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Foraging behaviour and habitat use of chick-rearing Australasian Gannets in New Zealand

  • Instituto de Investigaciones Marinas y Costeras (IIMyC), CONICET Universidad Nacional de mar del Plata Argentina

Abstract and Figures

Patchily distributed marine pelagic prey present considerable challenges to predatory seabirds, including Gannets (Morus spp.) departing from large breeding colonies. Here, for the first time, we used GPS data loggers to provide detailed spatial, temporal, and habitat metrics of chick-rearing Australasian Gannets (Morus serrator) foraging behaviours from two distant colonies in New Zealand. Our goal was to examine the extent to which Gannet foraging tactics vary across disparate habitats, and determine whether the observed differences are consistent with predictions derived from foraging studies of other gannet species. Foraging trip performance was highly consistent between colonies, and sexes, and no significant differences in any of the variables analyzed were observed. However, Gannets from Farewell Spit (FS) dove in shallower waters (0–50 m) than birds from Cape Kidnappers (CK, >50 m), which is consistent with previous dietary studies suggesting that FS Gannets feed mainly on coastal prey, whereas CK birds feed on species with a more oceanic distribution. Diving frequencies were similar in the two colonies suggesting that Gannets were foraging in habitats with similar levels of food availability. Further studies are needed to understand the relationship between prey availability, oceanography and geographic features, to better interpret foraging tactics of Australasian Gannets.
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Journal of Ornithology
ISSN 2193-7192
Volume 155
Number 2
J Ornithol (2014) 155:379-387
DOI 10.1007/s10336-013-1018-4
Foraging behaviour and habitat use of
chick-rearing Australasian Gannets in New
Gabriel E.Machovsky-Capuska, Mark
E.Hauber, Mariela Dassis, Eric Libby,
Martin C.Wikelski, Rob Schuckard,
David S.Melville, et al.
1 23
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Foraging behaviour and habitat use of chick-rearing Australasian
Gannets in New Zealand
Gabriel E. Machovsky-Capuska Mark E. Hauber Mariela Dassis
Eric Libby Martin C. Wikelski Rob Schuckard David S. Melville
Willie Cook Michelle Houston David Raubenheimer
Received: 11 March 2013 / Revised: 19 September 2013 / Accepted: 7 October 2013 / Published online: 23 October 2013
ÓDt. Ornithologen-Gesellschaft e.V. 2013
Abstract Patchily distributed marine pelagic prey
present considerable challenges to predatory seabirds,
including Gannets (Morus spp.) departingfrom large breeding
colonies. Here, for the first time, we used GPS data loggers
to provide detailed spatial, temporal, and habitat metrics of
chick-rearing Australasian Gannets (Morus serrator) for-
aging behaviours from two distant colonies in New Zea-
land. Our goal was to examine the extent to which Gannet
foraging tactics vary across disparate habitats, and deter-
mine whether the observed differences are consistent with
predictions derived from foraging studies of other gannet
species. Foraging trip performance was highly consistent
between colonies, and sexes, and no significant differences
in any of the variables analyzed were observed. However,
Gannets from Farewell Spit (FS) dove in shallower waters
(0–50 m) than birds from Cape Kidnappers (CK, [50 m),
which is consistent with previous dietary studies suggesting
that FS Gannets feed mainly on coastal prey, whereas CK
birds feed on species with a more oceanic distribution.
Diving frequencies were similar in the two colonies sug-
gesting that Gannets were foraging in habitats with similar
levels of food availability. Further studies are needed to
understand the relationship between prey availability,
oceanography and geographic features, to better interpret
foraging tactics of Australasian Gannets.
Keywords Foraging range Diving behaviour
Morus serrator Food sources GPS data loggers
Communicated by C. Barbraud.
G. E. Machovsky-Capuska (&)D. Raubenheimer
Faculty of Veterinary Science, Charles Perkins Centre, School of
Biological Science, University of Sydney, Sydney, Australia
G. E. Machovsky-Capuska
Coastal-Marine Research Group, Institute of Natural and
Mathematical Sciences, Massey University, Auckland,
New Zealand
M. E. Hauber
Department of Psychology, Hunter College, The Graduate
Center of the City University of New York, New York,
NY 10065, USA
M. Dassis
Facultad de Ciencias Exactas y Naturales, Instituto de
Investigaciones Marinas y Costeras, Universidad Nacional de
Mar del Plata-CONICET, Funes 3350 (7600),
Mar del Plata, Argentina
E. Libby
New Zealand Institute for Advanced Study, Institute of Natural
Sciences, Massey University, Private Bag 102 904 North Shore
MSC, Auckland, New Zealand
M. C. Wikelski
Max-Planck Institute for Ornithology, Vogelwarte Radolfzell,
Radolfzell, Germany
M. C. Wikelski
Department of Biology, University of Konstanz, Konstanz,
R. Schuckard D. S. Melville W. Cook
Ornithological Society of New Zealand, Nelson, New Zealand
M. Houston
Equine Parentage and Animal Genetics Service Centre,
Massey University, Palmerston North, New Zealand
J Ornithol (2014) 155:379–387
DOI 10.1007/s10336-013-1018-4
Author's personal copy
Nahrungssuchverhalten und Habitatnutzung
Australischer To
¨lpel wa
¨hrend der Jungenaufzucht in
¨ckenhaft verbreitete pelagische Beute stellt eine betra
chtliche Herausforderung fu
¨r nahrungssuchende Seevo
gel dar. Das gilt auch fu
¨lpel (Morus spp.), die aus
großen Brutkolonien zur Nahrungssuche auf See abfliegen.
In zwei weit voneinander entfernt liegenden Kolonien
Australischer To
¨lpel (Morus serrator) in Neuseeland wur-
den nun zum ersten Mal GPS-Datenlogger eingesetzt, um
¨hrend der Jungenaufzucht detaillierte Raum-Zeit-Daten
sowie Informationen zur Habitatnutzung nahrungssuchen-
der To
¨lpel zu erhalten. Ziel war es zum einen zu untersu-
chen, in welchem Ausmaß die Nahrungssuchstrategien der
¨lpel variieren zwischen verschiedenen Habitaten. Zum
anderen wurde bestimmt, ob die beobachteten Unter-
schiede konsistent sind mit Vorhersagen aus Studien zur
Nahrungssuche anderer To
¨lpelarten. Die Nahrungsflug-
Leistung war einheitlich zwischen den Kolonien und
Geschlechtern. Es konnten keine signifikanten Unter-
schiede zwischen den weiteren analysierten Variablen nach-
gewiesen werden. Allerdings tauchten To
¨lpel der
Farewell Spit Kolonie (FS) in flacheren Gewa
(0–50 m) als Vo
¨gel aus der Cape Kidnappers Kolonie (CK,
[50 m). Fru
¨here Nahrungsstudien besta
¨tigen dies und
deuten darauf hin, dass FS To
¨lpel hauptsa
¨chlich ku
nahe Beute fressen, wohingegen CK To
¨lpel mehr ozea-
nisch verbreitete Nahrung aufnehmen. Die
Tauchfrequenzen waren a
¨hnlich in beiden Kolonien, was
darauf schließen la
¨sst, dass To
¨lpel in Habitaten mit a
chen Beuteverfu
¨gbarkeiten auf Nahrungssuche gehen.
