Biological Conservation 110 (2003) 33–44
www.elsevier.com/locate/biocon
Control of introduced mammalian predators improves kaka
Nestor meridionalis breeding success: reversing the decline of a
threatened New Zealand parrot
Ron Moorhousea, Terry Greeneb,*, Peter Dilksb, Ralph Powleslandc, Les Morana,
Genevieve Taylora, Alan Jonesb, Jaap Knegtmansb, Dave Willsc, Moira Prydeb,
Ian Fraserb, Andrew Augustc, Claude Augustc
a
Department of Conservation, Science and Research Unit, c/o Nelson-Marlborough Conservancy Office, Private Bag 5, Nelson, New Zealand
Department of Conservation, Science and Research Unit, c/o Canterbury Conservancy Office, Private Bag 4715, Christchurch, New Zealand
c
Department of Conservation, Science and Research Unit, PO Box 10420, Wellington, New Zealand
b
Received 10 September 2001; received in revised form 15 May 2002; accepted 28 May 2002
Abstract
The kaka (Nestor meridionalis) is a threatened, endemic New Zealand parrot that is declining primarily because of predation by
introduced mammals. Numbers of female kaka surviving to sexual maturity more than compensated for adult female mortality at
three sites with predator control but not at three unmanaged sites. Nesting success at the sites with predator control was significantly greater (580%) than at unmanaged sites (438%) while predation on adult females was significantly less (5% c.f. 65%).
Predation was the most common cause of nesting failure at all sites. Stoats (Mustela erminea) appeared to be the main predator,
although evidence of possum (Trichosurus vulpecula) predation on eggs, nestlings and nesting females was also found. These results
suggest that control of stoats and possums can potentially reverse the decline of the kaka on the main islands of New Zealand.
# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Psittaciformes; Conservation; Mustelids; Brush-tail possums; Rats; Poison-bait stations; Trapping
1. Introduction
The kaka (Nestor meridionalis) is an endemic New
Zealand parrot (Heather and Robertson, 1996). Formerly widespread and abundant in native forest
throughout the New Zealand archipelago (Buller, 1888),
it is now common in only a fraction of its former range
(Bull et al., 1985; Heather and Robertson, 1996; Wilson
et al., 1998). Forest clearance has destroyed a significant
amount of kaka habitat (Heather and Robertson, 1996).
However, the fact that the kaka has become rare within
some large tracts of indigenous forest suggests that
introduced predators and competitors, now widespread
on the main islands of New Zealand, have also played a
part in its decline (Beggs and Wilson, 1991; Wilson et
al., 1998).
* Corresponding author. Fax: +64-3-315-1388.
E-mail address: [email protected] (T. Greene).
Kaka breed between October and July, however,
breeding intensity is strongly influenced by the size of
the seed crops produced by certain forest trees, this
being the principle food of nestlings and fledglings
(Moorhouse, 1991; Wilson et al., 1998). For example, in
Big Bush in the northern South Island, Wilson et al.
(1998) found that kaka only attempted to breed when
red beech (Nothofagus fusca) produced seed, an event
that occurred, on average, every 2.3 years. Nests are
located in tree cavities and the female alone incubates
the eggs and broods the nestlings. The usual clutch and
brood sizes are 4 and 2 respectively (Moorhouse, 1991)
although up to 5 young can be fledged (personal observation). The incubation and nestling periods are 23 and
60 days respectively (Moorhouse and Greene, 1995) and
fledglings usually do not become independent for
another 5–6 months (Moorhouse and Greene, 1995;
Wilson et al., 1998). Pairs can fledge two broods in one
year if seed crops are sufficiently abundant (personal
observation).
0006-3207/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0006-3207(02)00173-8
34
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
In Big Bush Forest in the northern South Island,
Wilson et al. (1998) found that only two of 20 nesting
attempts monitored over an 11-year period were successful. Only five young fledged, while predators, probably stoats (Mustela erminea), killed four of seven radiotagged nesting females. By comparison, on stoat-free
Kapiti Island, 14 of 35 nesting attempts were successful
producing 22 young without any female mortality
(Moorhouse, 1991). Wilson et al. (1998) concluded that
the kaka is continuing to decline on the main islands of
New Zealand, primarily because of predation by stoats
on nesting females. This hypothesis is consistent with
the discovery that kaka populations elsewhere had male
biased sex ratios (Greene and Fraser, 1998).
Although stoat predation appears to be the main
reason for the kaka’s decline, other introduced predators may also contribute. For example, Norway rats
(Rattus norvegicus) preyed on kaka nestlings on Kapiti
Island (Moorhouse, 1991) and the more widely distributed (Innes, 1990) ship rat (Rattus rattus) may also
be a predator of kaka eggs and nestlings on the main
islands.
