bred within c. 100 m of
the main path network of the island (AMM, NS and LP pers.
obs.), a large proportion of the colony was under observation. Furthermore, Bird Island has been permanently staffed
since the 1990s by South African National Park rangers,
who have never recorded this behaviour prior to the
incidents described here.
Parental investment theory predicts that nest defence
should increase with the advance of the breeding season
because the fitness value of a chick is higher than that of
an egg (Williams 1966). Activities such as nest defence can
incur a significant risk for adults in terms of injury or even
mortality and the probability of their own survival needs
to be measured against the likelihood of their offspring
surviving. The intensity of nest defence has been shown to
be positively correlated with the age of the nesting effort (in
Chinstrap Penguins Pygoscelis antarctica, Amat et al. 1996;
and King Penguins Aptenodytes patagonicus, Côté 2000).
The earlier breeding season for African Penguins during
2013 resulted in a large number of penguins attending
chicks during the time when Cape Gannets were fledging
and it is possible that aggressive interactions were heightened due to the advanced investment that these adults had
made at this stage.
The number of mortalities of Cape Gannets observed due
to these interactions is unlikely to have had any significant
effect on the gannet population. In seabirds, adult survival
rather than breeding success or fledging survival, generally
plays the major role in governing population growth.
Gannets, like most seabird species, typically experience
high fledgling mortality rates (e.g. Schreiber and Burger
2002; Makhado et al. 2006), which would likely mask any
appreciable effects that could arise from this phenomenon.
Prior to the onset of excessive guano harvesting in the
nineteenth and early twentieth centuries, African Penguins
nested predominantly in burrows (Frost et al. 1976), which
would have limited the frequency of interspecific aggression
80
N = 17
70
60
50
N = 35
40
2010/11
2011/12
BREEDING SEASON
2012/13
Figure 3: Proportion (%) of Cape Gannet chicks fledged for three
breeding seasons between 2010 and 2013
between these species to a small area around the entrance
of the burrow (e.g. for Spheniscus penguins, see Stokes
and Dee Boersma 2000). The loss of this burrowing
habitat for penguins has exposed them to heat stress and
increased predation, which has prompted conservation
managers to install artificial nests to ameliorate these effects
(Pichegru 2013; Lei et al. 2014). This study has highlighted
another consequence of this habitat displacement, the
potential for increased interspecific aggression encounters.
Acknowledgements — Thanks to Dr Claudio Velasquez from
Homebrew Films (Cape Town) for his footage of penguins
attacking a juvenile Cape Gannet made available on Youtube. We
are also grateful to SANParks for permission to conduct research
on Bird Island and to the rangers who conducted the penguin
nest checks. Two anonymous reviewers and the Associate Editor,
Dr Richard Sherley, are thanked for their useful comments on the
original manuscript.
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Received 30 April 2014, revised 24 July 2014, accepted 6 August 2014
Associate Editor: Richard Sherley