AfncaD JautD.ll of Marine Sctence 2005. 2/(1) 239 218
Printe(J n Safth Afnca
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AFRICAN JOURNAL OF
MARINE SCIENCE
ISSN 1811 232X
Foraging of a coastal seabird: flight patterns and movements of
breeding Cape gannets Morus capensis
NJ Adamsl* and RA Navarro2
l Formerly Percy FitzPatrick lnstitute of African Ornithology, University of Cape Town, Private Bag,
Rondebosch 7701,
South Africa: now School of Natural Sciences. Unitec, New Zealand, Private Bag 92025. Aucktand, New Zealand
2Avian Demography Unit, Department of Statlstica/Sciences, University of Cape Town, Private Bag
Rondebosch 7701
South Africa
. Corresponding
author, e-mail: [email protected]
Cape gannets Morus capensis are predatory seabirds in
the coastal waters of southern Africa where they feed
on commercially important fish species. Using a
combination of intensive monitoring at nest sites,
tracking of radio-tagged birds and diet sampling, we
determined the foraging ranges and foraging areas used
by breeding gannets, and whether links existed between
the broad-scalespatial distribution of foraging birds and
the distribution of prey or the predominant wind regime.
A totaf of 270 Cape gannets dispersing to forage from
Malgas lsland, South Africa, were tagged over three
consecutive breeding seasons. Modal durations of
foraging flights were six or 24 hours, depending on
whether birds returned on the same day they left or
remained at sea overnight. Few birds remained continuously at sea over two consecutive nights. Non-radiotagged birds more frequently undertook shorter foraging
trips than radio-tagged birds, indicating a behavioural
response to handling. Some 23% of radio-taggedgannets
triangulated throughout a complete foraging trip foraged
within a maximum of about 60km of Malgas lsland, 44%
foraged between about 60km and 120km of the island
and the remaining 33% flew beyond 120km, travelling a
total of at least 240km. Flight directions of gannets
departing from the island were non-random in two of the
three seasons. Return flight directions were non-random
in all three seasons. Most foraging flights were to the
south-west of the island, birds generally returning with
the prevailing wind and from the same general direction
in which they departed. Birds returning with saury
Scomberesox saurus did so significantly more frequently
from a west-south-west direction. Birds feeding on the
other two prey species were equally likely to return from
any direction. Under average conditions, the energy
benefit associated with returning under load with a
tailwind as opposed to a headwind was equivalent to
12o/.of the average stomach sample mass. The observed
distribution of flights probably reflects the large area of
suitable foraging habitat to the south and south-west of
the island and the energetic advantagesof returning with
the prevailing wind.
Keywords. Cape gannet, feeding, Morus capensls,radiotelemetry,
seabird,South Africa
Introduction
The foraging patternsof predatoryseabirdsshould reflect,
largely,the distribution,predictabilityand mobility of their
prey (Wanlesset a/. 1998).The study of the natureof these
predator-preyinteractionsin offshore and pelagic marine
environmentsis challenginggiven the difficultyof sampling
and making observationsat the appropriatespatial and
temporal scales. This problem is acute in volant seabirds
that nest on land, but have the ability to fly rapidly,and
feed on patchily distributed prey located at substantial
distancesfrom the breeding site.
Gannets (genus Morus) are one such genus of coastal
seabirds, typical of temperate and cool{emperate waters
of the northernAtlantic,southernAfrica and Australiaand
New Zealand that feed on surface shoalingfish and squid
(Nelson 1978). Substantialparts of their range overlapwith
waters exploited intensivelyby coastal-basedcommercial
p e l a g i cf i s h e r i e s( B e r r u t a
i n d C o l c l o u g h1 9 8 7 ,M o n t e v e c c h i
a n d M e y e r s1 9 9 5 ,H a m e re t a l . 2 0 0 0 ,B u n c e2 0 0 1 ) .S t u d i e s
of diets and prey harvests of northern gannets Morus
bassanusand Cape gannets M. capensisat breedingsites
in the North-West and South-East Atlantic Ocean have
demonstratedcorrelationsbetween relattve orooortionsof
specific prey species in their diet and interannual
fluctuationsrn the abundance of this prey determined by
fishery surveys and catches (Berrutiand Colclough 1987,
Montevecchiand Berruti 1991, Crawford and Dyer 1995,
M o n t e v e c c h ia n d M e y e r s 1 9 9 5 , C r a w f o r d 19 9 9 ) . T h e
strengthsof such correlationswere sensitiveto the spatial
scale over which fishing and survey data are collated.The
selection of the appropriatescales for analysis and their
correct interpretationrequires detailed knowledge of the
foragingpatternsand behaviourof seabirds.Such data are
i m p o r t a n t f o r a s s e s s i n g t h e i r t r o p h i c i n t e r a c t i o n sw i t h
commercialfisheries(Hameret al.2000) and understanding
Adams and Navarro
the causal Iinks between prey abundance and seabird
biologyas observedat the nest.The demonstrationof such
direct links would enhancetheir use as indicatorsof change
i n p o p u l a t i o n so f t h e i r m a r i n e p r e y t h a t m a y p r o v i d e
supplementaryinput in fishery managementmodels.
Cape gannetsare colonialseabirdsrestrictedto breeding
on small offshoreislandsoff the south-westernand southern
coasts of Africa (Nelson 1978, Crawford et al. 1983). The
presentstudy was conductedduring the australsummer on
gannets breedingat Malgas lsland (33'02'S, 17"56'E)
Saldanha Bay, South Africa (Figure 1). The island provides
nesting sites for tens of thousands of breeding pairs of
gannets (Crawford et al. 1983, Crawford 1999) and lies
within the Benguelaupwellingsystem. In common with the
world's other four eastern boundary current systems, the
Benguela system is characterisedby equatorwardsurface
which
flow,coastalupwelling,and high biologicalproductivity,
supports a large biomass of a number of pelagic shoaling
fish species (Crawford et al. 1987). Anchovy Engraulis
engrasicolus,sardine Sardlnops sagax, saury Scomberesox
saurus and horse mackerel Trachurus trachurus capensrs
are important constituentsof the diet of Cape gannets
(Berrutief a/. 1989).The proportionsof these prey species
in the diet of gannets have varied with environmentalor
fishinginducedchangesin their availability(Crawford1999).