¨hrende Untersuchungen zur Beziehung zwischen
¨gbarkeit, Ozeanografie und geografischen Ei-
genschaften sind no
¨tig, um die Strategien der Nah-
rungssuche Australischer To
¨lpel besser zu verstehen und
interpretieren zu ko
Marine pelagic resources of predatory seabirds can present
considerable challenges because prey is often widely and
patchily distributed in space and time (Weimerskirch
2007). Accordingly, successful foraging trips often range
over hundreds of kilometres and span several days (Hamer
et al. 2000; Rayner et al. 2010). In such circumstances,
members of breeding pairs of biparental species need
effective long-range foraging strategies to locate the food
source and integrated time-budgeting to balance self-
feeding, offspring-feeding, and the nutritional constraints
of the partner tending the nest (Weimerskirch et al. 1994;
Ropert-Coudert et al. 2004; Garthe et al. 2013).
Foraging area could differ between colonies of a single
species in relation to regional oceanographic differences,
intraspecific competition and food availability (Hamer
et al. 2000). Recent advances in bio-logging science,
through the development of increasingly miniaturized data
loggers, have provided growing details on foraging
behaviours and feeding ranges of marine predators,
including seabirds (Ropert-Coudert and Wilson 2005).
Amongst the three species of closely-related Gannets
(Morus spp.), the foraging behaviour of Northern (Morus
bassanus) and Cape Gannets (Morus capensis) has been
extensively studied using different data loggers. Austral-
asian Gannets (Morus serrator) have been considered to be
the southern hemisphere form of the Northern Gannet with
similar foraging characteristics, although recent work,
based at the breeding colony, suggested that these two
distinct species seem to occupy different breeding and
foraging niches (Stephenson 2005).
Australasian Gannets breed exclusively in southeastern
Australia and New Zealand (Nelson 1978). Despite the
recent positive population trends, the species remains the
second rarest member of the seabird group Sulidae (Nelson
2005). Within New Zealand, Gannets are distributed
among 26 breeding colonies on the east coast and only
three on the west coast, spanning a latitudinal range of
34–46°S (Nelson 2005). Australasian Gannets are known to
have a flexible diet of fish and squid, which ranges from
coastal to oceanic species with marked prey-use differ-
ences between different gannetries (Robertson 1992;
Schuckard et al. 2012). The foraging behaviour of this
species has been previously characterized using bird bands
(Wingham 1985), colour-marked on the chest (Wingham
1985), stable isotopes and capillary tubes (Ismar 2010),
direct observations (Wodzicki and Robertson 1955), aerial
and underwater filming (Machovsky-Capuska et al. 2011b,
2012,2013), regurgitations (Wingham 1985; Robertson
1992; Bunce 2001; Pyk et al. 2008; Schuckard et al. 2012),
necropsies (Machovsky-Capuska et al. 2011a) and data
loggers only in Australian colonies (GPS, Bunce 2005;
heart rate, Green et al. 2010).
Here, we report a study in which GPS data loggers were
used to examine and compare the behaviour of chick-
rearing Gannets during foraging trips in two Australasian
Gannet colonies from different geographic locations in
New Zealand, the Cape Kidnappers (7,300 breeding pairs,
east coast) and Farewell Spit (3,900 breeding pairs, west
coast) colonies. Recent studies of Northern Gannets
(Wakefield et al. 2013) with large sample sizes have
quantitatively assessed predictions about the effect of col-
ony size, interspecific competition, oceanographic
380 J Ornithol (2014) 155:379–387
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conditions, and food availability on Gannet foraging tac-
tics. While the number of colonies we studied was too low
to statistically test these hypotheses, our study will be the
first to provide detailed spatial, temporal, and habitat
metrics of the Australasian Gannet’s foraging behaviours
during the breeding season (chick-rearing stage) in two of
New Zealand’s growing gannetries. In particular, we seek
to (1) gain a better understanding of the foraging strategies
of Gannets and (2) identify and compare the main foraging
areas in which Gannets feed in the two regions.
Study area
The study was conducted during the chick-rearing periods
in January 2010 and 2011 on the Beach Colony of Cape
Kidnappers gannetry (CK), North Island, New Zealand
(39°380S, 177°050E) and in January 2012 at Farewell Spit
gannetry (FS), which is located at the northern end of the
South Island, New Zealand (40°330S 173°010E). CK has a
population of around 7,300 breeding pairs (Nelson 2005;
Ismar et al. 2010), whereas the FS gannetry has a population
estimated at 3,900 breeding pairs (Schuckard et al. 2012).
Capture and handling of birds
Adult Gannets rearing 2- to 5-week-old chicks were cap-
tured with a blunt-tip shepherd’s crook from nests located
in the periphery of the colony immediately after adopting
the sky pointing posture (Nelson 1978). Chick age was
similar for both colonies. Captured Gannets were banded
with individually numbered metal rings on their leg and
secondary covert feathers were collected for DNA sex
identification following Fridolfsson and Ellegren (1999).
The loggers were attached with Tesa tape to the four
central tail feathers as in Hamer et al. (2001). To aid in
their rapid identification, birds were also marked on the
chest with Sharpie markers
´millet et al. 2004). Cap-
turing, measuring and the attachment of loggers took
*10 min, whereafter birds were released at the edge of the
colony (Garthe et al. 2007a,b). Devices and tape strips
were retrieved soon after the birds arrived at the colony
following a single foraging trip. This study was conducted
under permits of Massey University Animal Ethics com-
mittee (09/76) and the New Zealand Department of Con-
servation (ECHB-23237-RES).
Data logger deployment
The GPS data loggers were manufactured by e-obs digital
telemetry in Germany ( and consisted
of a power supply (lithium polymer battery cell with
4.5 V), a flash memory SD-card, a GPS module (LEA 4S
by u-blox
), a radio transmitter (‘‘pinger’), an on-board
real-time clock, an antenna, and a mobile interface between
user and GPS-RF-tag (Base Station b5; e-obs). All com-
ponents were embedded into a heat-shrink tube for water-
proofing. Final size was 50 950 915 mm (length 9width
9height), weighing 45 g and representing around 2 % of
the adult body weight (Nelson 1978). To record data
related to position (latitude, longitude, and altitude), speed
and time, we deployed continuous (1-s intervals) or inter-
mittent (15-s intervals) loggers (Table 1).
Data analysis
Differences in foraging trip parameters were compared
between colonies. Following Gre
´millet et al. (2004),
maximum distance away from the colony (MCD), total
foraging path, foraging trip duration, flying time, resting
time and speed were estimated from the recorded GPS
data. The GPS continuous logger offered high resolution
data that allowed inferring diving behaviour from the
interruptions of GPS signals (Pichegru et al. 2007). In this
study, dive duration and dives per hour of trip were esti-
mated as signal interruptions B8 s, assuming mean dive
duration of 8 s for this species (Machovsky-Capuska et al.