Competition with introduced species may also be a
factor in the decline of kaka. Possums and introduced
wasps (Vespula spp.) compete with kaka for food and
possums may also compete for nest sites (Beggs and
Wilson, 1991; Moorhouse, 1997; Wilson et al., 1998).
Kaka have declined on the South Island’s West Coast
following the invasion of this region by possums and
increased on Kapiti Island following the eradication of
possums there (Veltman, 2000). Although there are no
published records of possums preying on kaka, they are
a major egg predator of the glossy-black cockatoo (Calyptorhychus lathami) (Garnett et al., 1999), a similar-sized
(c. 400 g) Australian parrot (Forshaw, 1989).
Parrots are the focus of increasing international conservation concern. Collar and Andrew (1988) found
evidence to suggest that 21.4% (71 species) of the 330
extant parrot species were in danger of extinction with
another of 9% (29 species) potentially threatened.
Although the main threats to parrots are usually habitat
loss and poaching for the illegal pet trade (op. cit.),
there is evidence that introduced pests pose a serious
threat to the survival of insular species (Gnam and
Rockwell, 1991; Wilson, 1993; Garnett et al., 1999). The
role of introduced species in the decline of a variety of
New Zealand birds, including parrots, has been well
documented (Lloyd and Powlesland, 1994; Clout et al.,
1995; Elliott et al., 1996; McLennan et al., 1996; Wilson
et al., 1998; Innes et al., 1999).
If introduced predators are responsible for the continuing decline of kaka populations then it should be
possible to reverse this by controlling these pests. In this
paper we attempt to test this hypothesis by comparing
kaka breeding success at six sites; three with, and three
without, predator control.
2. Study areas
Data on kaka breeding success were obtained from six
sites, three with predator control and three without
(Fig. 1, Table 1).
2.1. Rotoiti Nature Recovery Project (RNRP)
The RNRP is the site of an ongoing ecosystem
restoration project designed to benefit a number of
native species including kaka (Butler, 1998). Possums
and ship-rats were controlled between June 1997 and
June 2000 using poison bait stations placed in a grid
pattern at 100–150 m intervals. Beginning in June 1997
these stations were first baited with 1080TM
(sodium monofluoroacetate) and then with TalonTM
(brodifacoum).
During the 1997 kaka breeding season individual
kaka nest-trees were protected from introduced predators by sheathing a section of their trunks with sheet
metal. In addition, a ring of 25 Mk VI FennTM traps
baited with rabbit meat were set at a radius of 25 m
around each nest tree. These methods were discontinued
in subsequent breeding seasons after stoat trap-lines had
been established around the perimeter of, and within,
the RNRP. Three-hundred and two Mk VI FennTM
traps have been deployed since July 1998 (cleared and
Fig. 1. Map of New Zealand showing location of kaka study-sites.
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
Table 1
Sites from which data on kaka breeding success were obtained
RNRP Eglinton WEA
Forest type Beecha Beecha
Area (ha) 825
13,000
Control
Yes
Yes
Rotoroa Big Bush Whirinaki
Podocarpb Beecha
1100
600
Yes
No
Beechc
c.500
No
Podocarpd
4880
No
RNRP=Rotoiti Nature Recovery Project, WEA=Waipapa Ecological Area. Beech=Nothofagus spp., podocarp=members of the Podocarpaceae. ‘‘Control’’ means predator control. Data on kaka breeding
success in Big Bush were obtained by Wilson et al. (1998) from 1984 to
1996; data from the other sites were collected by the authors between
1996 and 2000.
a
Personal observation.
b
Leathwick (1987).
c
Wilson et al. (1998).
d
Morton et al. (1984).
re-baited weekly) and have killed 183 stoats between
then and June 2000. Since 34 of a sample of 54 stoat
livers were found to contain brodifacoum (D. J. Butler,
unpublished data) an unknown proportion of stoats
may also have died of secondary poisoning. Two hundred and nineteen ship rats and six ferrets (Mustela
furo), other potential kaka predators, were also killed in
FennTM traps between July 1998 and June 2000.
35
stoat irruptions when they were serviced weekly. Possums have been controlled with 1080TM dispensed from
a line of poison bait stations running along the valley
floor since 1994.
2.4. Rotoroa
This study-site was located 20 km from the RNRP at
the southeastern corner of Lake Rotoroa (Fig. 2). There
was no predator control at this site.
2.5. Whirinaki
This study-site lies about 100 km from the WEA
(Fig. 1). Although there was no systematic predator
control at Whirinaki during the period of our research,
recreational possum hunting did occur.
2.6. Big Bush
This unmanaged study-site (c. 500 ha) was less than 2
km from the RNRP (Fig. 2) (for a detailed description
see Wilson et al., 1998).
