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Evidence for fatal collisions and kleptoparasitism while plunge‐diving in Gannets

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Evidence for fatal collisions and kleptoparasitism while plunge‐diving in Gannets

Abstract and Figures

Plunge-diving is a highly successful strategy for dealing with the challenges confronting birds feeding on pelagic prey. We tested for evidence of fatal injuries due to collision between conspecifics in plunge-diving Australasian Gannets Morus serrator and Cape Gannets Morus capensis, respectively, by performing post-mortem examinations of carcasses recovered from New Zealand waters and analysing video footage of Cape Gannet foraging events from South Africa. We found evidence of accidental collisions between Gannets and also observed a case of attempted kleptoparasitism, in which a diving Cape Gannet targeted a previously captured fish in the beak of a conspecific.
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Short communication
Evidence for fatal
collisions and
kleptoparasitism while
plunge-diving in Gannets
GABRIEL E. MACHOVSKY CAPUSKA,
1,2
*SARAH
L. DWYER,
2
MAURICE R. ALLEY,
3
KAREN A.
STOCKIN
2
& DAVID RAUBENHEIMER
1,2
1
Nutritional Ecology Research Group, and
2
Coastal-Marine Research Group, Institute of Natural
Sciences, Massey University, Private Bag 102 904
North Shore MSC, Auckland, New Zealand
3
New Zealand Wildlife Health Center, Institute of
Veterinary, Animal and Biomedical Sciences, Massey
University, Private Bag 11-222, Palmerston North,
New Zealand
Plunge-diving is a highly successful strategy for dealing
with the challenges confronting birds feeding on pelagic
prey. We tested for evidence of fatal injuries due to colli-
sion between conspecifics in plunge-diving Australasian
Gannets Morus serrator and Cape Gannets Morus capen-
sis, respectively, by performing post-mortem examina-
tions of carcasses recovered from New Zealand waters
and analysing video footage of Cape Gannet foraging
events from South Africa. We found evidence of acci-
dental collisions between Gannets and also observed a
case of attempted kleptoparasitism, in which a diving
Cape Gannet targeted a previously captured fish in the
beak of a conspecific.
Keywords: accidental collisions, Australasian
Gannet, Cape Gannet, injuries, kleptoparasitism,
plunge-diving.
Pelagic prey of seabirds are able to evade predation by
descending to depths beyond the reach of diving birds.
Among the adaptations that have evolved for dealing
with these challenges is plunge-diving. This is a highly
specialized foraging technique, which often takes place
in high-density assemblages of conspecific and hetero-
specific predators, in which the bird locates prey from
the air and then plunges at high speed into the water
for pursuit and capture (Cunningham 1866). Plunge-
diving provides the advantage of surprise (Johnston
1989), by helping to prevent prey descending beyond
reach, and is considered to be one of the most accu-
rate foraging methods (Wanless et al. 2005). However,
some authors have also noted possible disadvantages
associated with plunge-diving. Feeding in high-density
assemblages can involve fierce competition (Camphuy-
sen & Webb 1999), and contact with the water at high
dive speeds can be hazardous (Zillmer 2003), particu-
larly for younger, less experienced birds (Tator et al.
1981).
An additional hazard associated with this foraging
strategy is that diving at high speeds into dense assem-
blages of conspecific and heterospecific predators, some-
times in poor visibility, presents the risk of collision and
associated injury or death. There is, furthermore, a two-
fold risk of collision, because a given bird is at risk of
both colliding and being collided with. Unintentional
collisions in the water column have been reported from
high-density feeding assemblages of water-fowl (Bailey &
Batt 1974). Surprisingly, however, we are not aware of
any existing records for any species of such collisions
leading to injuries or death.
Here we report a study in which we tested for evi-
dence of injuries due to collision in two plunge-diving
seabirds, the Australasian Gannet Morus serrator and
Cape Gannet Morus capensis. These birds feed in large
groups (Nelson 1978), usually in multi-species feeding
associations (MSFAs) involving dolphins Delphinus spp.
and Bryde’s Whales Balaeonoptera brydei (Burgess 2006),
and are therefore at risk from this type of injuries.
