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The breeding biology and factors affecting
reproductive success in rockhopper penguins
Eudyptes chrysocome at...
Article in Polar Biology · October 2004
DOI: 10.1007/s00300-004-0643-z
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Polar Biol (2004) 27: 711–720
DOI 10.1007/s00300-004-0643-z
O R I GI N A L P A P E R
Cindy L. Hull Æ Mark Hindell Æ Kirsten Le Mar
Paul Scofield Æ Jane Wilson Æ Mary-Anne Lea
The breeding biology and factors affecting reproductive success
in rockhopper penguins Eudyptes chrysocome
at Macquarie Island
Received: 15 October 2003 / Revised: 20 May 2004 / Accepted: 20 May 2004 / Published online: 22 July 2004
Springer-Verlag 2004
Abstract Adult mass changes, egg morphometrics, chick
growth rates, fledging masses, reproductive success and
reasons for reproductive failure were examined in
rockhopper penguins at Macquarie Island from 1993/
1994 to 1995/1996. Mean arrival masses, growth rates of
chicks and fledging masses exhibited inter-annual variability, while egg morphometrics, hatching success
(68.0±6.0%) and reproductive success (47.3±8.3%)
were constant between years. Reproductive failures
occurred primarily during incubation, with the majority
of eggs lost to great skuas. Logistic regressions revealed
that no variable significantly explained hatching success,
and only in 1994/1995 was fledging success significantly
correlated with the position of nest in the colony (those
in the centre were more successful than those on the
periphery). Reproductive success during this study was
relatively high, and therefore an assessment during poor
years would be instructive, particularly in relation to
aspects of the penguins’ foraging ecology.
Introduction
Penguins are a highly specialised group of seabirds
whose relatively low reproductive output may have
evolved to cope with the difficulties of exploiting their
food resource, and the constraints related to transporting food from feeding grounds to the nest site (Ricklefs
1983). However, reproductive success in a number of
C. L. Hull (&) Æ M. Hindell Æ K. Le Mar Æ P. Scofield
J. Wilson Æ M.-A. Lea
School of Zoology, University of Tasmania,
GPO Box 252-05, Hobart, Tasmania,
7001, Australia
E-mail: cindy.hull@hydro.com.au
Present address: C. L. Hull
Environmental Services, Hydro Tasmania,
GPO Box 355, Hobart, Tasmania,
7001, Australia
species exhibits large inter-annual and geographical
variability, which is apparently dictated by the distribution and abundance of prey ultimately due to environmental perturbations (Reilly and Cullen 1981;
Trivelpiece et al. 1983; Heath and Randall 1985; Brown
and Klages 1987; Boersma et al. 1990; van Heezik and
Davis 1990; Williams and Croxall 1991; Thompson
1993; Watanuki et al. 1993; Crawford and Dyer 1995).
Rockhopper penguins (Eudyptes chrysocome) are the
most widespread species of the crested penguins. They
range from islands near the Antarctic Polar Front to
islands near the subtropical convergence in the South
Atlantic and Indian Oceans (Marchant and Higgins
1990). They comprise three subspecies (E. chrysocome
chrysocome, E. chrysocome filholi and E. chrysocome
moseleyi), although their taxonomy requires further
examination (Ellis et al. 1998).
Rockhopper penguin populations have declined
substantially at a number of sites (Campbell Island,
Tristan da Cunha, the Antipodes and Falkland Islands,
Ellis et al.1998). These declines have prompted the
upgrading of their IUCN conservation status to vulnerable (Ellis et al. 1998). Declines in the order of 94%
have been recorded on Campbell Island (Pacific sector of
the Southern Ocean), a site that had a breeding population of 1.6 million birds in 1942 (Cunningham and
Moors 1994). In order to better understand, and hopefully prevent, such declines, data are required on the
species’ breeding biology and its relationship with foraging ecology (Pütz et al. 2001).
Members of the eastern subspecies, E. chrysocome
filholi, breed on Macquarie Island (5433¢57¢¢S,
15854¢57¢¢E), also in the Pacific sector of the Southern
Ocean. Although some aspects of their breeding biology
have been studied at this site (Warham 1963; St Clair
and St Clair 1996), no multi-year comparisons have been
made. The purpose of this study was to describe the
factors affecting reproductive success in rockhopper
penguins at Macquarie Island, assess inter-annual variability, and how these factors relate to aspects of their
foraging ecology.