3. Methods
2.2. Waipapa Ecological Area (WEA)
3.1. Finding and monitoring nests
The WEA (4013 ha, Fig. 1) is the site of an ongoing
ecosystem restoration project similar to that in the
RNRP. Kaka research was conducted in an 1100 ha
study-site within the WEA. Control of possums with
poison bait stations began in late 1993 and was expanded in December 1995. Stations are arranged on a 150m grid and were initially baited with 1080TM followed
by applications of TalonTM possum bait. From 1996 to
1997 anticoagulent toxins (TalonTM PB, TalonTM 50
WB, VenomTM B and Pest OffTM) were used, while during 2000–2001 a combination of 1080 in cereal baits and
WarfarinTM (in peanut butter paste and cereal pellets)
were used. Some control of mustelids probably occurred
through secondary poisoning (Murphy et al., 1998).
Between 13 and 54 adult kaka were radio-tagged at
each study site (total=178). By radio-tracking these
birds the outcome of between 10 and 31 nesting
attempts (113 in total) was determined at each site. With
the exception of the WEA, where nests were sometimes
checked at intervals of up to one month, at all other
study-sites nests were checked every 1–7 days. At each
study-site a sample of up to 10 nestlings was radio-tagged each season to allow estimation of post-fledging
survival. At the RNRP and Rotoroa, but not at the
other study-sites, the sex of radio-tagged fledglings was
determined using the measurement criteria described in
Moorhouse et al. (1999).
2.3. Eglinton Valley
3.2. Predator identification
The Eglinton Valley is the site of a stoat and possum
control programme originally intended to protect a
remnant population of Mohua (Mohua ochrocephala), a
hole-nesting native passerine (O’Donnell et al., 1996). A
40-km long stoat trap-line running the length of the
valley was in continuous operation from January 1998
to May 2001. This was comprised of 193 trapping stations spaced at 200 m intervals, a station being a wooden tunnel (200200600 mm) containing two Mk VI
FennTM traps with bait placed between them. Traps
were checked and re-baited monthly, except during
At Big Bush predators were identified by the caching
of kaka remains (stoats and ferrets are the only predators in New Zealand likely to cache prey the size of
adult or fledgling kaka; King, 1990; Lavers and Clapperton, 1990) or sightings of predators in the nest, or
nest-tree, after predation had occurred (Wilson et al.,
1998). At the other study-sites, kaka corpses that
showed signs of predation were also examined for
mammalian hair and dissected to reveal canine punctures in flesh and bone. The distance between paired
canine punctures was measured for comparison with the
36
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
Fig. 2. Location of the Rotoiti Nature Recovery Project, Big Bush and Rotoroa study-sites. Shaded areas are forest, unshaded areas are grassland,
or lakes (labelled), hatched areas are study-sites, solid lines are streams or rivers, double solid lines are roads.
intercanine distances of different mammalian predators
given in Ratz and Moller (in press).
At sites other than Big Bush, hair found on kaka
corpses or in nests where birds had been preyed on was
identified by its species, or genus-specific, scale pattern
(Day, 1966). Dead birds that showed no evidence of
predation were sent to a veterinary laboratory for
autopsy. The remains of eggs that had been preyed on
were compared to the photographs of feeding sign on
eggs left by different predators in Brown et al. (1996).
greater productivity; sites with predator control produced twice the number of fledglings per nest as the
most productive unmanaged site (Table 2).
Because of the close proximity of Big Bush and the
RNRP, the Big Bush data provide an estimate of kaka
nesting success before the implementation of predator
control in the RNRP. Nesting success in the RNRP was
significantly higher than it was in Big Bush irrespective
of the method of predator control employed (1-tailed
Fisher’s Exact test; RNRP 1997 vs Big Bush, P=0.001;
RNRP 1998–1999 vs Big Bush, P < 0.001).
4. Results
4.2. Predator abundance
4.1. Breeding success
If, by chance, the managed sites always had significantly lower predator numbers than the unmanaged
sites, then this, rather than predator control, might
explain the higher nesting success observed at these
sites. With respect to stoats, we only have comparable
abundance data from three study-sites, Big Bush, the
RNRP and Eglinton (Fig. 3). Because of the relatively
small number of trap-nights at Big Bush (150, c.f. 9060
in the RNRP and 11,580 in Eglinton) these data are too
Nesting success (% successful nests) at sites with predator control was significantly higher than at sites
without it (Table 2). Logistic Regression indicates that
predator control significantly increased nesting success
(Wald statistic=5.993, df=1, P=0.014) while siteeffects were insignificant (Wald statistic=4.837, df=4,
P=0.304). Higher nesting success was manifested in
37
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
imprecise to show if stoats were more, or less, abundant
there than they were in the RNRP and Eglinton (Fig. 3).
Despite their wide geographic separation, seasonal fluctuations in stoat abundance in the RNRP and Eglinton
were very similar and relatively high stoat numbers
occurred at both sites in the summers of 1999 and 2000
(Fig. 3).