Following Pettex et al. (2010), we calculated the average
bearing location of the dives from the colony to represent
the intended destination. For each day of deployment, we
computed the average bearing angle of dives between
foraging destinations to quantify the difference in their
daily bearing from the colony. Being coastal colonies, the
Gannets at both study sites did not have a full range of 360°
available for oceanic foraging trips. To evaluate the prob-
ability that the observed distribution of vectors would
occur under the null hypothesis of no difference in the
bearing direction of foraging trips on the same days, we
Table 1 Numbers and characteristics of the devices deployed in
chick-rearing adult Australasian Gannets (Morus serrator) at Cape
Kidnappers (CK) and Farewell Spit (FS), New Zealand
Colony Year Device type Number of birds
CK 2010 GPS continuous 3 4
GPS 15 s 2 2
CK 2011 GPS continuous 5 4
GPS 15 s 0 1
FS 2012 GPS continuous 4 4
GPS 15 s 2 1
Mmales, Ffemales
J Ornithol (2014) 155:379–387 381
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randomised the day assignments of Gannets 100,000 times
as part of a permutation test (Robson et al. 2004). Home
range areas were calculated applying the adaptive Kernel
method (Worton 1989) with 40, 50, 60, 70, 80, 90 and
95 % locations, using the Home Range Tools extension in
ArcGIS 9.8. Following Kie et al. (2010), a smoothing
factor of 80 % of reference bandwidth was applied to
estimate a reliable home range area. All kernel areas are
included on the maps; however, only 95 and 50 % values
were statistically analysed. As previously described, the
95 % Kernel area (K95) represents the general use area and
the 50 % Kernel area (K50) represents the core area or the
most intensively used area (Iversen and Esler 2006; Hamer
et al. 2007; Rodrı
´guez et al. 2013). K95 and K50 were
calculated for both general areas, grouping all birds from
each colony and individual areas for each animal.
For statistical comparisons data from the GPS units were
analysed using MATLAB 2009 and PASW Statistics v.18.
Data were initially tested using Levene’s test for homo-
scedasticity and Shapiro–Wilk’s test for normality, v
ttests were used for subsequent comparisons. Following
Firth et al. (2006) the K95/K50 ratios (the proportion of the
general area that were most intensively used) between
colonies were log
transformed and then compared using
ttests. We report data as mean ±standard deviation.
A total of 32 individual foraging trips were recorded from
CK in 2010 and 2011, and FS in 2012 (Table 1). Foraging
trip performance was highly consistent between the two
consecutive breeding seasons studied at CK colony, with
no significant differences in any of the variables analysed
(Table 2). Data from both years at CK were therefore
combined and pooled for multiple comparisons with data
collected from FS colony.
Foraging trip performance was highly consistent
between colonies and no significant differences in any of
the variables analysed were observed (Table 3). During
foraging trips, Gannets spent on average 23.5 % (±7.5) of
the time flying at CK and 29.0 % (±21.9) at FS, whereas
they rested on the water an average of 75.5 % (±7.4) of the
time at CK and 70.1 % (±21.9) at FS. Overall, plunge-
diving only accounted for\1 % of the foraging trip in both
From a total of 2,206 dives recorded, 521 dives were
from FS and 1,685 dives were from CK (808 dives in 2010
and 877 dives in 2011). No significant differences were
observed in the duration and frequency of the dives
between colonies (Table 3), Gannets from FS dove in
shallower waters (99.8 %, 0–50 m isobaths; Fig. 1) than
Gannets from CK (54.5 %, [50 m isobaths; Fig. 2) (Chi
square test, v
=481.25; df =1; p\0.0001).
The K95 and K50 used by Gannets from CK colony
were similar between years (ttest, t=-1.09, df =20,
p=0.29 and ttest, t=-1.32, df =20, p=0.20,
respectively) and were therefore combined for the analyses,
resulting in 4,964.1 and 755.7 km
, respectively (Fig. 1),
Table 2 Performance of foraging trips made by chick-rearing adult
Australasian Gannets at Cape Kidnappers in 2010 (n=11) and 2011
Parameter 2010 2011 tvalue p
Max. distance to
colony (km)
55.1 ±18.7 56.2 ±29.3 -0.10 0.92
Foraging path
length (km)
255.9 ±119.9 282.5 ±126.9 -0.48 0.64
Foraging trip
duration (h)
37.1 ±35.1 25.6 ±9.3 0.96 0.35
Speed (km h
) 8.5 ±3.4 11.4 ±4.5 -1.68 0.11
Flying time (h) 5.6 ±2.5 5.7 ±2.6 -0.08 0.93
Resting time (h) 31.5 ±35.4 19.8 ±8.6 0.96 0.35
Dive duration (s) 4.0 ±2.1 3.9 ±2.1 1.02 0.98
Dives per hour of
4.2 ±1.3 4.2 ±1.2 -0.01 0.92
Values are given as mean ±standard deviation
Table 3 Colony characteristics and foraging trip performance of
Australasian Gannets breeding at Cape Kidnappers and Farewell Spit
Parameter Cape Kidnappers Farewell Spit tvalue p
East coast
(North Island)
West coast
(South Island)
Population size 7,300 3,900
Sample size (n)21 11
Max. distance
to colony
55.6 ±23.3 40.2 ±28.2 -1.63 0.12
Foraging path
length (km)
267.9 ±120.6 184.6 ±188.9 -2.03 0.05
1,854.4 ±1,312.0 1,061.9 ±1,681.9 -1.45 0.16
167.2 ±131.4 108.0 ±190.6 -1.02 0.32
Foraging trip
duration (h)
31.9 ±26.8 14.7 ±10.7 -1.50 0.14
(km h
9.8 ±4.1 15.3 ±16.1 1.46 0.16
Flying time (h) 5.7 ±2.5 4.4 ±4.1 -1.23 0.23
Resting time
26.2 ±26.9 10.3 ±7.7 -1.88 0.07
Dive duration
4.1 ±2.2 3.9 ±2.1 1.92 0.05
Dives per hour
of trip
4.2 ±1.2 4.8 ±1.1 1.15 0.26
Values are given as mean ±standard deviation
382 J Ornithol (2014) 155:379–387
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whereas the marine areas used by FS Gannets were 3,786.2
and 689.1 km
for K95 and K50 respectively (Fig. 2).
Despite the CK areas having slightly larger values com-
pared to the FS areas, no significant differences between
colonies were found in either the K95 or the K50 area
distributions (Table 3). Individual kernel ranges showed a
high variability among Gannets in both colonies: K95
ranged from 417.94 to 5,158.86 km
(CV =70.75 %) and
K50 from 27.16 to 485.30 km
(CV =78.63 %) in CK
colony, and K95 from 0.74 to 5,922.69 km
(CV =158.38 %) and K50 from 0.04 to 533.61 km
(CV =176.47 %) in FS colony. Again, no significant
differences were found in the ratios K95:K50 areas
between CK (9.4 ±3.7 %, range =3.2–17.6 %) and FS
(9.3 ±8.0 %, range =3.8–29.3 %) (ttest, t=-0.89,
df =29, p=0.38).