712
Materials and methods
Fieldwork was undertaken at the rockhopper-penguin
colony at Brothers Point, on the east coast of Macquarie
Island during the 1993/1994, 1994/1995 and 1995/1996
breeding seasons. The colony comprised approximately
2,000 breeding pairs, and was located at the rear of a
small cove on steeply sloping ground. It was a small
colony (777 m2), at an altitude of 3–37 m, and covered
with loose boulders and patches of tussock grass, Poa
spp. Adult mass changes, egg and chick morphometrics
and reproductive success were quantified during the
3 years of the study.
Adult mass changes
Approximately 30 breeding adults [indicated by plumage
characteristics (Warham 1975), time of year (juveniles
do not return until after breeding commences, C. Hull,
personal observation) and, when breeding, by the presence of a brood patch] were weighed on the beach below
the colony at each of the following stages each year:
1. Male return to the island following the non-breeding
winter period at sea.
2. Female return to the island following the nonbreeding winter period at sea.
3. Male departure during incubation, following fasting.
4. Female departure during incubation, following fasting.
5. Male return from the 3-weeks foraging trip during
incubation.
6. Female return from the 2-weeks foraging trip during
incubation.
7. Guard stage—females as they departed and returned
from foraging (males remain at the nest).
8. Crèche stage—weekly measurements of both sexes as
they departed for and returned from foraging.
During each weighing session, adults were captured
by hand, weighed and sexed using a discriminant formula incorporating bill depth and length (Hull 1996).
Individuals were marked with a small patch of watersoluble paint to avoid re-weighing in that session, and
then released. Mass was compared between years using
Analysis of Covariance (ANCOVAs), with stage in the
breeding season as a covariate.
Nest monitoring
Breeding chronology, egg morphometrics, chick growth
rates, fledging masses, and reproductive success were
measured at 50 nests during each year of the study.
Nests on two transect lines that bisected the colony (one
in the upper third of the colony and one in the lower
third) were marked with individually numbered aluminium tags (2 cm2) attached to rocks prior to the
commencement of breeding, to minimise disturbance
(Hull and Wilson 1996). Tags were left in place between
seasons, and hence the same nests were monitored unless
a pair moved or a marker was lost. All tags were
removed at the end of the study.
Once pairs had formed, adults were banded with
metal flipper bands and sex determined from bill morphometrics (as above, Hull 1996). As metal flipper bands
were not available until after the 1993/1994 breeding
season was well established, only some birds were banded permanently during this year, with the remainder
banded with velcro bands, which were lost or removed
by the end of this season. The mass of breeding birds
was taken at the nest at the start of the 1993/1994 season, but not in subsequent seasons due to the disturbance this procedure caused (Hull and Wilson 1996).
Nests were monitored twice weekly prior to the hatching
of chicks, and once a week thereafter to determine
breeding status and parental attendance.
The masses, maximum lengths and widths of A (first
laid) and B (second laid) eggs were measured within
2 days of laying. Egg dimensions were compared
between the years using one-way ANOVAs. The
‘‘investment’’ by females in both A and B eggs was
determined by expressing egg mass as a percentage of
female mass during 1993/1994.
Initial chick masses were recorded within 2 days of
hatching and then once weekly thereafter until the end
of the creching period. A uniquely numbered aluminium
fish fingerling tag (10 mm·2 mm) was inserted into the
webbing of the right foot for identification at 1 week of
age. At 3 weeks of age, a small, individually numbered
velcro band was secured to the right flipper to assist with
identification, as once chicks entered a crèche it was
difficult to observe the fingerling tags in the mud of the
colony. Each week the velcro band was loosened to
allow for flipper growth, and just prior to fledging, the
fingerling tags and velcro bands were removed and
chicks were banded with a permanent metal flipper
band. Chick growth rates were compared between years
using Gompertz equations, which take non-linear
growth rates into account (Zullinger et al. 1984). Gompertz equations were selected as these are the best
available for comparison between species (Zullinger
et al. 1984), and best described growth in other species of
birds (Ricketts and Prince 1981). Three growth-related
variables were estimated: maximum mass (the asymptote, or maximum mass recorded for each individual);
growth rate (the shape of the curve of mass change); and
maximum growth rate (the time of maximum growth
rate) (see Ricketts and Prince 1981). Inter-annual variability was determined by comparing the amount of
overlap in the 95% confidence limits of variables.
The masses of fledging chicks were taken on the
beach as they departed the colony. A small patch of
water-soluble paint was applied to the bird’s breast to
avoid re-weighing. Chicks from each breeding season
were distinguished from first-year birds by their smaller
size, different call and different plumage (Marchant and
713
Higgins 1990). Fledging masses were compared between
years using one-way ANOVAs.