Develooments in electronics and assoctated miniaturisation have allowed the developmentof transmittersand
other remote-sensingdevices small enough to be carried
by free-flyingindividualseabirds (Wilson ef a/. 2002). This
has made possiblethe study of the foraging behaviourof
individualseabirds.In the presentstudy,utiltsingfavourable
coastaltopography,we used conventionalVHF telemetryto
examinesome foragingvariablesof breedingCape gannets.
Radio telemetryhas been used as a tool to elucidatethe
feeding behaviourof inshore feeding seabirds (Trivelpiece
e t a / . 1 9 8 6 . H e a t h a n d R a n d a l l1 9 8 9 ,W a n l e s se t a / 1 9 9 0 '
'1
1 9 9 1 , 1 9 9 8 , l r o n s 9 9 8 , L i t z o wa n d P i a t t2 0 0 3 ) a n d s o m e
offshore seabirds (Anderson and Ricklefs 1987, Hanison
The relativelysmall
and Stoneburner198'1, Lewisel al.2OO2).
unit cost of VHF transmittersallowed deployment of transmitterson a large number of birds.
The aims in this study were: (1) to definethe spatialscale
over which breeding Cape gannets, in particular,travel to
feed, (2) to establishwhether birds utilisespecificforaging
areas withintheir potentialforagingarea, and (3) if so, to test
whether such patternscould be accountedfor by distribution
of preferred prey species or the prevailingwind regime.
diameter)with a flexible antenna extended approximately
25cm long beyond the casing.The total packageweighed
a m a x i m u mo f 2 0 9 , a p p r o x i m a t e l1y% o f a d u l tg a n n e tb o d y
mass. Transmitterhousingswere tied ventrallyto the base
of the shafts of two tail feathers using cotton carpet thread
Cyanoacrylateester glue was applied to the knot to bond
the thread to the feather shaft and to preventslipping.The
proximalsection of the antenna was tied and glued to the
feather shaft while the distal end was allowed to project
beyondthe tail. In thrs position,the transmitterhousingwas
completelycovered by the under-tailcoverts.
Receivingequipment consisted of three S-elementand
one 11-elementverticallypolarised paired Yagi antennae
(null-peaksystem)mountedon a rotatingmast and coupled
to radiotransceivers(Yaesu50W 2m FM mobile).A compass
rose. with zero set to true north, allowed direct reading of
the bearings.The null-peaksystem has the abilityto localise
transmitterst2' (Kenward1987).At longer range, precrsron
was reducedto t5'. During1987/1988,radio{aggedgannets
were tracked from one receiving station located on
Malgaskopa
, s m a l l h i l l ( 1 1 3 m ) l o c a t e do n t h e m a i n l a n d
directly behind the island. Flight directionswere obtained
t o a n d f r o m M a l g a s l s l a n d o n l y . D u r i n g 1 9 B B i 91 8 9 a n d
Material and Methods
Telemetry
Transmitterswere attached (see below) for a period of one
to three days to a total of 270 birds attendingchicks,with
individualbirds each being trackedfor a completeforaging
'1988/1989,radio{rackingsessions
trip. ln 1987/'1988and
were conductedfor a total of three to five days at monthly
intervalsfrom Octoberto March.During1989/1990'sesstons
were restrictedto November and February,but lasted six
and seven days respectively.Transmitters(150-l 52MHz)
were housed in waterproofcasings (50mm long x 15mm
18"
F i g u r e 1 : M a p o f t h e s o u t h - w e s t e r nC a p e c o a s t o f S o u t h A f r i c a
s h o w i n gl o c a t i o no f M a l g a s l s l a n da n d t r a c k i n gs t a t i o n s C o n c e n t r i c
circles represent the maximum potential reception range of
t r a n s m i t t e r sf r o m t r a c k i n g s t a t i o n s ,i n d i c a t e db y f i l l e d t r i a n g l e s
A f r i c a n J o u r n a l o f l \ , 4 a r i nS
e c i e n c e 2 0 0 5 . 2 7 ( 1 J : 2 3 92 1 8
1989/1990,birds were tracked from a combinationof four
of a possible five receivingstations, includingMalgaskop
(Figure1). These stationswere locatedon high pointsalong
the South-WesternCape coast (Figure '1) and allowed
triangulationof bird positionsat sea.
Handling procedure
A t l e a s t o n e a d u l t g a n n e t i s n o r m a l l yi n a t t e n d a n c ea t a
nest site throughoutthe chick-rearingperiod,with partners
a l t e r n a t i n gf o r a g i n ga n d a t t e n d a n c ed u t i e s ( N e l s o n1 9 7 8 ) .
Adult gannetswere caught between09:00 and '16:00,after
having been recentlyrelievedfrom attendanceduties.After
weighing, the transmitterwas attached and the bird then
released in vicinityof the nest site. Gannets generallyleft
for sea within 3O-minutes.lmmediatelyfollowingreleaseof
the adult, the chick was weighed. The marked nest was
checked subsequentlyat 30-minuteintervalsfrom dawn to
dusk until the radio-taggedadult returned. Chicks >5009
w e r e w e i g h e da t 3 - h i n t e r v a l s( 0 9 : 0 0 ,1 2 : 0 0a n d 1 5 : 0 0 ) .O n
their returnto the nest site, adult gannetswere caught and
induced to regurgitatetheir stomach contents, collected
d u r i n g f o r a g i n g a t s e a . R e g u r g i t a t i o nw a s i n d u c e d b y
invertingthe bird over a bucketand applyinggentlepressure
to the stomach. Once stomach contents had been recovered, radio transmitterswere removed.A final weighingof
t h e a d u l t a n d c h i c k a l l o w e du s t o e s t a b l i s ha n y a d d i t i o n a l
mass gain.Adultscan feed largechickssubstantialamounts
of food within a few minutes of arriving back at the nest.
Therefore,this procedurealloweddeterminationof foragingtrip duration and chick meal-size.We recognise that the
total amount of food collectedon a foragingtrip was underestimatedon account of digestionand assimilationby the
adult. However,the results are consideredto be the best
estimate of the foraging performanceof gannets, against
which tagged and control birds may be compared.
Flight directions and locations
Radio-taggedgannetswere scannedfrom receivingstations
at 15-minuteintervalsfrom dawn to dusk (05:00-20:00 in
mid-summer).Gannetsare visual predatorsand do not feed
at night (NJA, unpublisheddata, see also Garthe er ar.