The bearing angles of departing birds deployed on the
same day at FS (n=4 groups, eight birds) showed that the
majority of tracked FS Gannets foraged southeast of the
colony (Chi square test, v
=7.36; df =2; p\0.05),
which corresponds to both a general use of area (K95) and
a core foraging area (K50) almost fully included within the
0–50 m isobaths (Fig. 2). However, CK Gannets (n=6
groups, 18 birds) dispersed along north-eastern bearing-
angles of their colony (Chi square test, v
=13.71; df =2;
p\0.001), used deeper areas (K95, 35.8 %, and K50,
51.9 % overlapping the 50–100 m isobaths) and ranged
into areas of 1,000 m isobaths (K95, 23.3 % overlapping
100–1,000 m isobaths, Fig. 1). A permutation test revealed
that the average angle of bearing between Gannets
deployed on the same day was not significantly different
than random (n=10, p[0.05).
Fig. 1 Locations of the diving
activities by Australasian
Gannets (Morus serrator)
foraging from Farewell Spit,
New Zealand. Star the location
of the colony, dots the positions
of the dives and kernel polygons
the foraging home ranges.
Isobaths expressed in meters
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There were no significant differences between the sexes in
the foraging performance and neither in the areas in which
Gannets concentrated their foraging activity (Table 4).
Seabirds, including Gannets, spend most of their lives over
the open ocean foraging in diverse marine environments
(Lack 1968). These challenges are particularly pronounced
for breeding Gannets, which are face additional foraging
demands to feed their growing offspring and also increased
risk of injury associated with diving more fre-
quently (Machovsky-Capuska et al. 2011a). Spatial and
temporal fluctuations in prey concentrations create chal-
lenges for foraging (Weimerskirch 2007; Machovsky-
Capuska et al. 2011b). Here, for the first time, we report
foraging behaviour and home range in Australasian Gan-
nets from two different colonies in New Zealand using
bird-attached data loggers. As in previous studies on
Gannets with similar devices (Garthe et al. 2003,2007a,b;
´millet et al. 2004; Moseley et al. 2012), we did not find
any detectable effect of our work on the birds’ behaviour
on land.
Trip duration and time spent flying and resting were
similar between colonies and consistent with previous
findings for the same species by Bunce (2005), and for the
congenerics Cape Gannets Moseley et al. (2012) and
Northern Gannets (Garthe et al. 2007a). The similarities in
their performance suggest that these three geographically
different species balance their foraging behaviour in a
similar manner.
Australasian Gannets from CK covered foraging dis-
tances that were similar in range to those of their
Fig. 2 Locations of the diving
activities by Australasian
Gannets foraging from Cape
Kidnappers, New Zealand. Star
the location of the colony, dots
the positions of the dives and
kernel polygons the foraging
home ranges. Isobaths
expressed in meters (m)
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conspecifics from FS (Table 3). Wingham (1985) sug-
gested a larger foraging range (mean =268 km for CK)
than reported by us, although that study was based on a
mark–recapture method involving birds marked with paint
on their chests. In contrast, the results from our study are
similar to those distance ranges (mean =52.7 km) recor-
ded by Bunce (2005) using GPS loggers in Australia,
presumably reflecting similarities in the foraging behaviour
of these marine predators in different habitats off distant
coastal areas within their natural distribution.
Wild and laboratory foraging animals exploit their
environment in a way that reflects the distribution of food
sources (Gre
´millet et al. 2004). Individual kernel ranges
showed a high variability among Gannets in both colonies,
suggesting that individual experience and memory could
serve as an orientation factor for patch detection as it has
been proposed to be important to Atlantic Gannets (Hamer
et al. 2007; Pettex et al. 2010) and Cape Gannets (Gre
et al. 2004). Overall, foraging range sizes used by Aus-
tralasian Gannets were similar between colonies, and most
individuals tended to concentrate their at-sea activities
similarly in areas of *10 % of the maximum explored
range (Figs. 1,2). Bearing angles of departing birds
deployed on the same day in both colonies also corre-
sponded to foraging areas exploited by Australasian Gan-
nets. These concentrated areas around FS (Golden, Tasman
and Admiralty Bays) and CK (Hawke Bay) are well known
as high primary marine productive zones for their blooms
in nutrient-rich diatoms during the breeding season of
Gannets (Heath 1985; Paul et al. 2001).
Bathymetry has been suggested to be an important for-
aging parameter related to the habitat use of the prey
captured by Gannets (Hamer et al. 2000; Garthe et al.
2007a). Our results showed that Gannets from FS dived in
shallower waters (0–50 m) than birds from CK ([50 m).
This result is consistent with findings by Schuckard et al.
(2012), who showed that Australasian Gannets on FS feed
mainly on coastal species (pilchard and anchovy Engraulis
australis), and also by Robertson (1992), who showed that
Gannets at CK fed on species with a more oceanic distri-
bution (saury Scomberesox saurus, khawai Arripis trutta
and cubiceps Cubiceps caeruleus). The diving frequency
during trips documented in our study for both colonies (CK
4.2 and FS 4.8 dive h trip
) was higher than that previ-
ously reported for Australasian Gannets (2.6 dive h trip
Green et al. 2010), Cape Gannets (3.8 and 2.8 dive h trip
Moseley et al. 2012) and for Northern Gannets (1.35 dive h
; Lewis et al. 2004). It has been suggested that dive
frequency may be used as a good proxy for prey encounter
rate in this species (Lewis et al. 2004), especially given the
high success in prey capture (72 %; Machovsky-Capuska
et al. 2012). Diving frequencies were similar in both
Table 4 Foraging performance of male (M) and female (F) Australasian Gannets breeding at Farewell Spit (M=6 and F=5) and Cape
Kidnappers (M=9 and F=11), New Zealand
Parameter Colony Males Females tvalue p
Max. distance to colony (km) FS 36.4 ±36.6 44.8 ±16.2 -0.47 0.65
CK 62.7 ±27.9 49.7 ±18.1 -1.26 0.22
Foraging path length (km) FS 209.6 ±246.9 154.6 ±104.9 0.46 0.65
CK 281.2 ±119.6 257.0 ±126.0 -0.44 0.67
K95 (km
) FS 1,254.7 ±2,307.5 830.5 ±543.5 -0.40 0.70
CK 2,288.9 ±1,573.2 1,498.9 ±990.8 -1.37 0.19
K50 (km
) FS 103.6 ±211.3 113.3 ±186.9 0.08 0.94
CK 190.0 ±127.5 148.5 ±137.7 -0.69 0.50
Foraging trip duration (h) FS 16.4 ±12.5 12.7 ±6.7 0.32 0.75
CK 39.3 ±38.8 25.9 ±8.3 -1.11 0.28
Speed (km h
) FS 10.4 ±7.4 10.3 ±4.4 1.12 0.30
CK 9.4 ±3.8 10.1 ±4.5 0.38 0.70
Flying time (h) FS 5.1 ±5.5 3.2 ±2.4 0.75 0.47
CK 5.8 ±2.6 5.6 ±2.4 -0.25 0.80
Resting time (h) FS 11.3 ±8.7 9.5 ±7.1 -0.77 0.46
CK 33.4 ±39.3 20.3 ±7.7 -1.08 0.29
Dive duration (s) FS 4.2 ±1.5 3.9 ±1.8 0.63 0.54
CK 4.4 ±2.2 3.7 ±2.5 0.45 0.85
Dives per hour of trip FS 4.0 ±0.4 4.7 ±1.4 -0.97 0.37
CK 4.5 ±0.7 4.1 ±1.5 0.72 0.49
Values are given as mean ±standard deviation
J Ornithol (2014) 155:379–387 385
Author's personal copy
colonies, suggesting that Gannets were foraging in habitats
with similar levels of food availability.