Reproductive success was compared between years
and transects, of nests at different locations (centre of
the colony or peripheral, the latter being defined as the
first three nests from the edge of transect lines, cf. Ainley
et al. 1983), and in different microhabitats using
v2 analysis. Microhabitats were categorised as either:
–
–
–
–
Exposed (no shelter provided by rocks or tussock);
In a rock cavity;
In the lee of rocks; or
In tussock.
The causes of nest failures were categorised as:
– Deserted (where the adults and nest contents had
disappeared);
– Predation of egg (where the adults were present with
no egg and it was not in the surrounding area);
– Broken egg;
– Egg rolled away (where it was observed at the side of
the nest);
– Never hatched (but remained in the nest);
– Chick died (when found dead in the nest);
– Predation of chick (when the chick had disappeared
and was not in the surrounding area, but adults were
present);
– Unknown.
There were probably some instances when eggs or
chicks were taken by predators which prompted the
abandonment of the nest, and hence, some of the
desertions described may represent cases of predation.
The reasons for the demise of eggs or chicks were
compared using v2 analysis.
Binary logistic regressions were run to determine if
the explanatory variables: female and male mass (1993/
1994 only); egg length; egg width; egg mass; whether the
male or female had successfully bred or attempted to
breed in the previous year (conducted only in the final
season due to limited data in 1993/1994); position in
colony; and nest microhabitat, had an effect on hatching
and fledging success. As the same nests were monitored
each year, logistic regressions were conducted separately
for each year to avoid pseudoreplication.
Data are presented as means±standard deviations
throughout.
Results
Adult mass change
The masses of 656 male and 827 female rockhopper
penguins were measured during the 3 years of this study.
On average, males were heavier (3.4±0.6 kg) than females (2.9±0.5 kg, or 86% of males) in all years (1993/
1994: F1, 504=94.2, P<0.0001; 1994/1995: F1, 543=38.0,
P<0.0001; 1995/1996: F1, 427=51.2, P<0.0001). However, both sexes exhibited considerable variability in
their masses across the breeding season (Table 1). The
Table 1 The mean mass±SD (kg) of rockhopper penguins during all stages and years of the study. Only females forage during guard
stage; sample sizes are given in parentheses
Stage in breeding season
1993/1994
1994/1995
1995/1996
Male return after winter
Male departure after fasting
Male return (incubation)
Female return after winter
Female departure after fasting
Female return (incubation)
Early guard—departure
Early guard—return
Late guard—departure
Late guard—return
Males
Early crèche—departure
Early crèche—return
Mid crèche—departure
Mid crèche—return
Late crèche—departure
Late crèche—return
Females
Early crèche—departure
Early crèche—return
Mid crèche—departure
Mid crèche—return
Late crèche—departure
Late crèche—return
3.4±0.3
2.3±0.2
3.5±0.6
3.1±0.5
2.5±0.5
2.7±0.4
2.5±0.2
2.8±0.2
2.3±0.2
2.5±0.3
3.7±0.4
2.6±0.3
4.1±0.4
3.6±0.3
(120)
(19)
(29)
(119)
3.0±0.3
2.6±0.3
2.9±0.3
2.6±0.2
2.7±0.2
(36)
(22)
(10)
(9)
(11)
4.1±0.3
3.0±0.4
3.4±0.4
3.9±0.3
2.5±0.4
3.3±0.2
2.8±0.1
3.1±0.3
2.4±0.2
2.7±0.2
(86)
(11)
(12)
(46)
(15)
(15)
(16)
(16)
(30)
(30)
2.4±0.04 (4)
2.7±0.4 (13)
3.0±0.2 (5)
3.0±0.2 (5)
2.8±0.3 (14)
2.9±0.2 (12)
2.8±0.3
3.2±0.5
2.9±0.3
3.5±0.4
2.8±0.4
3.0±0.3
(20)
(16)
(15)
(13)
(6)
(10)
3.4±0.4
3.4±0.5
2.7±0.3
3.2±0.3
(12)
(14)
(14)
(15)
2.4±0.3
2.7±0.3
2.4±0.2
2.7±0.3
2.4±0.2
2.5±0.3
2.7±0.3
2.8±0.3
2.6±0.3
3.0±0.2
2.6±0.2
2.8±0.3
(25)
(28)
(14)
(18)
(9)
(5)
2.8±0.4
2.8±0.2
2.7±0.3
2.9±0.3
a
Hull (1999a)
(162)
(3)
(24)
(83)
(36)
(29)
(10)
(14)
(9)
(13)
(13)
(18)
(2)
(11)
(9)
(18)
Average
mass loss (%)
Average mass
gain (%)
Average quantity
of food brought
ashore (g)a
9.1±15.7 (13)
31.5
43.0
6.6±15.9 (15)
± 11.5 (38)