1999).All departingbirds could be trackedfrom Malgaskop
for a minimum of 45 minutes. Ground speeds of flying
gannets were determinedby timing birds flying between a
p r o m i n e n tr o c k a n d M a l g a s l s l a n d o n a c a l m d a y ( w i n d s
< S k m h r ) f r o m M a l g a s k o p .B a s e d o n t h i s a v e r a g ef l i g h t
speed, maximum detection distance from the island was
a b o u t6 0 k m ( F i g u r e1 ) . B i r d sr e m a i n i n gw i t h i nt h i s d i s t a n c e
from Malgas lsland could be trackedfor the whole duration
of their foragingtrip from Malgaskop.Flightdirectionsfrom
the island were recorded as the last bearing before the
signal was lost for outgoing birds and the first bearing
recordedfor incoming birds. Gannets were located at sea
by triangulationof bearings to radiotagged birds taken
s i m u l t a n e o u s l yf r o m a t l e a s t t w o r e c e i v i n g s t a t i o n s .
Experimentationwith a test transmitterindicatedthat we
could detect transmittersfrom receiving stations on the
Cape Peninsula(Table Mountainand Constantiaberg)at a
rangeof 117km.
Because receivingstations for triangulatingthe position
of gannets beyond the 60km radiotrackingradius from
Malgas lslandwere by necessityon high pointsto the south
of Malgas lsland,these extendedforaging paths represent
a biased sample of all possibletracks. We were unable to
fix the positionsof gannetstravellingnorth of SaldanhaBay,
birds travellingfar offshoreor far to the south of Constantiaberg.
Diet samples
Diet samples were analysed immediatelyafter collection.
Each sample was weighed and then sorted into its component fish species.Diet samplesretrievedfrom Cape gannets
were frequentlywell preservedand only rarely could prey
s p e c i e sn o t b e i d e n t i f i e d .
Weather data
Measurementsof wind directionand speed are measured
routinelyat Port Control,Saldanha Bay, 4km from Malgas
lsland. Wind speed and direction is recorded continually.
The day was divided into four 4-h periods and four hourly
recordswere extractedfor the days on which radio-tagged
birds were monitored.
Transmitter and handling erfecfs
Handling and attaching of devices on seabirds has the
potentialto affectboth behaviourand foragingpedormance.
These effects were tested by comparing foraging-trip
durationsand meal masses of radio-taggedbirdswith those
of a controlgroup of marked birds,the nests of which were
checked regularlyat 30-minuterntervals.These biros were
marked by dabbing dye on feathersof the chest and back
with a paintbrushon a 2m-longpole. Birdswere not handled
until capturedfor stomach sampling at the conclusionof a
foraging excursion.
Statistical methods
Spatial distributionsof incoming and outgoing flights were
tested for randomnessby comparingobservedfrequencies
of flight directions grouped into seaside sectors of 20'
againstexpectedfrequencies,assumingthat gannetsutilise
each sector randomly.Cells closest to the shorelinewere
grouped to avoid zero values in some months. The G
statisticfor the log-likelihoodratio goodness-of-fittest for
e x p e c t e d a n d o b s e r v e d d i s t r i b u t i o n sw a s c o m p u t e d .
Significancelevels were set at p < 0.05.
The Benguelaupwellingsystemis a complexand dynamic
r e g i o n ( S h a n n o n ' 1 9 8 5 )a n d f i s h s u r v e y d a t a i n d i c a t e
s u b s t a n t i a cl h a n g e s i n m e s o s c a l ef i s h d i s t r i b u t i o n( 1 0 0 1 000km) between seasons and between years (Hampton
1987).Accordingly,we tested for consistentflight patterns
at the smallesttemporalscalecompatiblewith an appropriate
sample size in each cell. Pooling data over seasonal and
interannualscales may reduce the chance of detecting
patterns in foraging. We tested for significantdifferences
among observedflight directionfrequencydistributionsfor
six definedseasons(earlysummer[October-December]
and
Adams and Navarro
l a t e s u m m e r [ J a n u a r y - M a r c h ]f )o r 1 9 8 7 / 1 9 8 8 ,1 9 B B / 1 9 8 9
and 1989/1990using the 1, test. By examtningthe adjusted
standardised deviates calculated for each cell in the
contingencytable (Everitt 1977), it was possibleto identify
for each 20' flight sector where observed and expected
valuesdifferedsignificantly.
Positivevaluesindicatedeviates
greater than expected, negative values indicate deviates
lower than expected, adjusted deviates >3.291 are significantlydifferentfrom expectedvalues at p < 0.001 (Everitt
'1977).
The significantp value was set conservativelyto
account for repeatedcomparisonsof the same dataset.
Under the assumptionthat potentialprey speciesshowed
consistent mesoscale patterns of distribution,a similar
approach was used to test for the effects of prey type on
flight directionon the combined dataset.
In the absence of other factors. if wind directionaffects
flight patterns of Cape gannets, it was predicted that
gannets on their outward flightsfrom Malgas lsland would
head at an angle to the wind that optimisesflightefficiency.
However, given that other factors may influence flight
directions,such wind effectsmay only be significantat high
wind speeds. In addition,it was predictedthat the deviation
b e t w e e n o u t g o i n g f l i g h t d i r e c t i o n a n d p r e v a i l i n gw i n d
directionwould decreasesignificantlywith increasingwind
speed. These predictionswere tested by examining the
correlationbetween both the deviation in flight directions
and the outgoing flight direction,and the wind directionat
time of departure. This analysis was performed at two
differentwind speed categories(> or < 20km h-i). Patterns
in the variancesin directionof outgoingflight directionswind directionagainstwind speed - were examined(onetailed variance ratio F{est, Zar 1984).
Significantdifferencesin the distributionof foraging{rip
durations(log-likelihood
ratiotest) and meal mass (nonparametric,unpairedtwo sample analysis)were tested between
control birds and birds handled for attachment of transmitters. Stomach sample mass was not corrected for
digestion.Accordingly,the data were separatedinto samples recovered from birds (1) the same day that they
departed from the colony and (2) that remained away
overnrgnr.