Colony size has been suggested to increase intraspecific
competition and interference on food sources by diffusing
them farther away and subsequently augmenting the dis-
tance that Gannets need to travel for food (Lewis et al.
2001; Camphuysen 2011). Northern Gannets from larger
colonies travelled longer distances than conspecifics from
smaller colonies (Garthe et al. 2007a; Wakefield et al.
2013). However, it is not clear whether this pattern is
generally applicable to all three Gannet species, because
different studies on Cape Gannets showed opposite patterns
to one another (Pichegru et al. 2007; Moseley et al. 2012).
In our study, despite the fact that CK has almost twice the
number of breeding pairs than FS, and hence likely gen-
erates greater competition for food, no significant differ-
ences in foraging ranges were observed between these two
colonies. Similar findings were reported by Moseley et al.
(2012) from different sized colonies of the Cape Gannet.
However, interpretation of this result is subject to the
caveat that our sample sizes were small, and we were
unable to collect data from both colonies in the same
breeding season. On this basis, we suggest that the influ-
ence of intraspecific competition in foraging performance
requires further investigation for this species.
Sex is also known to influence Gannet foraging behav-
iour (Lewis et al. 2001; Ismar et al. 2010; Mullers and
Navarro 2010; Stauss et al. 2012). Again, subject to the
caveat of small sample sizes, we detected no statistical sex
differences in foraging trip parameters and in the use of
particular foraging areas. These results are consistent with
the findings of Bunce (2005) for Australasian Gannets and
Lewis et al. (2002) and Garthe et al. (2007a) for Northern
Gannets. In spite of the difficulties that monomorphic
species such as Gannets present (Nelson 1978), we still
need more data on fine-scale foraging behaviour and
predatory tactics to further compare patterns between the
sexes of the Australasian Gannet.
Our work has provided extensive quantitative details
towards gaining a better understanding of the relationship
between prey availability, oceanography and geographic
features, and variability in the foraging tactics of Austral-
asian Gannets across different spatial and temporal scales.
This work, and future extensions, will also be informative
regarding the assessment of the impact of commercial
fisheries in Gannet foraging areas around New Zealand. As
a next step, a wider range of Gannet colonies within New
Zealand and also between New Zealand and Australia
should be included in the comparison, as was done for
numerous colonies of the Northern Gannet by Lewis et al.
(2001) and Wakefield et al. (2013).
Acknowledgments We acknowledge T. Fettermann, S. Clements,
A. Boyer, L. Meynier, L. van Zonneveld, T. Greenawalt, E. Martı
K. and S. Machovsky, J. Melville and S. Ismar for assistance in the
field. We also thank the Napier Department of Conservation office for
the permission to use the ranger’s house during field work and the
Cape Kidnappers landowners and farm managers for access to their
property. The Department of Conservation, Golden Bay kindly
allowed use of their house at Farewell Spit and transport was provided
by Paddy Gillooly of Farewell Spit Ecotours. We thank E. Martı
S. Dwyer, R. Mullers, P. Battley, J. Waas, C. Moseley, L. Pichegru
and F. Bairlein for helpful comments on early versions of the man-
uscript. This research was funded by National Geographic Waitt
Grant, Massey University and Faculty of Veterinary Science Research
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... The characteristics of foraging trips have been extensively studied especially in the Northern gannet (e.g., Garthe et al. 1999Garthe et al. , 2011Garthe et al. , 2017Hamer et al. 2000), but only few studies have looked at differences between the sexes (Lewis et al. 2002;Stauss et al. 2012) or between different breeding stages or breeding seasons (Hamer et al. 2007;Pettex et al. 2012;Wakefield et al. 2015;Carter et al. 2016). Only a handful of studies are available on that topic for Australasian gannets breeding in Australia (Bunce 2001b;Angel et al. 2015b) or New Zealand (Machovsky-Capuska et al. 2014). In this study, therefore, we will test the following predictions regarding the foraging behaviour of size monomorphic Australasian gannets from a New Zealand colony: ...
... toolbox, as reported in the previous studies (Rodríguez et al. 2013;Machovsky-Capuska et al. 2014). Furthermore, the percent overlap of the foraging areas of the different sexes, breeding stages, and breeding seasons was calculated. ...
... Moreover, there was great interindividual variability in foraging behaviour, even within the same season, and sample sizes were possibly too small to detect statistically significant differences between the sexes. These results are in line with those from Machovsky-Capuska et al. (2014) from Cape Kidnappers and Farewell Spit, another New Zealand colony of the Australasian gannet. In other studies, on Northern gannets, researchers also did not detect significant differences between the sexes in foraging behaviour ), or mainly detected differences in the use of specific foraging areas (Lewis et al. 2002;Stauss et al. 2012), with females foraging more in offshore waters, while males foraged closer to the colony (Stauss et al. 2012). ...
Full-text available
Marine top predators forage in environments that show potentially extreme temporal and spatial variation in prey availability, with reproductive success being crucially linked to food supply. Multiple factors of interannual and sexual variation, as well as variation across breeding stages, can shape patterns of spatial use in foraging seabirds, yet studies that address all of these variables simultaneously are rare. We present spatial assessment of foraging patterns by µGPS tracking of a sexually size monomorphic, long-lived species, the Australasian gannet (Morus serrator). The study spanned the incubation and chick-rearing stages in three consecutive breeding seasons. Our findings revealed high interannual variability in foraging distances and trip durations, but no consistent differences between birds across different breeding stages or the sexes. The exception was that core foraging areas were different for female and male Australasian gannets, although trip durations or distances were similar for both sexes. Our results also indicate bimodality in foraging distance and trip duration in this species, while highlighting interannual variability in the extent of bimodality. These findings contribute to a scarcely documented type of foraging behaviour in the seabird family of the Sulidae. Overall, these spatial use patterns provide a baseline for understanding the evolution of sex-specific foraging differences in biparental seabirds, and the extent to which these differences might help in securing breeding success across years of variable food availability.