21.7
38.5±17.5 (10)
4.7
145.7±65.6 (32)
28.0
8.5
7.4
0.4
215.4±123.8 (14)
18.5
3.4
144.3±80.1 (11)
15.8
180.4±103.3 (15)
1.4
143.3±87.7 (18)
13.9
211.5±128.4 (24)
8.5
202.0±133.8 (19)
6.2
160.2±90.0 (27)
11.2
6.1
(19)
(21)
(15)
(18)
4.4
10.4
5.9
714
Table 2 Dates of events in the
breeding season of rockhopper
penguins during each year of
the study. Medians are given,
except for ‘‘first male return’’,
when first sightings are given –
not witnessed
Event
1993/1994
1994/1995
1995/1996
First males return
Females return
A eggs laid
B eggs laid
Males depart
Males return
Females depart
Females return
Chicks hatch
Chicks crèching
Chicks fledge
Forage for moult
Return to moult
Depart the island
Duration of male fast (days)
Duration of female fast (days)
Duration of incubation (days)
Duration of chick rearing (days)
17/10
20/10
14/11
19/11
29/11
6/12
7/12
20/12
20/12
14/1
24/2
25/2
–
–
43
48
31
66
16/10
18/10
13/11
17/11
27/11
5/12
5/12
19/12
21/12
15/1
21/2
25/2
6/3
25/4
43
48
32
64
14/10
20/10
14/11
17/11
27/11
6/12
6/12
15/12
18/12
13/1
22/2
25/2
10/3
–
41
49
29
69
greatest mass losses occurred during fasting in the
incubation period (males lost on average 31.5% of
arrival mass or 81.0 g day 1, and females 28.0% of arrival mass or 94.2 g day 1, Table 1).
The largest mass gains occurred when birds returned
from foraging trips during incubation (males gained
43.0% of departure mass or 7.5 g day 1, and females
21.7%, or 7.3 g day 1, of their departure masses,
Table 1). The smallest gains in mass were made by males
during crèche stage (4.8 g day 1), and by females during
guard stage when they lost 1.0 g day 1. These masses
include food brought ashore, although this amounts to
very little during the incubation period (Hull 1999a,
Table 1).
The masses of males and females differed significantly
between years (males: F2, 652=60.5, P<0.0001; females:
F2, 823=64.9, P<0.0001, Table 1), with both sexes
lightest in 1993/1994, and heaviest in 1995/1996. The
source of this variation was arrival masses, which were
significantly different between years in both sexes (males:
F2, 365=146.2, P<0.0001; females F2, 245=78.8,
P<0.0001).
Nest monitoring
Rockhopper penguins showed considerable synchrony
in breeding chronology between individuals and years
(Table 2). Only 9% of banded males and 4% of banded
females on these transects were found to undertake a
Table 3 The number of male
and female rockhopper
penguins that attempted to
breed during the 3 years of the
study, and in relation to the
number of birds known to be
alive (n=202 individuals, 87
males and 77 females)
breeding attempt during each one of the breeding seasons (Table 3), with few birds attempting to breed more
than once during the 3 years of the study.
Twenty-seven (54%) males and 24 (48%) females
remained at the same nest site over 2 years. Only five
(10%) males and two (4%) females remained at the same
nest site over the 3 years of the study. There were 15
cases of mate changes in the 3 years, 8 of which (53%)
occurred following a nest failure in the previous season.
The size of eggs and hatching masses of A and
B chicks were constant between years (Table 4), with
A and B eggs weighing 3.1% and 4.2% of female mass,
respectively. During the 3 years of the study, five pairs
of birds laid only an A egg. Hatching success of A and
B eggs combined was 68.0±6.0% and constant between
years (v22=0.5, P>0.05, Table 4). Fledging success
(number of chicks fledged from eggs hatched) was
69.3±6.9% and constant between years (v22=0.5,
P>0.05). Eight chicks (11%) fledged from A eggs.
Maximum masses, maximum growth rates and growth
rates of chicks were constant between the 3 years
(Fig. 1, Table 4). On average, chicks fledged at 82.1% of
adult mass, but there were significant inter-annual differences in these masses (F2, 175=6.5, P<0.002), with
chicks from 1995/1996 being significantly heavier than
those from the other 2 years (Table 4).