Results
Both the control and handled groups showed a trimodal
distributionof foragingtrips for birds returningfrom foraging
on the same day they departed, those remainingat sea
overnightbefore returningor those occasionallyremaining
a t s e a f o r t w o n i g h t s T h e f r e q u e n c y d i s t r i b u t i o n so f
foraging-tripdurations of handled and control birds were
stgnificantlydifferent (l' = 625.4, df = 11, p < 0. 0001,
Figure 2). Control birds more frequentlywent on shorter
t r i p s t h a n t h e h a n d l e db i r d s .
ln contrast,there were no significantdifferencesin the
meal mass between tagged and non-tagged birds that
returnedwithina day (Z= -1 .67561,p = 0 0938)or between
these two groups of birdswhen they stayedaway overnight
( Z = - 1 1 6 8 ' 1 ,p = 0 . 2 4 2 7 ) .
The directions of flight combined for all three years
showed a non-randomdistribution.Birdsgenerallydeparted
and returnedon a constant heading, concentratingin the
south-westernsector.When examinedon an annual basis.
the spatialdistributionof flightsof departingbirds was nonrandom in the breeding seasons of 1987/'1988and 1989/
1 9 9 0 , b u t n o t s i g n i f i c a n t l yd i f f e r e n t f r o m r a n d o m i n
1 9 B B /918 9 ( F i g u r e3 , T a b l e 1 ) . T h e f r e q u e n c yd i s t r i b u t i o n
of arrivalflights was non-randomin all seasons sampled.
Departureflights in 1989/1990 and late summer '1988/
1 9 8 9 w e r e d o m i n a t e db y f l i g h t sb e t w e e n 1 9 1 " a n d 2 1 0 ' ;
(a)
Radiotaggeogannets
n=270
30
20
10
s
z
s
LIJ
f
(b)
uJ
c
'J'
.6'
4
b'
b'
Si
controlganners
n=2052
t
I
30
20
10
s . A t 3 p 6 r l q , 9 $ . p
F O R A G T N GT R t P D U R A T I O N ( h )
F i g u r e 2 : F o r a g i n gt r i p d u r a t i o n so f ( a ) r a d i o - t a g g e da n d ( b ) c o n t r o l
gannets
T a b l e 1 : R e s u l t s o f l o g - l i k e l i h o o dr a t i o g o o d n e s s - o f - f i t e s t t o
a s s e s s w h e t h e r t h e s p a t i a l d i s t r i b u t i o n so f f l i g h t s t o a n d f r o m
M a l g a s l s l a n d w e r e d i f f e r e n tf r o m r a n d o m
B r e e d i n gs e a s o n
G Statistic
df
All years
19 8 7 / 8 8
19 8 8 / 8 9
19 8 9 / 9 0
Outgoingflights
14.96
5
3 1. 0 7
5
7.53
5
18.14
5
< 0.05
All years
1987/88
19 8 8 / 8 S
19 8 9 / 9 0
lncoming flights
4 9 . 19
5
30.88
5
tz.1a
5
13.96
5
<
<
<
<
< 0.05
< 0.05
NS
0.005
0.05
0.05
0.05
A f r i c a n J o u r n a l o f M a r i n e S c i e n c e2 0 0 5 . 2 7 ( 1 ) . 2 3 9 2 4 8
1987/19BB
\
2a
fromisland
fl Departing
!
Arrivingat island
0 1 0? 0 k m
F i g u r e 3 : F l i g h t d i r e c t i o n so f g a n n e t s d e p a r t i n ga n d a r r i v i n g a t
M a l g a s l s l a n dl u m p e di n 2 0 ' s e c t o r s f o r t h r e ec o n s e c u t i v eb r e e d i n g
s e a s o n s , 1 9 8 7 / 1 9 8 8 ,1 9 8 8 / 1 9 8 9a n d 1 9 8 9 / 1 9 9 0 .F r e q u e n c yo f a
p a r t i c u l a rf l i g h t d i r e c t i o ni s i n d i c a t e db y l e n g t h o f b a r s .
f l i g h t sd u r i n ge a r l ys u m m e r1 9 B B / 1 9 8a9n d 1 9 8 7 / '91B Bw e r e
dominatedby directionsfrom about 251-270". During late
summer 1987i1988, flightswere concentratedin the region
211-250".
The 12 test for seasonaldifferencesamong flight patterns
was restrictedto '1987/1988and 1988/1989. Inclusionof
the 1989i90data resultedin violationof the test requirement
Ihat <20"k of the cells in the contingency table of expected
frequencyoccurrenceshould be <5 and no cell should have
an expected frequency of <1. Flight direction frequency
distributions
varredsignificantlybetweenseasons(X, = 59.2,
df = 15, p < 0.0001).Adjustedresidualscalculatedfor each
contingency table cell indicated gannets flew out more
frequentlythan expected in the 231-250" sector and 2112 3 0 ' s e c t o r i n e a r l y s u m m e r 1 9 8 7 / 1 9 8 8a n d l a t e s u m m e r
1987/1988 respectively(Table 2). In late summer '198B/
1989, birds flew out more frequentlythan expected in the
<190'sector.
Althoughonly a relativelysmall proportionof the variance
was explained,the directionof outgoingflightsof individual
birds was correlatedwith the direction of their incoming
f l i g h t s( r , = 0 . 2 6 4 ,n = 1 5 9 , p < 0 . 0 0 1 ) .T h i s i n d i c a t e st h a t
birds showed a tendencyto returnon the reciprocalbearing
to the directionin which they departed.
The maximum distanceradio{agged birds travelledfrom
the islandin a singleforagingtrip of 6-24h was highlyvariable
(<10km - >12okm, Figure 4). Of all birds tracked in the
receptionarea (see Figure 1), 23oh remainedwithin receiving
distanceof Malgaskopfor the durationof the foragingtrip
(within about 60km of the island), 44% reached their
maximum range within the area north of a line due west of
Constantiaberg
to the southern limit of the receivingrange
from Malgaskop(60-120km from Malgas lsland),and 33%
of birds travelledto areas south of Constantiaberg
(>120km
from Malgas lsland, Figures4, 5). At least one bird moved
into False Bay to forage.Gannetsavoidedthe inshorearea
1Okmfrom the coast betweenSaldanhaBay and Table Bay.
Distancescovered by birds whose position was fixed within
30 minutes of dusk and one hour of dawn were generally
short and indicatedthat there was littleor no movementat
night. There was no significantcorrelationbetween the
durationof a foragingtrip and foraging range (p > 0.05)
Averageflightspeed of Cape gannetswas 63km h-1(range:
56-67km h 1) indicatingthat feeding areas were potentially
nearlyall withintwo hours flying time of the breedingisland.