... It displays reverse sexual dimorphism (females larger than males) and recent studies suggest individuals exhibit sex-related differences in habitat use [48][49][50] and prey selection [47]. Furthermore, inter-colony differences in diving behaviour and habitat selection have been documented [51,52]. However, little is known of the factors influencing foraging behaviour and behavioural consistency in this species. ...
... Open Sci. 7: 181423 Geographical variation has previously been reported in the foraging behaviour of gannets [51,52,83,84] and other marine predators, reflecting spatial differences in resource availability or habitat accessibility [32,85,86]. The results of the present study are consistent with these findings and reveal the substantial differences in oceanographic regimes and habitats available to individuals from the PD and PE gannet colonies [49,52]. ...
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Foraging is a behaviour that can be influenced by multiple factors and is highly plastic. Recent studies have shown consistency in individual foraging behaviour has serious ecological and evolutionary implications within species and populations. Such information is crucial to understand how species select habitats, and how such selection might allow them to adapt to the environmental changes they face. Five foraging metrics (maximum distance from the colony, bearing from the colony to the most distal point, tortuosity index, total number of dives and mean vectorial dynamic body acceleration were obtained using GPS tracking and accelerometry data in adult Australasian gannets (Morus serrator) from two colonies in southeastern Australia. Individuals were instrumented over two breeding seasons to obtain data to assess factors influencing foraging behaviour and behavioural consistency over multiple timescales (consecutive trips, breeding stages and years) and habitats (pelagic, mixed pelagic and inshore, and inshore). Colony, breeding stage and year were the factors which had the greatest influence on foraging behaviour, followed by sex. Behavioural consistency, measured as the contribution of the individual to the observed variance, was low to moderate for all foraging metrics (0.0-27.05%), with the higher values occurring over shorter timescales. In addition, behavioural consistency was driven by spatio-temporal factors rather than intrinsic characteristics. Behavioural consistency was higher in individuals foraging in inshore than pelagic habitats or mixed pelagic/inshore strategy, supporting suggestions that consistency is favoured in stable environments.
... Gannets Morus spp., in particular, have been extensively studied with respect to both their foraging behaviour and food preferences. Based on the diversity of prey they consume, gannet dietary patterns have often been described as generalist, opportunistic or flexible feeders (Bunce & Norman, 2000;Lewis, Sherratt, Hamer, Harris, & Wanless, 2003;Machovsky-Capuska et al., 2014;Montevecchi, 2007;Ropert-Coudert et al., 2009;Wanless, Harris, Lewis, Frederiksen, & Murray, 2008). Although a few studies have highlighted the importance of prey quality in different gannet populations (Bunce, 2001;Grémillet et al., 2008;Tait et al., 2014), the extent to which gannets are dietary generalists or specialists in terms of the foods that they exploit and diets that they compose from those foods remains to be established. ...
... Individual foraging trips were extracted using BirdLife International's "marine IBA" R package (Lascelles et al., 2016). Previous studies have shown that gannet dive durations are primarily between 3 and 8 s (Machovsky-Capuska, Vaughn, Würsig, Katzir, & Raubenheimer, 2011;Machovsky-Capuska et al., 2012); in this study, dive locations were inferred from interruptions of between 3 and 8 s in GPS signals from our high-resolution loggers since interruptions exceeding >10 s are likely to be related to loss of satellite signal reception (Machovsky-Capuska, Vaughn et al., 2011;Machovsky-Capuska et al., 2014;Moseley et al., 2012;Pichegru et al., 2007). ...
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1.Our understanding of the niche concept will remain limited while the quantity and range of different food types eaten remains a dominant proxy for niche breadth, as this does not account for the broad ecological context that governs diet. Linking nutrition, physiology and behaviour are critical to predict the extent to which a species adjusts its nutritional niche breadth at the levels of prey (“prey composition niche”, defined as the range of prey compositions eaten), and diet (“realized nutritional niche” is the range of diets composed through feeding on the prey). 2.Here we studied adult‐chick rearing Australasian gannets (Morus serrator) to propose an integrative approach using sea surface temperature anomalies (SSTa), geographic location and bathymetry over different years, to explore their relationship with the nutritional composition of prey and diets (i.e., prey composition and nutritional niche breadth), habitat use and foraging behavior. 3.We found that gannets feed on prey that varied widely in their nutritional composition (have a broad prey composition niche), and composed diets from these prey that likewise varied in composition (have a broad realized nutritional niche), suggesting generalism at two levels of macronutrient selection. 4.Across seasons, we established “nutritional landscapes” (hereafter nutriscapes), linking the nutritional content of prey (wet mass protein to‐lipid ratio ‐P:L‐) to the most likely geographic area of capture and bathymetry. Nutriscapes varied in their P:L from 6.06 to 15.28, over time, space and bathymetry (0 to 150 m). 5.During warm water events (strong positive SSTa), gannets expanded their foraging habitat, increased their foraging trip duration and consumed prey and diets with low macronutrient content (wet mass proportions of P and L). They were also constrained to the smallest prey composition and realized nutritional niche breadths. 6.Our findings are consistent with previous suggestions that dietary generalism evolves in heterogeneous environments, and provide a framework for understanding the nutritional goals in wild marine predators and how these goals drive ecological interactions and are, in turn, ultimately shaped by environmental fluctuations.
... During the presence of larger groups of foraging dolphins (>100 individuals), workups further intensify gannet activity, as characterized by high-altitude U-shaped dives into dense schools, enabling high (95%) prey capture success rates (Machovsky-Capuska et al., 2011a, 2013a. Animal-borne GPS loggers showed that during single foraging trips from their colonies, gannets have a high diving frequency of 4.8 dives h -1 that are confined within small spatiotemporal areas (Machovsky-Capuska et al., 2013b). It is therefore likely, that these high-performance diving events could be associated with larger dolphin groups that enable gannets to undertake successive multiple dives in a workup while fish schools remain concentrated (Thiebault et al., 2016). ...
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Prey detection and subsequent capture is considered a major hypothesis to explain feeding associations between common dolphins and Australasian gannets. However, a current lack of insight on nutritional strategies with respect to foraging behaviours of both species has until now, prevented any detailed understanding of this conspecific relationship. Here we combine stomach content analysis (SCA), nutritional composition of prey, a multidimensional nutritional niche framework (MNNF) and videography to provide a holistic dietary, nutritional, and behavioural assessment of the feeding association between dolphins and gannets in the Hauraki Gulf, New Zealand. Dolphins consumed ten prey species, including grey mullet (Mugil cephalus) as the most representative by wet mass (33.4%). Gannets preyed upon six species, with pilchards (Sardinops pilchardus) contributing most of the diet by wet mass (32.4%) to their diet. Both predators jointly preyed upon pilchard, jack mackerel (Trachurus spp.), arrow squid (genus Nototodarus), and anchovy (Engraulis australis). Accordingly, the MNNF revealed a moderate overlap in the prey composition niche (0.42) and realized nutritional niche (0.52) between dolphins and gannets. This suggests that both predators coexist in a similar nutritional space, while simultaneously reducing interspecific competition and maximizing the success of both encountering and exploiting patchily distributed prey. Behavioural analysis further indicated that dolphin and gannets feeding associations are likely to be mutually beneficial, with a carouselling foraging strategy and larger pod sizes of dolphins, influencing the diving altitude of gannets. Our approach provides a new, more holistic understanding of this iconic foraging relationship, which until now has been poorly understood.