Reproductive success was constant between years at
47.3±8.3% (v22=1.5, P>0.05). The majority (98.0% of
A eggs and 79.7% of B eggs) of nest failures occurred
during the incubation period (Table 5). Egg losses
Attempted to breed all 3 years
Attempted to breed during 2 years
Attempted to breed during 1 year
Successfully bred for 3 years
Successfully bred for 2 years
Successfully bred for 1 year
Males
Known
to be alive (%)
Females
Known
to be alive (%)
8
32
62
4
6
45
9.2
36.8
71.3
4.6
6.9
51.7
3
25
72
3
5
44
3.9
32.5
93.5
3.9
6.5
57.1
715
Table 4 Morphometric details
of rockhopper penguin eggs and
chicks, with results of statistical
comparisons between years
shown. Maximum masses,
growth rates and maximum
growth rates are derived from
the Gompertz equations
*Significant case, P>0.05
Dimension
Mean±SD
F
A length (cm)
A width (cm)
A mass (g)
B length (cm)
B width (cm)
B mass (g)
Clutch mass (% female, 1993/1994 only)
Hatching masses A egg
Hatching masses B egg
Maximum mass 95% CL
1993/1994
1994/1995
1995/1996
Growth rate 95% CL
1993/1994
1994/1995
1995/1996
Maximum growth rate 95% CL
1993/1994
1994/1995
1995/1996
Fledging masses
1993/1994
1994/1995
1995/1996
6.5±0.4
4.9±0.2
88.2±13.8
7.1±0.3
5.3±0.3
115.2±14.8
8.0±4.97
67.5±8.4
82.6±8.5
F3,
F3,
F3,
F3,
F3,
F3,
–
–
F2,
2.36–2.59
1.92–2.51
2.46–2.82
–
–
–
19
25
27
0.38–0.49
0.25–0.41
0.32–0.43
–
–
–
19
25
27
4.23–4.60
3.96–5.01
4.55–5.08
–
–
–
19
25
27
2.2±0.2
2.3±0.4
2.4±0.2
F2,
–
–
occurred throughout the incubation period, but peak
losses of A eggs occurred from 6 to 16 December and
B eggs from 26 November to 16 December (Fig. 2).
There were no significant differences in reproductive
success between transects (v21=0.2, P>0.05); however,
peripheral nests (22.0%) were significantly less successful than central nests (55.3%, v21=4.2, P<0.05). There
were no significant differences in the reproductive success of nests found in various microhabitats (v23=2.5,
P>0.05).
n
138=0.6
136=1.9
136=1.9
127=0.5
127=0.2
126=0.1
22=0.05
175=6.5*
141
141
139
130
130
129
39
8
25
35
34
109
The logistic regressions revealed that none of the
explanatory variables were significant predictors of
hatching success (1993/1994: 6.2, df=7, P>0.05; 1994/
1995: 9.8, df=5, P>0.05; 1995/1996: 3.4, df=5,
P>0.05). During 1994/1995, the model was significant
at predicting fledging success (11.9, df=5, P<0.05), but
not during the other years (1993/1994: 7.4, df=7,
P>0.05; 1995/1996: 12.7, df=9, P>0.05). The most
important explanatory variable during 1994/1995 was
the position of the nest in the colony (t=2.4, P<0.02).
Discussion
This 3-years study allows an assessment of the factors
affecting reproductive success, and the relationship between breeding biology and aspects of the foraging
ecology of individuals. In particular, any dietary differences or changes in foraging zones used across years
(Hull 1999a, b, 2000) can be assessed. Such information
is useful in understanding the reasons for the decline of
the species at some sites.
Adult mass changes
Fig. 1 Growth rates of rockhopper penguin chicks from hatching
to prior to fledging. Sample sizes 1993/1994=191, 1994/1995=239,
1995/1996=300
Rockhopper penguins exhibited inter-annual differences
in arrival masses during these 3 years. This suggests
different foraging success during the winter or the period
just prior to breeding, with prey possibly more abundant
or accessible during these periods in 1995 compared to
those during 1993.
Studies of other penguin species have found that
arrival mass correlates with subsequent reproductive
success (Drent and Daan 1980; Williams and Stone
716
Table 5 Observed causes of
reproductive failures in
rockhopper penguins during the
3 years of the study
Total laid
Deserted Predation
of egg
A eggs (150) 2 (1.3)
B eggs (79)
5 (6.3)
Egg broken Rolled Never
Chick
away
hatched died
107 (71.3) 1 (0.7)
48 (60.8) 2 (2.5)
1981; Chastel et al. 1995); however, this was not found in
the current study. Two factors affect the relationship
between arrival mass and reproductive success: the
degree of pelagic foraging (Chastel et al. 1995), and the
extent to which a species is a capital or income breeder
(Drent and Daan 1980). Pelagic foragers and capital
breeding species are more susceptible to reproductive
failure than income breeders if food is in short supply
during the early part of the breeding season (Drent and
Daan 1980). Rockhopper penguins conform to the
characteristics of an income breeder, by relying less on
reserves built up prior to the start of the breeding season, and instead continually replacing lost body condition throughout the season (Drent and Daan 1980;
Chastel et al. 1995). This is evident in the mass data from
1993/1994 where individuals attained masses similar to
other years, although they began the season with lower
body masses.