Given the inherentbias in the data because of the limited
trackingrange,these data shouldbe interpretedwith caution.
ln all, 57"k of all winds recordedat Malgas lsland during
the study period were from a south-westerlyto southerly
direction.The strongestwinds were also recorded blowing
from this direction(Figure 6).
Examinationof the frequency at which differentsectors
were used by returning birds that fed on different prey
indicatedthat those eating saury returnedsignificantlymore
frequently in lhe 231-250' sector (p . O.OO1,adjusted
r e s i d u a l= 4 , F i g u r e7 ) .
There was some suggestionthat wind influencedgannet
flights, because variabilityin direction of departure flights
in relationto prevailingwind directionwas significantlylower
a t h i g h w i n d s s p e e d s( > 2 0 k mh 1 ) t h a n a t l o w w i n d s p e e d s
( F = 2 1 5 6 , p < 0 . 0 5 ) . H o w e v e r ,t h e r e w a s n o s i g n i f i c a n t
Adams and Navarro
correlationbetweenflightdirectionof departinggannetsand
wind directionwhen all data were consideredat wind soeeds
> 2 0 k m h 1 ( p > 0 . 0 5 ) . l t i s c o n c l u d e dt h a t b i r d s d i d n o t
select a specific heading relativeto wind direction.
In summary, it is consideredthat the longer periods of
absence of radio{agged Cape gannets from nests reflect
a d i r e c t b e h a v i o u r a lr e s o o n s e t o h a n d l i n o r a t h e r t h a n
inefficientprey capture.
ln all. 67ok of all birds tracked at sea remarnednorth of
C o n s t a n t i a b e rw
g ,i t h i na b o u t1 2 0 k mo f t h e i rn e s t so n M a l g a s
lsland.The specificconcentrationof birds recordedforaging
a m a x i m u mo f 9 1 - 1 1 O k mf r o m M a l g a sl s l a n dm a y b e p a r t l y
an artefact of the two southern receiving stations being
located on the coast directly opposite this feeding area.
However.these measurementsare consistentwith at-sea
observationsof Cape gannets of unknown status marked
with coloureddye at Malgas lslandand a recentstudy using
GPS tracking.These indicateda longshoreforaging range
o f 2 0 0 k m d u r i n g s u m m e r ( B e r r u t i1 9 8 7 ) a n d a m a x i m u m
Discussion
BreedingCape gannets showed a behaviouralresponseto
handlingfor the attachmentof transmitters.The birds under
study increasedthe average duration away from the nest
compared to control birds (see also Adams and Klages
1999). Northern gannets handled as part of a study of
activity-specificmetabolic rates showed similar adverse
responsesto handling(Brrt-Friesenet a/. '1989),with manipulated birds spending less time at the nest than control
birds. However, northern gannets fitted with platform
terminaltransmitters,in a similarlyshort handlingprocedure
as the present study, showed no differencein foraging{rip
durationsfrom those of control birds in a study examining
foraging ranges and feeding locations in the North Sea
(Hamer et al. 2000). The magnitude of such effects in
coastalseabirdsappearscase, speciesand breeding-status
dependent. For example, alcids feeding chicks were less
sensitive to such disturbancethan pre-breedingor incubating birds (Wanless ef a/. 1985) and the frequency of
chick feeding by two species of auks, Uria aalge and Alca
torda,were adverselybut differentially
affected(Wanlesset
a / . 1 9 8 5 ,1 9 8 8 ,1 9 B g ) .O n t h e o t h e rh a n d ,a t t e n d a n c eb e h a viours of the black-legged kittiwake Rissa triactyla were
unaffectedby handling (lrons 1998).
In addition to the behaviouraleffects associated with
handling(see above),the devicesthemselves,particularlyif
2.5okor more of body mass, may affect locomotoryefficiency
and cost by adding mass and increasingdrag (Gessaman
and Nagy '1988,Obrechtel a/. 19BB).However,it is suggested
that the tail-mountedtransmitters,weighing a maximum of
1% of body mass and completelycovered by the undertail
coverts, would have a negligibleeffect on the ability of
gannetsto catch prey. Lack of any significantdifferencesin
meal mass of the taggedand controlCape gannetssupports
t h i s c o n c l u s i o n . S i m i l a r l y ,i n t e r n a l a n d s m a l l e x t e r n a l
temperatureloggersattachedto the legs of northerngannets
did not detrimentallyaffect their foraging efficiency (Garthe
el a/. 1999).
S
33'
Figure 4: Flightpaths and foragingranges of gannetstrackedat
i n t e r v a l st h r o u g h o u ta c o m p l e t ef o r a g i n g e x c u r s i o n
Table 2: Adjusted residualsof chi-squaredratio goodness-of-fittest for outgoingflight directlonsover four consecutivepartialbreedingseasons.
A l l d i r e c t i o n sn o r t h o f d u e w e s t a n d < 1 9 0 ' w e r e g r o u p e d t o i n c r e a s es a m p l e s i z e i n r n d i v i d u acl e l l s .T h e r e m a i n i n gf l i g h t sw e r e g r o u p e d
i n 2 0 " s e c t o r s . P o s i t i v ev a l u e s i n d i c a t ed e v i a t e s g r e a t e r t h a n e x p e c t e d ,n e g a t i v ev a l u e s i n d i c a t ed e v i a t e s l o w e r t h a n e x p e c t e d . C r i t i c a l
v a l u e s a r e f r o m t h e S t u d e n t ' st - d i s t r i b u t i o nw i t h i n f i n i t ed e q r e e s o f f r e e d o m
F l i g h t d i r e c t i o ns e c t o r
<190'
191,210"
211 230"
231-250"
251-270"
>270'
1987/88
1987/88
1988/89
1988/89
Early summer
Late summer
E a r l ys u m m e r
Late summer
2.3
,1.4
41'
0.2
0.5
-t.o
08
3.6'
-2.0
LC
-2.6
. A d j u s t e d d e v i a t e s > ! 3 . 2 9 1 a r e s i g n i f i c a n t l yd i f f e r e n tf r o m e x p e c t e dv a l u e s
at p < 0.001
o.2
0.8
-2
0
0.5
2.8
3.91.4
- 0.6
- 1.4
-0 5
African Journal of lvlarine Science 2005. 27(1\'. 239 248
distance of 104km (Gremillet et al. 2004) respectively;
although the later study, not restrictedby the topographic
constraintsof VHF telemetry,indicatedthat some individual
birds travelled considerablyfarther than this (Gremilletel
al. 2004). The range in travellrngdistances recorded for
Cape gannets is similar to earlier estimates for northern
(Kirkhamet al. 1985, Tasker et a/. 1985) and Australasian
gannet M. serrator (Wingham 1985). Mean maximum
foraging range of northerngannets estimatedfrom activity
budgetswas 128km for birds that averaged 13h away from
nests on Hermaness,Shetland, UK (Garthe ef a/. 1999).