... When possible, prey type (zooplankton or fish) was identified by direct observation or inferred from the presence of other predators in the same area as the whales and known to have a specific prey preference. For example, Australasian gannets (Morus serrator) dive to feed on schooling fish (Machovsky-Capuska et al., 2014), whereas New Zealand storm petrels (Fregetta maoriana) and fluttering shearwaters (Puffinus gavia) feed on surface zooplankton (Gaskin & Rayner, 2013). When possible, video footage of the feeding behavior of the whales was also recorded. ...
Large predators typically feed on proportionally sized prey but the world's largest animals, baleen whales, bulk feed on plankton and small fishes. While most baleen whales migrate to feed on polar aggregations of nutritious zooplankton prey, Bryde's whales (Balaenoptera edeni brydei and B. e. edeni) inhabit less productive warm‐temperate waters with variable prey abundance and quality. Off New Zealand, Bryde's whales target both fish and zooplankton, some with lower calorific value. We use multisensor tags (n = 4) and visual observations from drones and boats (n = 52) to reveal that Bryde's whales employ specialized feeding tactics matched to prey type. Zooplankton‐feeding at the surface involved multiple head‐slaps that presumably aggregate zooplankton followed by a side‐lunge. Whales exploiting plankton patches swam in tight circles, performing up to 33 lunges (M = 5.5 ± 6.1) per feeding bout. In contrast, whales targeting fish performed faster vertical lunges. With both prey types, whales concluded lunges with a ~90° roll probably to minimize prey escape at the surface. The diet plasticity and dynamic behaviors of Bryde's whales are key to increasing their foraging efficiency. This may be essential for the whales to meet energetic demands year‐round with a variety of prey in New Zealand waters.
... Seed seawater exposure would also depend on the part of the seabird to which it attached. Australasian gannet (Morus serrator) foraging trips involve bouts of short dives interspersed with periods of resting on the surface of the water which represent 70-80% of the foraging trip time (Machovsky-Capuska et al. 2014). Therefore, seeds attached to areas immersed during resting, such as the legs and abdomen, would experience seawater exposure for up to 30 hours in seawater, whereas the upper parts of the body would have shorter seawater immersion. ...
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Lepidium oleraceum (Brassicaceae) is a threatened New Zealand plant closely associated with seabird colony environments. We hypothesise that this spatial coincidence arises from seabird dispersal of L. oleraceum seed and/or guano stimulating germination. To test the possibility of seabird epizoochory, we used laboratory studies to examine L. oleraceum seed adhesion and tolerance to seawater immersion. To test for stimulation of germination by guano we measured seed germination in increasing concentrations of seabird guano or inorganic fertiliser (to separate a nutrient effect from a possible effect from some other unknown constituent). Seeds developed a sticky mucilage layer after 50 seconds in freshwater and 50% were able to adhere to dry paper for over 24 months. A higher proportion of seeds germinated after soaking in freshwater (91% ± 3.1%) than seawater (44% ± 4.9%), and seawater germination was comparable to unsoaked controls (46% ± 3.1%). A comparison of germination rates under different concentrations of guano and fertiliser showed no significant differences associated with treatments except germination was inhibited above 1 gNL-1. We conclude that seabirds are probable dispersers of L. oleraceum seed through external attachment, but germination is not promoted by guano presence.
... This summary is provided as an illustration and is not exhaustive. central-place forager has the complex challenge of balancing its own nutritional needs with the needs of their offspring by provisioning them with food obtained while foraging often through regurgitations (Machovsky-Capuska et al. 2014 ). These predators therefore provide the rare opportunity to estimate the amounts and proportional nutritional composition of consumed prey and overall diets by collecting undigested regurgitations that can then be analysed for chemical composition (Tait et al. 2014; Machovsky-Capuska et al. 2016a, b). ...
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Carnivorous animals are assumed to consume prey to optimise energy intake. Recently, however, studies using Nutritional Geometry (NG) have demonstrated that specific blends of macronutrients (e.g. protein, fat and in some cases carbohydrates), rather than energy per se, drive the food selection and intake of some vertebrate and invertebrate predators in the laboratory. A vital next step is to examine the role of nutrients in the foraging decisions of predators in the wild, but extending NG studies of carnivores from the laboratory to the field presents several challenges. Biologging technology offers a solution for collecting relevant data which when combined with NG will yield new insights into wild predator nutritional ecology.
Territorial protandry, or early season male-biased settlement at breeding sites, is a widespread phenomenon in a range of animal breeding systems. While protandry is common across several avian lineages and has been linked with increased reproductive success of earlier breeding males in terrestrial species, the selective advantage of breeding protandry has only rarely been studied in seabirds. We assessed the seasonal changes in the sex ratio at the breeding site and sexspecific correlates of arrival date with reproductive success during 2 breeding seasons of a colonial seabird, the Australasian Gannet (Morus serrator), at Cape Kidnappers, New Zealand. We found no biases in overall sex ratios of adults and fledglings but detected a male sex bias during nest site establishment, and a significantly higher probability of reproductive success for earlier-settling males. In contrast, the reproductive success of females did not correlate with the timing of arrival. Our findings provide an assessment of the sex differences in reproductive correlates of the timing of breeding settlement in gannets and are consistent with selective advantages as suggested by indirect selection hypotheses. This study contributes to our understanding of the fitness benefits of protandry, and its linkages with sex differences in breeding philopatry and mate fidelity, in a long-lived seabird species with obligate and extended biparental care.
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Technological advances in recent years have seen an explosion of tracking and stable isotope studies of seabirds, often involving repeated measures from the same individuals. This wealth of new information has enabled the extensive variation among and within individuals in foraging and migration strategies (movements, habitat use, feeding behaviour, trophic status, etc.) to be examined in unprecedented detail. Variation is underpinned by key life-history or state variables such as sex, age and breeding stage, and residual differences among individuals (termed ‘individual specialization’). This variation has major implications for our understanding of seabird ecology because it affects the use of resources, level of intra-specific competition, and niche partitioning. In addition, it determines the responses of individuals and populations to the environment, and the susceptibility to major anthropogenic threats. Here we review the effects of season (breeding vs nonbreeding periods), breeding stage, breeding status, age, sex, and individual specialization on variation in foraging and migration strategies, and the consequences for population dynamics and conservation.