The masses of adult rockhopper penguins exhibited
considerable variability across the breeding season.
Males reached their lowest masses after long periods of
fasting, while females were at their lowest masses during
foraging trips in guard stage. Although females are
foraging at this time, they presumably deliver the
majority of the food to the chick, retaining a minimal
amount of food for self-maintenance. This, combined
with the fact that only one parent is foraging, makes this
stage the most energetically taxing phase of chick rearing
8 (5.3)
2 (2.5)
2 (1.3)
1 (1.3)
Predation Unknown
of chick
2 (1.3) 1 (0.7)
2 (2.5) 14 (17.7)
27 (18.0)
5 (6.3)
(Ricklefs 1983; Bost and Jouventin 1991; Wilson et al.
1991; Culik 1994; Edge et al. 1999). Female rockhopper
penguins bring ashore 147–227 g (5.2–9.6% of adult
mass) of food during early and late guard stage,
respectively (Hull 1999a). Generally, other species of
penguin feed their chicks 23.3–32.0% of food they obtain during a foraging trip (Green and Gales 1990).
Given the large mass losses during this period, it is
possible that rockhopper penguins supply their chicks a
higher percentage of food obtained from a foraging trip
during guard stage than other species.
Males were, on average, heavier than females
throughout the breeding season, but lost similar quantities of mass to females (males 1.0 kg and females
1.1 kg from pre-breeding to crèche, see Table 1). This
pattern contrasts with that of female little penguins
(Eudyptula minor), who lose significantly more mass
than males during the breeding season. These larger
mass losses in female little penguins have been attributed
to the production and raising of two chicks (Dann et al.
1995). Although the relative egg mass of rockhopper
penguin eggs (8.0% of female mass) was similar to that
of little penguins (10% of female mass, Dann et al.
1995), rockhopper penguins did not ever successfully
retain a second chick to fledging, thereby reducing the
energetic investment required from adults, particularly
females, during guard stage.
Nest monitoring
Fig. 2 Frequency distribution of the dates when rockhopper
penguin A and B eggs were lost
Like other species of sub-Antarctic eudyptid penguins,
the breeding chronology of rockhopper penguins was
highly synchronous between individuals and years
(Gwynn 1953; Warham 1972; Williams and Croxall
1991). This synchrony may have evolved as a mechanism
to reduce the impact of terrestrial predators (Liddle
1994), maximise chick survival by timing fledging when
prey abundance is highest (Liddle 1994), and/or when
climatic conditions are most favourable (Croxall and
Prince 1980). The last suggestion finds support in the
correlation between the commencement of breeding and
latitude, with rockhopper penguins breeding 10 days
later for every 1C decline in sea-surface temperature
(Warham 1972). The royal/macaroni penguin species
complex exhibits a similar trend, with the more southerly macaroni penguin (Eudyptes chrysolophus) breeding
later than royal penguins (Croxall and Prince 1980).
During the 3 years of this study, only 3% of males
and 8% of females attempted to breed during all years.
This low rate is explained in part by the lack of complete
permanent banding during the first season. It is also
717
possible that some rockhopper penguins were disturbed
by investigators, either through handling or through the
attachment of flipper bands, and did not attempt to
breed.
These figures should then be viewed as conservative,
although the actual percentage of birds attempting to
breed each year may still be low. Long-term avian
studies reveal that only a small proportion of individuals
produce a disproportionate number of the next generation (Clutton-Brock 1988; Ollason and Durnett 1988;
Thomas and Coulson 1988; Wooller et al. 1989). For
example, only 20% of male and 18% of female little
penguins were responsible for producing all the following generation (Dann and Cullen 1990).
Skipping a breeding season in a long-lived seabird
may minimise the risk of mortality, with the risk ultimately being mediated by body condition (Chastel et al.
1995; Weimerskirch et al. 1995). Non-breeding in adult
seabirds is related to food shortage and large-scale
environmental perturbations (Coulson 1984; Ainley
et al. 1988; Chastel et al. 1993). While some individuals
may skip a breeding attempt during years where they
have not attained sufficient condition, different classes of
breeders may also exist, as reported in king penguins
(Aptenodytes patagonicus) (Jiguet and Jouventin 1999).