All these measurementsand estimates reflect the coastal
feeding habits of gannets and foraging trips that are
^ A
><
;()
z
ul
l
4
LIJ
x.
n 2
1O
to
60
go llo nto nto nro ^so
MAXIMUM
D I S T A N C(Ek m )
10
F i g u r e 5 : M a x i m u m d i s t a n c e sa t t a i n e db y i n d i v i d u a gl a n n e t sf r o m
M a l g a s l s l a n d d u r i n g f o r a g i n g t r i p s f o r w h i c h r a d i o l o c a t i o nd a t a
w e r e a v a i l a b l et h r o u g h o u ta c o m p l e t ef o r a g i n gt r i p
\r.l
------{
/
E
(
z
Z
t.ll
f
LU
t
/
18"
F i g u r e 6 : R e l a t i v ef r e q u e n c yo f o c c u T r e n c eo f w i n d d i r e c t i o na n d
s t r e n g t h r e c o r d e da t M a l g a s l s l a n d d u r i n g d a y s t h a t b i r d s w e r e
t r a c k e db y r a d i o t e l e m e t r yL.e n g t ho f b a r r e p r e s e n t sf r e q u e n c yw i t h
w h i c h a o a r t i c u l a rw i n d s o e e d w a s r e c o r d e d
180 200 220 240 260 2BO 300 320 340
( 'N
)
)TRECTTO
F i g u r e 7 : N u m b e r s o f C a p e g a n n e t s r e t u r n i n gt o M a l g a s l s l a n d
w i t h s i n g l e s p e c i e s p r e y l o a d s i n r e l a t i o nt o a r r i v a l d i r e c t i o n
Y
246
restrictedusuallyto 36h or Iess.These valuesare somewhat
less than earlier estimates of foraging ranges of Cape
gannets of 270-360km based on distributionat sea and
recoveryof bandedadults(Rand 1959) and of 470km based
on parental nest attendance and estimated flight speed
( J a r v i s1 9 7 1 ) .T h e y a r e a l s o l e s s t h a n t h e m a x i m u mr a n g e
of 540km and the mean distanceto farthest point on any
one trip of 232km measured for northern gannets using
satellitetelemetery(Hamer et al. 2000). Significantly,
foragingtrips of these birds nesting on Bass Rock, SouthEast Scotland,rangedfrom 13h to B4h (Hamer et a|.2000).
T h e f o r a g i n g r a n g e o f g a n n e t s o n i n d i v i d u a fl o r a g i n g
excursionsfrom Malgas lslandover the durationof the study
did not correlate with trip duration. This observation is
consistentwith data that show gannets attending chicks
may spend <50% of their time at sea actuallyflying (Adams
a n d K l a g e s 1 9 9 9 ,G a r t h ee t a / . 1 9 9 9 ) ,p r o v i d i n gt h e m w i t h
considerable flexibility to respond to variation in food
a v a i l a b i l i t ya m o n g f o r a g i n g t r i p s a n d s t i l l m a i n t a i n a
consistentfood delivery rate to chicks (Burger and Piatt
' 19 9 0 , A d a m s
a n d K l a g e s 1 9 9 9 ) . l t i s a l s o c o n s i s t e nw
t ith
the trimodal distributionof foraging-tripdurationscoupled
with littlemovementat night.The presentresultis in marked
contrastto Hamer et al. (2000), and that of Gremilletel a/.
(2004),who demonstratedthat the trip durationof northern
gannets and Cape gannets explainedmost of the variation
in distance travelled.This may reflect marked differences
in the spatial and temporal predictabilityof prey between
and within the two marine systems,althoughdemonstrated
disturbanceeffectsand the bias owing to the limitedtracking
range requrresome caution in interpretation.
M u c h o f t h e p a t c h i n e s si n s e a b i r d d i s t r i b u t i o ni n t h e
s o u t h e r n B e n g u e l a u p w e l l i n g s y s t e m d e t e r m i n e df r o m
transects carried out at sea, which is probably related to
p r e y p a t c h i n e s sw, a s a t s c a l e so f 0 . 3 - 1 0 k m( S c h n e i d ear n d
D u f f y 1 9 8 5 ) .T h i s i s a t a s c a l et o o s m a l l t o b e i n v e s t i g a t e d
using VHF or indeed current satellitetelemetrytechnology.
N o n - r a n d o md i s t r i b u t i o no f f o r a g i n g f l i g h t s o f s e a b i r d s
reflecting movement at larger spatial scales has been
consideredto reflectthe locationof favouredfeeding areas
with respect to nesting sites (Harrison and Stoneburner
1 9 8 1 , A n d e r s o n a n d R i c k l e f s' 1 9 8 7 ,H a m e r e t a l . 2 0 0 0 ) .
Such feeding areas may represent spatially predictable
m e s o s c a l e( 1 0 - 1 0 0 k mw i d e ) o c e a n o g r a p h i fce a t u r e s( s e e
Hunt and Schneider1987), which are presumedto
concentrate primary production and seabird prey (Heath
a n d R a n d a l l 1 9 8 9 , J o u v e n t i ne t a l . 1 9 9 4 ) . I n s p i t e o f a
generallyconsrstentpatternof dispersalmaintainedover a
number of seasons, we are unaware of any bathymetric
f e a t u r e si n t h e s o u t h e r nB e n g u e l as y s t e m( S h a n n o n1 9 8 5 )
that concentrate seabird prey in such a spatially and
temporally predictableway Enhanced primary production
is associatedwith well-developedand spatiallypredictable
u p w e l l i n gc e l l s l o c a t e do f f t h e C a p e P e n i n s u l aa n d C a p e
C o l u m b i n e( S h a n n o n1 9 8 5 ) .1 2 0 k mt o t h e s o u t h a n d 3 0 k m
to the north respectivelyof the study site.A closeassociation
betweenthese sourcesof enhancedprimaryproductionand
a b u n d a n c eo f s e a b i r d p r e y s p e c i e s a t e q u i v a l e n ts p a t i a l
scales has not been demonstrated.