Owing to the difficulties involved in detecting collisions
by direct observation, we analysed video footage of
Gannet foraging events from South African waters and
performed post-mortem examinations of carcasses
recovered from New Zealand waters.
MATERIAL AND METHODS
Gannet carcasses
During 2009 and 2010, 50 Australasian Gannet car-
casses were opportunistically collected from the waters
of the Hauraki Gulf, New Zealand. Gannets were
inspected for signs of injury due to collision while
plunge-diving, including physical injuries to the beak,
head or neck. Photographs were taken of any such
injuries, and the birds were subjected to post-mortem
examination following avian necropsy protocols (Work
2000). Stomach contents were analysed and fish were
removed from the oral cavity, oesophagus and stomach
for species identification. Otoliths were isolated and
diagnostic features were used to enable identification
to the lowest possible taxonomic level using published
guides (Smale et al. 1995) and the reference collection
*Corresponding author.
Email: g.machovsky@massey.ac.nz
ª2011 The Authors
Ibis ª2011 British Ornithologists’ Union
Ibis (2011), doi: 10.1111/j.1474-919X.2011.01129.x
held at Massey University, Albany. Digestion codes
described in Meynier et al. (2008) were assigned to
retrieved fish.
Video footage analysis
Only aerial footage of Australasian Gannets plunge-
diving in the Hauraki Gulf was included in this behavioural
analysis. A total of 40 min of high-resolution video footage
was collected in October 2009 using a Canon XH A1S
handycam with 20-mm zoom on board Dolphin Explorer,
a 20-m dolphin tour catamaran, at 5 m observer eye-
height. Additionally, 10 min of aerial and 15 min of
underwater video footage (25 frames s) of Cape Gan-
net foraging was analysed frame by frame using ADOBE
PREMIERE PRO CS4. This footage, collected on 4 June
2008, 24 and 30 June 2009, and 8 July 2009 from the
waters of Port Saint Johns, Eastern Cape, South Africa,
was loaned from Earth-touch (http://www.earth-touch.
com). The following categories of accidental collisions
were recorded: (1) Gannets colliding with Gannets
(G-G), or (2) Gannets colliding with Sharks, Whales
and or Humans (G-SWH). The G-G category was
classified into two sub-categories: (a) collision while
powered by underwater momentum alone (i.e. no wing
flapping), or (b) collision during underwater wing
flapping. Numbers of Gannets diving were recorded to
estimate the frequency of accidental collisions.
RESULTS
Gannet carcasses
Two of the 50 Gannet carcasses examined had injuries
consistent with death due to accidental collision. Both
carcasses were found floating in the water, one on 2 May
(G2M) 2010 (G17M) and the other on 17 May 2010
(at 3635.409¢S, 17501.369¢E and 3634.030¢S,
17520.400¢E, respectively). The carcasses considered to
be fresh, based on the presence of eye moisture and
absence of rigor mortis (Stockin et al. 2007). The nec-
ropsy of bird G2M revealed a circular wound 3–4 mm in
diameter, penetrating approximately 3.5 mm into the
left dorsal side of the cranium (Fig. 1a). This injury
extended through the cranium into the meninges of the
right cerebral cortex and cerebellum, producing periph-
eral haemorrhaging and a severely reddened left frontal
lobe (Fig. 1b). The peripheral diameter of the wound
closely fitted the circumference of the bill at 3.5 mm
caudal to the tip of an adult Australasian Gannet
(Fig. 1c), suggesting penetration by a Gannet bill as a
likely cause of the injury. A large Jack Mackerel Trachu-
rus novaezelandiae measuring 25 cm occupied almost the
entire oesophagus from immediately below the pharynx
caudally, while a second smaller (15 cm) fish of the same
species was present in the proventriculus. The fish were
intact with no flesh digested, suggesting that they were
ingested shortly prior to the Gannet’s death.