Sex differences in foraging behavior are typically studied in size-dimorphic taxa. Data on sex-specific behavior in monomorphic taxa are needed to test theories of reproductive investment. It has been suggested that in seabirds foraging niche separation may be related to decreased intersexual competition for food between cooperating pair-bonded individuals. Alternatively, sex differences in foraging niches may be driven by different nutritional requirements of females associated with the reproductive costs of egg production and oviposition. To assess these possibilities, we studied a size-monomorphic colonial seabird, the Australasian Gannet (Morus serrator) at the Cape Kidnappers gannetry, New Zealand. We recorded maximum dive depths, and distinct diet composition of incubating females as indicated by stable isotopic signatures. Results suggested greater female foraging effort during early times of incubation, indicated by significantly deeper maximum dives. Sex-specific foraging patterns across other breeding stages were more variable. Nitrogen stable isotopic values showed that incubating females occupied a different trophic position compared to males at the same breeding stage, and also from those of gannets of both sexes at later stages of parental care. Overall, the data are consistent with cost-of-oviposition compensation in females necessitating male-bias in parental care in biparental breeders. Further research is needed to unravel the implications of nutritional needs for the evolution of sex differences in behavior in this and other monomorphic taxa.
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The diet of the Australasian gannet (Morus serrator) at Farewell Spit, New Zealand, was studied by the analysis of 70 regurgitations collected from the 1995 to 2001 breeding seasons. Surface schooling pilchard (Sardinops neopilchardus) was the main prey, followed by anchovy (Engraulis australis). The composition of the diet was similar in most seasons examined except in 1996 in which anchovy was the main prey item. Such a change in diet could be linked with a pilchard mass mortality in New Zealand in August 1995. The estimated annual prey consumption by birds at the Farewell Spit gannetry was 852 tonnes. Although annual catches of pilchard and anchovy by commercial fsheries in the area are still relatively small, an increase may interfere with prey availability, and in turn, increase competition between marine predators and infuence the breeding success. Our analyses of diet are consistent with previous studies showing that Australasian gannets as fexible foragers and they highlight their importance as bioindicators of fsh stocks in New Zealand.
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Field observations around the largest Northern Gannet Moms bassanus colony in the North Sea, the Bass Rock, showed that 66% of all Gannets foraged in areas with very low densities of conspecifics, more than 100 km from the colony. When one forager found prey, even distant Gannets responded by joining the finder to obtain a share of the bounty but, because of the low densities of Gannets far from the colony, feeding opportunities were typically exploited by small flocks, with relatively few competing birds. Intraspecific competition was thus less intense than it would have been nearer the colony. Searching and feeding tactics of Gannets, as well as foraging associations with other top predators, were different between sea areas. Low numbers of Gannets per flock occurred within inshore multi-species feeding associations, where Gannets hampered feeding opportunities for other seabirds (and themselves) by plunge-diving into compact schools of small prey fish. Larger flocks of competing Gannets formed in situations where an escape response in prey fish was absent (discards behind commercial trawlers) or weakened (fish schools herded by marine mammals). The association of Gannets with marine mammals was typically an offshore phenomenon, despite the abundance of cetaceans in inshore waters. Behind trawlers, Gannets focused mostly on roundfish, between 22 and 30 cm in length. Discards were, however, a fairly unimportant source of food during the breeding season and natural feeding opportunities were widespread.
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Some predators face the problem of locating and capturing foods while at the same time avoiding a number of environmental hazards and even predation on themselves. These challenges can be more extreme for some species than for others (Raubenheimer 2010). For example, a number of marine predators forage specifically within the air-water interface (Thewissen & Nummela 2008). The need to function in both media imposes major constraints, evolutionary pressures and physiological trade-offs on the individual's morphology, physiology and sensory systems (Kröger & Katzir 2008). It has been suggested that air-breathing marine animals physiologically prepare for dives of a specific depth by loading oxygen prior to submergence (Thompson & Fedak 2001). These animals include penguins, which also apparently prepare their dives before entering the water with the aim of increasing prey capture success (Wilson 2003). Australasian gannets (Morus serrator; hereafter gannets) are highly specialised marine predators that feed mainly on pelagic fish and squid at the air-water interface (Robertson 1992; Machovsky-Capuska et al. 2011a; Schuckard et al. 2012). Diving often occurs in multi-species-feeding associations (MSFA) that involve high densities of marine predators increasing competition for prey capture (Machovsky-Capuska et al. 2011 a, b). Gannets detect prey from the air and perform rapid plunge-dives to capture prey underwater using either U-or V-shaped dive profiles that have a significantly different level of prey capture success (95% vs. 43%, respectively, Machovsky-Capuska et al. 2011b; Machovsky-Capuska et al. 2012). We were therefore Notornis, 2013, Vol. 60: 255-257 0029-4470 © The Ornithological Society of New Zealand, Inc.
Regurgitations analysed over three seasons (1978-80) indicated that Pilchards Sardinops neopilchardus, both numerically and by weight, were the most common prey species of the Australasian Gannet at Motukaramarama. The next most common species were: numerically, Anchovies Engraulis australis and by weight, Jack Mackerel Trachurus novaeselandiae. It is thought that relative abundance of prey analysed reflects their abundance. Prey between 11-20 cm comprised 77% of the Gannet's diet. A further two prey species: Sprat Sprattus antipodum and Kahawai Arripis trutta were recorded, bringing the known food of the Gannet to 15 species. The average adult Gannet regurgitated food equal to 11% of its body weight (or 259 g and 2,000 Kj), whereas the average daily energy requirements were estimated at 2,844 Kj/day, or 353 g of food. An average feeding range of 268 km (range = 86-450 km) was estimated from the time adults spent away from the colony, resighting of marked birds and from recoveries of banded, breeding adults.
Predators utilize a variety of behavioral techniques to capture elusive prey. Behavioral flexibility is essential among generalist predators that pursue a diversity of prey types, and capture efficiency is expected to be intense during the breeding season for parents that engage in self- and offspring-provisioning. We studied the foraging behavior of parental northern gannets in the northwestern Atlantic (Gulf of St. Lawrence) when they were feeding on Atlantic mackerel almost exclusively. Data-loggers recorded short (mean duration: 6.3 s), high speed (inferred vertical speeds of up to 54.0 m*s- 1, equivalent to 194 km*h- 1), and shallow dives (mean depth: 4.2 m; maximum: 9.2 m). Dives tended to occur in bouts, varying between 0.3 and 4.6 per hour (mean = 1.6). During foraging, overall flight heights ranged from 0 to 70 m, with no clear preferences for height. Most plunge-dives were initiated at flight altitudes of 11-60 m (mean ± SE = 37.1 ± 2.8 m; range 3-105 m except for 1 of 162 dives that was initiated at the sea surface). Dive depth and flight altitude at plunge-dive initiation were positively and significantly correlated, though it appears that low flight altitudes were sufficient to reach dive depths at which mackerel were present. Almost all dives were V-shaped indicating that a high acceleration attack is the most effective strategy for gannets feeding on large rapid-swimming prey such as mackerel that owing to thermal preferences does not occur below the thermocline and are thus well available and essentially trapped in the water depths exploited by northern gannets.
The paper reviews the physical oceanography of the seas around New Zealand as known up to 1982 and includes: deep‐ocean water characteristics and mean flow; fronts, tides, and coastal and continental shelf oceanography; and waves and tsunamis.