At a larger, regional scale (macroscale:100 1 000km;
H u n ta n d S c h n e i d e r1 9 8 7 ) ,p o p u l a t i o nos f a d u l ts a r d i n ea n d
Adams and Navarro
anchovyaround SouthAfricaare generallyrestrictedto cold
coastalwaters over the continentalshelf (Armstronget a/.
1 9 8 7 .C r a w f o r de t a l . 1 9 8 7 ,H a m p t o n1 9 8 7 ) .T h e r e f o r et,h e
generally south-west mainly longshore movement of
gannets may reflect,partly.the locationof the shelf (Figure
1 ) . B i r d sf l y i n gw e s t w o u l d s o o n e n c o u n t e tr h e e d g e o f t h e
s h e l f b r e a k ( d e p t h> 2 0 0 m ) a n d t h e l i m i to f t h e d i s t r i b u t i o n
of sardine and anchovy. Of note is the more westerly
distributionof the flight directionsof gannets returningwith
saury. In contrastto anchovy and sardine, saury are
associatedwith oceanic water and thermal fronts (Dudley
e t a l . 1 9 8 5 ,B e r r u t i1 9 B B ) T
. h e c o n t i n e n t asl h e l f b r e a k a n d
associatedoceanicwater is in closest proximityto foraging
g a n n e t sd u e w e s t o f M a l g a s l s l a n d( F i g u r e1 ) .
Althoughwind speed and directionhas a major influence
o n l a r g e - s c a l ef o r a g i n g m o v e m e n t s o f s o m e p e l a g i c
seabirds(Jouventinand Weimerskirch1990, Weimerskirch
et al. 2000), there was little evidence of such influences
for Cape gannets departing from Malgas lsland (but see
Gr6millet et al. 2004). Nevertheless,on account of the
direction and persistenceof the prevailingwind, gannets
often left quarteringinto the wind and food-ladengannets
O
5
z
t-o_
l
ct)
z
O
t
LIJ
z
LJJ
200
400
600
800
"?k g )
PAYLOAD MASS (x 10
F i g u r e 8 : T o t a l e n e r g y r e q u i r e m e n t s( c a l c u l a t e df r o m P e n n y c u i c k
1 9 8 9 )f o r a 1 0 0 k mf l i g h tb y a C a p e g a n n e t( m a s s = 2 . 6 3 k 9 ,w i n g
s p a n = 1 . 7 3 m ,f l i g h ts p e e d= 1 7 2 m s 1 ) c a r r y i n gp a y l o a d sr a n g i n g
from zero, through mean accumulatedmass (0.358k9) to the
m a x i m u m r e c o r d e d ( 0 . 9 5 k 9 )u n d e r v a r i o u s w i n d c o n d i t i o n s( N W :
no wind, TW: tailwind, HW: headwind). The most frequently
r e c o r d e dw i n d s p e e d a t M a l g a s l s l a n d w a s 6 . 5 m s r
A f r i c a nJ o u r n a lo f M a r i n e S c e n c e 2 0 0 5 . 2 7 ( 1 ) 2 3 9 2 4 8
247
BERRUTIA
, . 1 9 8 7 - T h e u s e o f C a p e g a n n e t si n m a n a g e m e n t
of the purse seine fishery of the Western Cape. Ph.D. thesis,
U n i v e r s i t yo f N a t a l ( P i e t e r m a r i t z b u r g 3) :0 4 p p .
BERRUTI, A. 19BB
D i s t r i b u t i o no f a n d p r e d a t i o n o n s a u r y
Sconrberesox saurus scombroldes in continental shelf waters off
the Cape Province, South Africa. S. Afr. J. mar. Scl. 6:
183-192.
B E R R U T I ,A . , A D A M S , N J . a n d S . J A C K S O N 1 9 8 9 - T h e
B e n g u e l ae c o s y s t e m .6 . S e a b i r d s .l n O c e a n o g r a p h ya n d M a r i n e
B i o l o g y .A n A n n u a l R e v i e w . B a r n e s , M . ( E d . ) . A b e r d e e n ;
U n i v e r s i t yP r e s s 2 7 . 2 7 3 - 3 3 5 .
B E R R U T I ,A . a n d J . C O L C L O U G H1 9 8 7
C o m p a r i s o no f t h e
abundance of pilchard in Cape gannet diet and commercial
catches off the Western Cape. South Africa. In The Benguela
a n d C o m p a r a b l e E c o s y s f e m s .P a y n e , A . L L . , G u l l a n d , J . A .
and K. H. Brink (Eds). S. Afr. J. mar. Scr.5: 863-869.
B I R T - F R I E S E NV,. 1 . , I V O N T E V E C C HW
I , A . . C A I R N S .D . K . a n d
S . A . M A C K O 1 9 8 9 - A c t i v i t y - s p e c i f im
c e t a b o l i cr a t e s o f f r e e living northern gannets and other seabirds. Ecology 70(2).
357-367.
B U N C E , A . 2 0 0 1 - P r e y c o n s u m p t i o no f A u s t r a l a s i a ng a n n e t s
( M o r u s s e r r a t o r )b r e e d i n gi n P h i l l i pB a y , s o u t h e a s tA u s t r a l i a ,a n d
p o t e n t i a lo v e r l a p w i t h c o m m e r c i a lf i s h e r i e s ./ C F S J . m a r . S c t .
58: 904 915.
B U R G E R ,A . E . a n d J F . P I A T T 1 9 9 0 - F l e x i b l et i m e b u d g e t si n
breeding common murres: buffers against variable prey
a b u n d a n c e .S t u d .a v i a n B i o l . 1 4 . 7 1 - 8 3 .
C R A W F O R D , R . J . M . 1 9 9 9 - S e a b i r d r e s p o n s e st o l o n g { e r m
changes of prey resources off southern Africa. In Proceedings
of the 22'"' lnternational OrnithologicalCongress, Durban. 1998.
A d a m s , N . J . a n d R . H . S l o t o w ( E d s ) .J o h a n n e s b u r gB. i r d l i f e
S o u t h A f r i c a :6 8 8 - 7 0 5 .
CRAWFORD, R. J. lV. and B. M. DYER 1995
R e s p o n s e sb y
f o u r s e a b i r ds p e c i e st o a f l u c t u a t i n ga v a i l a b i l i t yo f C a p e a n c h o v y
Engraults capensls off South Africa. /bls 137: 329-339.