Post-mortem examination of bird G17M revealed a
3–4-mm-diameter ·4-mm-deep circular penetrating
wound in the left side of the neck (Fig. 2). The region
of the first cervical vertebra connected to the occipital
side of the skull was severely reddened, containing a
6·10-mm-wide area of peripheral haemorrhaging.
Two fresh and undigested Pilchards Sardinops neopil-
chardus, measuring 17.3 and 18.4 cm, occupied almost
the entire oral cavity and the oesophagus, from imme-
diately below the pharynx caudally. A third large fresh
Jack Mackerel was present in the proventriculus and
four additional Pilchards, which could not be classified
as fresh due to the partial absence of flesh, were found
in the stomach.
(a) (b) (c)
Figure 1. (a) A 3- to 4-mm-diameter circular penetrating wound on the left dorsal surface of the cranium of a male Australasian
Gannet Morus serrator. (b) Peripheral haemorrhage and a severely reddened left frontal lobe of the brain. (c) The head wound
diameter exactly fitted the dimensions of the bill tip of an adult Australasian Gannet at the inferred depth of penetration. Photographs
by M. Alley.
ª2011 The Authors
Ibis ª2011 British Ornithologists’ Union
2G.E. Machovsky Capuska et al.
Video footage analysis
No accidental collisions were recorded during detailed
analysis of aerial video footage of Australasian or Cape
Gannets foraging. However, a large number of Gannets
were seen manoeuvring and repositioning during the
momentum of plummeting into the water, although we
were unable to distinguish repositioning associated with
prey capture and collision avoidance.
In contrast, analysis of 15 min of underwater footage
revealed 3375 Cape Gannets diving and 25 cases of colli-
sions. Of these, 20 were from G-G events and five from
G-SWH events. The estimated frequency of collisions
per dive was 0.007, while the frequency of collisions
between Cape Gannets was 0.006. Of the impacts
between Cape Gannets, 18 took place during what
appeared to be the wingflapping stage, with two clear
cases of collision during the underwater momentum
phase (see Supporting Information, Appendix S1).
A descending bird orientating towards a second bird
holding a captured fish in its beak provided evidence of
kleptoparasitism by foraging Cape Gannets (see support-
ing Appendix S2). The birds made contact, competing
for the previously captured fish. The interaction lasted
for 6 s, whereafter both Gannets, still joined at the beak,
disappear from the frame. Such events might heighten
the risk of accidental collision, through stimulating
plunging Gannets to orient towards other birds in the
water column.
DISCUSSION
The advantages of plunge-diving as a foraging strategy
include the benefit of surprise (Johnston 1989) and the
accuracy of approach to prey (Wanless et al. 2005).
Plummeting into the water is a highly effective strategy,
as evidenced by the success of four families of seabirds
(Sulidae, Phaenthonidae, Laridae and Pelecanidae) that
feed in this way (Nelson 1978). Our analysis of video
footage demonstrated that collisions between diving
Gannets occur infrequently in MSFAs (0.007 collisions
per dive) and necropsy results of carcasses suggest that
collisions between foraging Gannets can potentially
result in severe head and neck trauma.
Post-mortem analysis of Australasian Gannets G2M
and G17M revealed penetrating wounds as might result
from the high-speed impact of an adult Australasian
Gannet beak. The momentum gained during a plunge-
dive allows Gannets to reach a depth of 10 m without
wing flapping, at which point they achieve neutral buoy-
ancy (Wilson et al. 1992). Velocities generated by this
momentum are higher than in the wing-flapping phase
(Ropert-Coudert et al. 2009) and are likely to be suffi-
cient to cause significant damage to an object upon
impact. Furthermore, our observations indicate that
these Australasian Gannets were involved in foraging
during or shortly prior to death, as would be expected if
accidental collision in MSFAs was the cause of the inju-
ries. Complete digestion of fish takes a Gannet between
2 and 6 h (Davies 1956), suggesting that the undamaged
fish with skin intact, found in the beak and oesophagus
of these birds, were ingested well within this period.