Some king penguins lay an egg every year, while others
are frequent or infrequent intermittent breeders. These
distinctions may be related to age classes, different adult
quality or alternate breeding strategies (Jiguet and Jouventin 1999). Dann and Cullen (1990) found that only
31% of fledged little penguin chicks ever attempted to
breed during their lifetime. The quality of individuals
appears to be of primary importance in lifetime reproductive output (Coulson 1968; Dann et al. 1995).
If only a small proportion of breeding-age rockhopper penguins return to the island each year to breed,
estimates of the population size and food-consumption
rates based on the number of breeding birds observed at
a colony, may be a significant under-estimate. While the
number of birds breeding each year will be related to the
size of a population, this relationship may be complex
and exhibit variability related to factors such as prey
availability.
Rockhopper penguins, like other eudyptid penguins,
exhibit reversed egg size dimorphism (Slagsvold et al.
1984) and, relative to other birds, a low total investment
in clutches (Lack 1968; Williams 1990). This small
investment may be due to the difficulties in obtaining
sufficient food during the breeding season (Lack 1968),
and hence penguins have adjusted to a minimum
reproductive output (Ricklefs 1983). As this is a relatively long-lived species (Marchant and Higgins 1990),
this may be a low-risk breeding strategy that is advantageous when faced with unpredictable food resources.
Egg-failure rates of rockhopper penguins in this and
other studies (Williams 1980) were higher than those of
Magellanic (Spheniscus magellanicus), little and chinstrap (Pygoscelis antarctica) penguins (Reilly and Cullen
1981; Croxall et al. 1988; Boersma et al. 1990; Dann and
Cullen 1990). Nest failures in the current study occurred
primarily during incubation, generally around the time
of laying. Some previous studies have found peak egg
losses occurred in rockhopper penguins closer to laying
(Williams 1980; Lamey 1993), while others have found
them close to hatching (St Clair and St Clair 1996),
which may reflect a site or annual difference.
Rockhopper penguins in this study only ever fledged
one chick, as has been found previously (Warham 1975),
although rockhopper penguins on the Falkland Islands
have an average reproductive success of 0.72 chicks per
pair, and it was thought that it was not unusual that, at
least in some years, rockhopper penguins raised two
chicks (Pütz et al. 2001). Eleven percent of chicks in the
current study hatched from A eggs. Both A and B eggs
of rockhopper penguins are viable, although there are
inherent differences in their embryonic metabolism
(Williams 1980; Brown 1988). Only when the B egg is
lost do rockhopper penguins raise a chick from an A egg
(Gwynn 1953), which appears to be the case in this
study, and reinforces the contention that A eggs have
some insurance role for rockhopper penguins (St Clair
and St Clair 1996).
The majority (71%) of A and B egg losses in the
current study were due to great skuas (Catharacta skua).
Predation by skuas, aggressive behaviour among conspecifics and movements of newly hatched siblings have
also been listed as the most important causes of egg
failure in rockhopper penguins (St Clair and St Clair
1996). Individuals of this species may be more vulnerable to predation by skuas because of their: (1) smaller
size of eggs, which are presumably easier for predators
to remove; (2) small adult size, which reduces their
ability to defend their nest from predation as effectively
as larger species; (3) attempting to incubate two eggs; or
(4) the small size of this colony.
Reproductive success was similar in all 3 years of this
study, and higher than has been found in other penguins
(Carrick and Ingham 1970; Williams and Stone 1981;
Davis et al. 1989). Only royal penguins at a small westcoast colony on Macquarie Island, and rockhopper
penguins at Tristan da Cunha and the Falkland Islands
have exhibited a higher reproductive success (54, 51 and
35–61%, respectively, Williams and Stone 1981; Hindell
et al. 1995; Pütz et al. 2001).
Reproductive success, however, is not necessarily a
good indicator of survival, as some seabirds exhibit an
inverse relationship between chick growth and postfledging mortality up to the age of 2 years (Hamer et al.
1991). King-penguin chicks that fledged during a bad
year were resighted less than those fledged during a good
year, suggesting that their survival was near zero, or they
were delayed in returning to the colony (Olsson 1997). If
this relationship between fledging mass and survival of
chicks exists in rockhopper penguins, survival rates from
birds fledged during 1994/1995 could be lower than
during the other years of the study.
