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

Abstract

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.

Full text

1 23
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
Zealand
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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ORIGINAL ARTICLE
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 
Seabirds
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
e-mail: g.machovsky@sydney.edu.au
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,
Germany
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
123
J Ornithol (2014) 155:379–387
DOI 10.1007/s10336-013-1018-4
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Zusammenfassung
Nahrungssuchverhalten und Habitatnutzung
Australischer To
¨lpel wa
¨hrend der Jungenaufzucht in
Neuseeland
Lu
¨ckenhaft verbreitete pelagische Beute stellt eine betra
¨-
chtliche Herausforderung fu
¨r nahrungssuchende Seevo
¨-
gel dar. Das gilt auch fu
¨rTo
¨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
wa
¨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
To
¨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
¨ssern
(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
¨sten-
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
¨hnli-
chen Beuteverfu
¨gbarkeiten auf Nahrungssuche gehen.
Weiterfu
¨hrende Untersuchungen zur Beziehung zwischen
Beuteverfu
¨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
¨nnen.
Introduction
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
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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.
Methods
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
Ò
(Gre
´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 (http://www.e-obs.de) 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
TM
), 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.
2011b).
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
MF
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
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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
2
and
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
10
transformed and then compared using
ttests. We report data as mean ±standard deviation.
Results
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
colonies.
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
2
=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
2
, 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
(n=9)
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
-1
) 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
trip
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
Geographic
location
East coast
(North Island)
West coast
(South Island)
Population size 7,300 3,900
Sample size (n)21 11
Max. distance
to colony
(km)
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
K95
individuals
(km
2
)
1,854.4 ±1,312.0 1,061.9 ±1,681.9 -1.45 0.16
K50
individuals
(km
2
)
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
Speed
(km h
-1
)
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
(h)
26.2 ±26.9 10.3 ±7.7 -1.88 0.07
Dive duration
(s)
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
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whereas the marine areas used by FS Gannets were 3,786.2
and 689.1 km
2
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
2
(CV =70.75 %) and
K50 from 27.16 to 485.30 km
2
(CV =78.63 %) in CK
colony, and K95 from 0.74 to 5,922.69 km
2
(CV =158.38 %) and K50 from 0.04 to 533.61 km
2
(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
2
=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
2
=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
(m)
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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).
Discussion
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;
Gre
´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
´millet
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
-1
) was higher than that previ-
ously reported for Australasian Gannets (2.6 dive h trip
-1
;
Green et al. 2010), Cape Gannets (3.8 and 2.8 dive h trip
-1
;
Moseley et al. 2012) and for Northern Gannets (1.35 dive h
trip
-1
; 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
2
) 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
2
) 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
-1
) 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
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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
´nez,
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
´nez,
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
Funds.
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