Although we assumed high-velocity impact was
required to penetrate the 2.5-mm-thick skull of Austral-
asian Gannet G2M, the penetration to soft tissue
observed in the neck of G17M could plausibly have
resulted from less force. One possibility is that the bird
was injured during aggressive interactions with other
(a) (b)
Figure 2. (a) A 3- to 4-mm-diameter circular penetrating wound in the left side of the neck of a male Australasian Gannet Morus
serrator. (b) The injury extended 4 mm deep into the neck. Photographs by S. L. Dwyer.
ª2011 The Authors
Ibis ª2011 British Ornithologists’ Union
Collisions and kleptoparasitism in Gannets 3
Gannets while foraging below or above or below the
water. We did observe, during the analysis of underwater
video footage, an aggressive interaction associated with
attempted kleptoparisitism. Whether sufficient force to
create such a wound could be generated by the neck
muscles of a swimming bird is uncertain. Alternatively,
this injury might have resulted from the interactive
effects of plunge-diving and kleptoparasitism, whether
deliberate or accidental.
As far as we are aware, this study is the first to report
attempted kleptoparasitism in Gannets. Kleptoparasitism
is potentially a profitable way of obtaining food. It
might, on the other hand, involve fierce interactions and
fighting over prey (Nilsson & Brönmark 1999), with
associated fitness costs and even a risk of fatal injuries
(Broom & Ruxton 2003).
Aerial video footage analysis indicated no accidental
above-water collisions between Gannets, possibly due to
adept manoeuvring by the birds in flight. A small num-
ber of underwater collisions were recorded among Cape
Gannets, with a total frequency of 0.007 accidental colli-
sions per dive. The majority of underwater collisions (23
of 25) were observed during the slower wing-flapping
phase of the dive, with only two observed in the fast
momentum phase of the plunge. As with the lack of
observed aerial collisions, this might reflect the evolution
of motion-sensitive mechanisms for collision avoidance.
Although these mechanisms in animals have evolved to
prevent collisions (Horridge 1987), every fast-moving
animal is at risk of injury by impact with objects (Ashby
1960). In Gannets, the risk of accidental collisions is
clearly density-dependent (Masotomi et al. 2007) and
could be related to the very small degree of binocular
parallax and the absence of invariance features in their
field of view, in which case birds may not be able to
detect their height and velocity with sufficient accuracy
(Lee & Reddish 1981).
Finally, our analysis revealed several collisions of div-
ing birds with marine mammals and predatory fishes,
but post-mortem analysis revealed no cases of damaged
beaks, broken skulls or broken necks that might be
expected from such collisions. Even though the ratio of
fatal injuries due to collisions was two in 50 carcasses,
our video footage analysis provided evidence that acci-
dental collisions between Gannets are not uncommon.
Our priorities for the future are to use the ongoing sur-
vey of Gannet carcasses to obtain a more accurate quan-
titative estimate of the risk of injury resulting from
collisions, and to better understand the relationship
between Gannet vision, the detection of prey and the
need to avoid dangerous collisions with other foragers.
We thank Ruedi Nager and two anonymous reviewers who pro-
vided comments that improved an earlier version of the manu-
script. Birds examined as part of this study were collected under
Department of Conservation permit AK-26359-FAU and with
Iwi permission. We thank Dr L. Meynier for assisting in the
organization of the post-mortem analysis, J. Jouma’a and A. De
Plaa for assistance in the field and during necropsies, and the
staff and crew of Dolphin Explorer for providing the filming plat-
form. We also thank Earth-touch for the loan of the videos.
Aspects of this work were funded by the Massey University
Research Fund (MURF). G.E.M.C and S.L.D. are recipients of
the Institute of Natural Sciences at Massey University (INS)
doctoral scholarship, and D.R. is part-supported by the National
Research Centre for Growth and Development, New Zealand.
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Received 29 July 2010;
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Associate Editor: Niall Burton.
