Hunting between the air and the water : the Australasian gannet (Morus serrator) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Auckland, New Zealand
Australasian gannets (Morus serrator) are the second rarest member of the seabird
group Sulidae. Among the three species of gannets worldwide, they are the only
species that regularly breeds in southeastern Australia and New Zealand. Like all
gannets, M. serrator face considerable challenges in foraging, relying on sparsely
and patchily distributed pelagic prey, which move in a 3D environment. Whereas
most predators are specialise hunters in one media, gannets have to hunt within a
complex air-water interface. The aim of the present thesis is to examine the hunting
strategies of Australasian gannets, with particular emphasis on how these birds use
both aerial and aquatic adaptations to locate and capture prey.
The acquisition of information concerning food sources was analysed using
GPS data loggers, field observations and high resolution video footage. I tested the
hypothesis that gannets obtain information of food resources from their partners
using bill fencing as referential signals analogous to the waggle dance in honeybees
(Apis mellifera) (Chapter 2). Results did not support this hypothesis but suggested
that Australasian gannets use a combination of strategies, probably including
memory that facilitates their return to locations where prey was previously captured
(Chapter 3) and local enhancement to locate active feeding sites (Chapter 2).
The impact of intraspecific competition for local resources was studied
between large (Cape Kidnappers, 7,300 breeding pairs) and small (Farewell Spit,
3,900 breeding pairs) colonies in New Zealand using GPS data loggers (Chapter 3).
Results indicated that gannets from the larger colony invested more in foraging
(greater foraging times and foraging distances). This is consistent with previous
studies of other gannet species, suggesting that M. serrator experience intraspecific
competition for food when living in large colonies.
Pelagic prey are able to evade predation by descending to depths beyond the
reach of diving birds. Among the adaptations evolved by gannets for dealing with this
challenge is plunge-diving, where the bird uses gravity in the aerial phase of the hunt
to gain speed and momentum for descending into the water column. I conducted a
fine scaled analysis using videography of the aerial and aquatic phases of this highly
specialised hunting strategy. Analysis of the aerial phase (Chapter 4) showed that
the initiation of plunge dives are synchronised among members of foraging groups,
suggesting a form of group-level behaviour in which gannets might benefit from the
sensory experiences (prey detection) of conspecifics. The analysis also showed that
gannets adapt the aerial phase of their dives in presence vs. absence of
heterospecific predators. In the aquatic phase (Chapter 5), gannets perform short
and shallow V-shaped dives and long and deep U-shaped dives in pursuit of pelagic
fish and squid. My findings revealed that gannets adjusted their dive shape in
relation to the depth of their prey rather than prey type, as previously hypothesised.
Although the maximum number of prey captured per dive by the gannets was higher
than previously reported, reaching up to five fish in a single U-shaped dive, the
results presented herein suggest that the two dive profiles were equally profitable.
To examine the role of underwater vision in prey capture, I used underwater
video footage, photokeratometry and infrared video photorefraction (Chapter 6).
Analysis of video footage confirmed that there are two distinct phases in the
underwater component of plunge dives in Australasian gannets, an initial phase in
which the bird is propelled through the water column by the momentum of the plunge
(M phase) and a phase in which it is actively propelled by wing flapping (WF phase).
The highest prey capture rate was observed during the WF phase, a result that
suggests the use of vision in underwater prey pursuit. I therefore used
photokeratometry and video photorefraction to test whether gannets are able to
adapt optically in the transition from aerial to aquatic media. My measurements
showed that underwater visual accommodation in the gannets was attained within 2 -
3 frames (80 - 120 ms) of submergence, a remarkably short timescale in relation to
the optics of most vertebrate eyes.
The preceding chapters demonstrate some highly effective behavioural and
sensory capacities used by gannets in foraging. In Chapter 7 I demonstrate
evidence of fatal injuries due to collision between conspecifics in plunge-diving
Australasian and Cape gannets (M. capensis). The analysis also revealed a case of
attempted underwater kleptoparasitism, in which a diving bird targeted a previously
captured fish in the beak of another gannet. This novel observation suggests a
further challenge for hunting gannets, namely to retain prey following the capture.
Appendix 1 and 2 removed due to copyright restrictions:
Machovsky Capuska, G.E., Huynen, L., Lambert, D., Raubenheimer, D. (2011), UVS is rare in seabirds, Vision Research, 51, 1333-1337
Shuckard, R., Melville, D.S., Cook, W., Machovsky Capuska, G.E. (2012), Diet of the Australasian gannet (Morus serrator) at Farewell Spit, New Zealand, Notornis, 59, 66-70