The lack of inter-annual differences in the majority of
breeding parameters suggests that the 3 years of this
718
study were similar in prey availability or abundance
during the chick-rearing period, or that adults were able
to compensate for any differences in prey stocks. Adults
returned to the colony with the lowest masses in 1993/
1994, but this did not affect their reproductive success or
the mass of fledging chicks. The quantity of food
brought ashore was higher during guard and crèche
stages and least digested during 1993/1994, compared to
the other years of this study. This indicates that prey was
more plentiful or accessible and located closer to the
colony later in the breeding season in this year (Hull
1999a). This contrasts with 1995/1996 when arrival
masses were highest and chick-fledging masses greatest.
The quantity of food brought ashore and other dietary
variables were similar in 1995/1996 to other years, but
birds undertook shallower and shorter dives with less
bottom time during this year (Hull 1999a, 2000). This
suggests that prey were more available at shallower
depths, increasing foraging efficiency and reducing foraging time (due to less bottom time, see Hull 2000).
As the quantity of food brought ashore was constant,
adults may instead have allocated more food for their
own maintenance when foraging.
The logistic regressions revealed that none of the
variables measured in this study explained hatching
success, but the position of the nest in the colony
explained fledging success during 1994/1995. Peripheral
nests may be more vulnerable to predation from great
skuas (Ainley et al. 1983; Frere et al. 1992; Emslie et al.
1995; Barbosa et al. 1997), while central nests may also
benefit from by-product mutualism of breeding neighbours (Murphy and Schauer 1996). The presence and
defence of eggs and nestlings by surrounding neighbours
can assist nest protection through predator swamping,
mutual vigilance or mutual defence (reviewed by Wittenberger and Hunt 1985).
As the logistic regressions did not successfully predict
hatching and reproductive success during most years,
other factors, such as food abundance and availability,
are probably important contributory variables that were
not assessed in these models (cf. Ashmole 1971; Boersma
et al. 1990; Hamer et al. 1991). In grey-headed (Diomedea chrysostoma) and black-browed (D. melanophris)
albatrosses, and gentoo (P. papua) and macaroni penguins, decreases in reproductive success have been correlated with smaller and less frequent meals brought
ashore, and therefore prey abundance or their accessibility, but the nature of the response varies with the
species (Croxall et al. 1999). The most direct and
straightforward response is found in species with a
specialised diet, which have little opportunity to preyswitch, or which have restricted access to prey (by either
restricted foraging ranges or limited feeding habitat).
However, rockhopper penguins may be unable to rear
chicks if they switch prey species. Cunningham and
Moors (1994) speculated that rockhopper penguins on
Campbell Island switched to a diet higher in fish, which
may be less suitable than a predominantly euphausiid
diet, resulting in lowered reproductive success.
Other important factors influencing a species’ ability
to cope with low prey abundance are the costs of foraging, whether the predator can modify an aspect of
their foraging, and the tolerance of chicks to fasting
(Croxall et al. 1999). However, the majority of reproductive failures in this study occurred during incubation,
and hence differences in prey abundance can only be
related to reproductive success if it affected the adults’
ability to continue the breeding attempt, particularly
during incubation.
Although there were subtle differences in some
breeding parameters, the overall high reproductive success found during these 3 years suggests they may have
been good ones where prey was relatively abundant. It is
possible that differences in reproductive success may not
be detected until a very poor season occurs, as is the case
with yellow-eyed (Megadyptes antipodes) and Magellanic Penguins (Boersma et al. 1990; van Heezik and
Davis 1990). Poor years may occur during El Niño
Southern Oscillation events, which have been found to
affect reproductive success in seabirds (Croxall et al.
1988), or during the Antarctic Circumpolar Wave, which
operates on a periodicity of 4–5 years (White and Peterson 1996; Guinet et al. 1998). Although differences
were found in sea-surface temperatures around Macquarie Island during this study (Hull et al. 1997; Hull
1999b), there were no large warm-water anomalies (see
Fig. 1 of White and Peterson 1996).
The high adult survival of rockhopper penguins, and
the consequent capacity to spread breeding over a long
lifetime are probably part of the adaptive strategy of
species facing highly variable environmental conditions
(Chastel et al. 1993). Assessment of the breeding
parameters during poor years will be instructive in
determining the relationship between aspects of the
breeding biology of these species and marine factors.
Rockhopper penguins are relatively restricted in their
foraging range (Hull 1999b); therefore, a decrease in
food availability could have substantial consequences on
reproductive success.
Acknowledgements We thank Melissa Giese and Di Moyle for
valuable comments on drafts of the manuscript, and the Antarctic
Scientific Advisory Committee, the SeaWorld Research and Rescue
Foundation and the Trans-Antarctic Association for their financial
support. Work was carried out under Macquarie Island special
permits MI/34/94, MI/3/95 and MI/13/96.
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