C R A W F O R D ,R . J . M . , S H A N N O N , L V a n d D E P O L L O C K
1987 - The Benguela ecosystem. 4. The malor fish and
invertebrate resources. ln Oceanography and Marine Biology.
An Annual Review 25. Barnes.M. (Ed.). Aberdeen;University
Acknowledgements Financialsupportfor this prolectwas
Press:353 505.
provded by theformerFoundation
for Research
Development,
now
C R A W F O R D ,R J . M . , S H E L T O N ,P A . C O O P E R .J a n d R . K
National
Research
Foundation.
undertheauspices
of the Benguela
B R O O K E 1 9 8 3 - D i s t r i b u t i o np, o p u l a t i o ns i z e a n d c o n s e r v a t i o n
E c o l o g yP r o g r a m m eW
. e t h a n k t h e W e s t e r nC a p e N a t u r e
of the Cape gannet Morus capensls.S. Afr. J. mar. Sci.1.
Conservation
Board,National
ParksBoard,SouthAfricanNational
1 5 3 - 1 74 .
DefenceForceandtheCapeTownCityCouncil
for allowing
access
DUDLEY S. F. J., FIELD,J. G, and P. A. SHELTON1985 to areas undertheir jurisdiction.
The NationalParks Boardof
D i s t r i b u t i o na n d a b u n d a n c eo f e g g s , l a r v a e a n d e a r l y j u v e n i l e s
gavepermission
Trustees
for research
to be conducted
at Malgas
of saury Scomberesox saurus scombroldes (Richardson) off the
lslandand providedlogistical
support.The WesternCapeNature
S o u t h - W e s t e r nC a p e , S o u t h A f r i c a , 1 9 7 7 1 7 8S
. . Afr. J. mar Sci.
Conservation
Boardprovided
transport
to and fromMalgaslsland.
3.229-237.
TheTableMountain
AerialCableway
Companyprovided
additional
EVERITT, B. S. '1977 The Analysts of Contingency Tables.
logistical
support.
L o n d o n ,C h a p m a n& H a l l : 1 2 8 p p .
^
^
^
;
.
.
,
,
.
G A R T H E ,S . , G R E M I L L E TD
. . a n d R W . F U R N E S S1 9 9 9 - A t s e a f o r a g i n g e f f i c i e n c yi n c h i c k r e a r i n g n o r t h e r n g a n n e t s S u / a
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, . and S J. WEEKS 2004 - Offshore
A N D E R S O ND.. J . a n d R E R I C K L E F 1
S9 8 7- R a d i o { r a c k i n g
d i p l o m a c y ,o r h o w s e a b i r d s m i t i g a t ei n t r a - s p e c i f i c o m p e t i t l o n :
maskedand blue-footed
boobies(Su/aspp.)in the Galapagos
a case study based on GPS tracking of Cape gannets from
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neighbourrngcolonies. Mar. Ecol. Prog. Ser. 268. 265-279.
ARMSTRONG
M,. J . B E R R U TAI ,. a n dJ C O L C L O U G1H9 8 7
H A M E R , K , C , , P H I L L I P S ,R . A . , W A N L E S S ,S . , H A R R I S .M . P .
Pilchard
distribution
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a n d A . G . W O O D 2 0 0 0 - F o r a g i n gr a n g e s , d i e t s a n d f e e d i n g
Benguela
and Comparable
Ecosyslems.
Payne.A. l. L..Gulland.
l o c a t i o n so f g a n n e t sM o r u s b a s s a n u si n t h e N o r t h S e a . e v i d e n c e
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returningto the colony frequentlydid so while flying with a
tail wind. The coincidentflight and wind patternwill confer
some energetic benefitsto food-ladengannets. Based on
p r o g r a m m e sd e v e l o p e db y P e n n y c u i c k( 1 9 8 9 ) ,w e u s e d t h e
a p p r o a c ho f S c h a f f n e r( 1 9 9 0 ) ,a n d q u a n t i f i e dt h e e n e r g y
savings inherent in the observed flight patternsin relation
to prevailingwind conditionsVariationsin the wind strength
and payload have little effect on the total energy costs of
a 1 0 0 k m f l i g h tw h e n b i r d s a r e f l y i n gd o w n w i n d( F i g u r eB ) .
However, with headwinds of the order experienced at
M a l g a s l s l a n dd u r i n gt h e s u m m e r ,e n e r g y c o s t s i n c r e a s e d
s u b s t a n t i a l l yp,a r t i c u l a r l ya t h i g h l o a d s e q u i v a l e n t o t h e
maximum recorded for gannets (Figure B). For a payload
equivalentto the mean mass accumulatedover a foraging
t r i p ( 0 . 3 5 8 k 9 )t,h e t o t a l e n e r g y c o s t o f a 1 O O k mf l i g h tw i t h
and againstthe wind of the most frequentlyrecordedspeed
( 6 . 5 m s 1 ) i s 9 . 9 8 x 1 0 5 k Ja n d 2 2 . 4 x 1 0 5 k Jr e s p e c t i v e l y .
However, because birds feeding chicks are central-place
foragersand must returnto the nest, it is more appropriate
to examine differencesin energy costs of a 200km roundtrip. The differencebetweenbirds returningunder load with
a h e a d w i n d v e r s u st h e o o o o s i t ec o n d i t i o ni s 2 . 0 3 x 1 O s k J
(30 7 r 105-28.7x 1O5kJ).
Assuminga metabolisableenergy
content of 4.B9kJ g 1 wet mass for gannet prey (Adams el
a l . 1 9 9 1 ) , t h i s d i f f e r e n c ei s e q u i v a l e n tt o 4 1 9 o f f i s h .
S u m m e d o v e r t h e d u r a t i o no f a b r e e d i n gs e a s o n ,e n e r g y
s a v i n ga c c r u e db y f e e d i n gu p w i n do f n e s t i n gs i t e s m a y b e
considerableThere are very few suitablebreedingsites off
the SouthAfrican coast and constraintsother than the local
wind regime may be more critical in dictatingwhether or
n o t g a n n e t s w i l l b r e e d a t a p a r t i c u l a rs i t e . H o w e v e r ,
Pennycuicket a/. (1990) noted that colonies of breeding
white-tailed tropicbirds Phaethon leptzirus are usually
downwind of favoured feedino areas.
7
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