SUPPORTING INFORMATION
Additional Supporting Information may be found in the
online version of this article:
Appendix S1. Accidental collisions between Cape
Gannets (see, for instance, the sequence extracted from
the video of Cape Gannets while foraging in South African
waters). http://www.earth-touch.com/result.php?i=Gan-
nets-steal-the-shoal.
Appendix S2. Kleptoparasitism between Cape Gan-
nets (see, for instance, the sequence extracted from the
video of Cape Gannets while foraging in South African
waters). http://www.earth-touch.com/result.php?i=Gan-
nets-plunge-deep-to-snatch-sardines-.
Please note: Wiley-Blackwell are not responsible for
the content or functionality of any supporting materials
supplied by the authors. Any queries (other than missing
material) should be directed to the corresponding author
for the article.
ª2011 The Authors
Ibis ª2011 British Ornithologists’ Union
Collisions and kleptoparasitism in Gannets 5
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... This can be achieved by group foraging, and relying on information obtained by observing the foraging of other individuals to find food sources ( Jones et al., 2018;Valone, 1989). However, group foraging can also have disadvantages, such as risk of kleptoparasitism, competition for the same prey and interference between predators reducing prey accessibility (Machovsky-Capuska et al., 2011a;Safina, 1990;Shealer and Burger, 1993). Alternatively, targeting prey that require less chase and handling time can increase efficiency as this increases the time and energy a predator has left to search for more prey. ...
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We studied the foraging distribution and the formation of multi-species feeding associations of seabirds and marine mammals off the British east coast. The local top-predator community comprised c. 34 species of seabirds, two pinnipeds and eight cetaceans. It appeared that multi-species feeding associations (MSFA's),with rather low species richness and diversity, were commonly formed around fishing vessels, were attracted by or otherwise associated with cetaceans (MSFA's with a generally low but more variable species richness and moderate diversity) and occurred over natural resources, apparently mainly fish shoals (MSFA's with the highest species richness and diversity). Small, short-lived MSFA's were the commoner type, particularly those over natural prey (sandeels and small clupeoids). Black-legged Kittiwakes Rissa tridactyla acted as catalysts in these flocks, Common Guillemots Uria aalge and Razorbills Alca torda as diving producers, apparently driving up fish towards the surface. The specific role of all other species joining in is described in general terms. Typical associations with marine mammals were those in which the cetaceans (mainly White-beaked Dolphins Lagenorhynchus albirostris and Harbour Porpoises Phocoena phocoena) operated as 'beaters' for Northern Gannets Morus bassanus and Black-legged Kittiwakes. The functioning of MSFA's is decribed from two angles. First of all, MSFA's are prominent phenomena on the sea surface, guiding seabirds using visual cues for food finding. Secondly, perhaps more importantly, the differentiation of feeding methods deployed in MSFA's may facilitate seabirds to reach prey that would otherwise be unavailable for them. It is suggested that the frequent associations of Kittiwakes and auks are the most sustainable system, because commensalism is the underlying mechanism rather than competition. The participation of scroungers in these flocks (Herring Gulls Larus argentatus as common nearshore examples) normally ruined the MSFA formation in no time. Usually, a MSFA would usually collapse as soon as the auks gave up their synchronised feeding activities. This underlines the essential role of Alcidae in these formations. The MSFA's are common and prominent in the North Sea and they deserve further study in the context of when interspecific relationships and the structuring of seabird communities.
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This paper provides the first report of stomach contents of common dolphin (Deiphinus sp.) from New Zealand waters. We examined 53 stomachs from 42 stranded and 11 by‐caught common dolphin from the North Island of New Zealand between 1997 and 2006. Although the diet of by‐caught and stranded common dolphin comprised a diverse range of fish and cephalopod species, the prevalent prey were arrow squid Nototodarus spp., jack mackerel Trachurus spp., and anchovy Engraulis australis. Stranded dolphins that originated from coastal waters, and dolphins by‐caught within neritic waters, fed on both neritic and oceanic prey. Moreover, this mixed prey composition was evident in the diet of common dolphin by‐caught in oceanic waters, suggesting inshore/offshore movements of common dolphin on a diel basis.
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