With respect to possums the available data shows that
these declined in abundance at all three managed sites
following the implementation of possum control and
that their pre-control abundance in the WEA was similar to that in Whirinaki (Fig. 4). The higher possum
abundances recorded in Whirinaki and the WEA
relative to Eglinton and the RNRP probably reflect the
greater carrying capacity of podocarp compared to
beech forest (Cowan, 1990).
There was no significant difference in rat abundance
between the RNRP and Rotoroa and temporal trends
between these two sites were similar (Fig. 5).
4.3. Causes of nesting failure
If predator control was responsible for the higher
nesting success observed at managed sites then predation would be expected to have been the primary cause
of nest failure at unmanaged sites. The most common
cause of nesting failure at unmanaged sites was egg
mortality (Table 3). Most such failures (12 of 20)
occurred in Big Bush where it was not clear if females
did not get enough food to sustain incubation, deserted
the nest after being disturbed by predators, or the clutch
had been preyed on (P. R. Wilson, personal communi-
Table 2
Comparison of kaka breeding success at sites with (bold type), and without, predator control
No. of nesting attempts
No. successful nests
% Successsful nests
95% Confidence limits
No. chicks fledged
No. chicks fledged/nest
RNRP
WEA
Eglinton
Big Bush
Rotoroa
Whirinaki
14
12
86
68–100
35
2.5
31
27
87
75–99
70
2.3
25
20
80
64–96
55
2.2
20
2
10
1–31
5
0.25
10
1
10
0–45
4
0.4
13
5
38
11–65
14
1.1
Big Bush data are from Wilson et al. (1998). Data were collected between 1996 and 2000 except for Big Bush (1984–1996).
Fig. 3. Relative abundance of stoats at three kaka study-sites. The maximum 95% confidence limits are shown for Big Bush data; 95% confidence
limits for RNRP and Eglinton are <0.2.
38
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
cation). However, in the 11 such cases recorded at other
sites eggs had either preyed on, or removed, from nests.
In most cases there was no evidence to suggest what
had eaten, or removed, the eggs. However, stoat,
possum and ship-rat hair was found in three nests in
which eggs had been preyed on and eggshell remains in
a fourth closely resembled possum feeding sign.
Although we can’t be sure that the hairs found did not
Fig. 4. Relative abundance of possums at five kaka study-sites and the maximum 95% confidence limits for each site. Arrows indicate when possum
control began at each site.
Fig. 5. Relative abundance of ship rats at three kaka study-sites. Maximum 95% confidence limits are shown for the RNRP and Rotoroa; 95%
confidence limits for Eglinton are <0.2.
39
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
pre or post-date the actual predation or scavenging, hair
from these predators was not found in litter samples
from nine successful nests.
Predation of nesting females was the next most common cause of nesting failure (Table 3). Only two of the 15
radio-tagged females that died during the course of this
study (13%) showed no evidence of having been killed by
a predator. Of the remainder, apart from one bird that
appeared to have been shot by a poacher and two that
were killed by unknown predators outside the breeding
season, the remaining 10 displayed injuries consistent with
predation and their remains were found either in their
nest chamber or cached underground near their nest tree.
Although both stoats and ferrets are large enough to
cache an adult female kaka, only stoats have sufficient
climbing ability to reach most kaka nests (King, 1990;
Lavers and Clapperton, 1990). Evidence of stoat predation (caching of remains, sightings of stoats in the nesttree and tooth punctures of the appropriate size and
spacing) was found in 8 of the 10 cases of nesting female
mortality. In the other two cases the females’ remains
displayed tooth-punctures larger than could be inflicted
by a stoat and possum fur was found in one nest chamber.
We suspect that possums killed these birds.
The incidence of predation on nesting females was
significantly higher at sites without predator control;
65% (11/17) of radio-tagged breeding females were killed by predators at unmanaged sites compared to 5%
(2/38) at sites with predator control (Fisher’s Exact
Test, P=0.001).
Nestling mortality was the third most common cause
of nesting failure (Table 3). In all cases the entire brood
was found dead with injuries consistent with having
been either preyed on or scavenged. Evidence of stoat
predation, or scavenging (caching of dead nestlings,
sightings of stoats in the nest-tree or tooth punctures of
the appropriate size and spacing), was found in five of
the seven instances of brood failure. One of the
remaining two broods appeared to have been preyed on
by a possum while the predator, or scavenger, of the
third could not be identified.
4.4. Post-fledging survival
The survival of a sample of radio-tagged fledglings
was monitored at all sites but their sex was determined
only at the RNRP and Rotoroa. Because so few fledglings were produced at the unmanaged sites estimates of
fledgling survival are not precise enough to allow
meaningful comparison between unmanaged and
managed sites (Table 4). The remains of 20 of the 25
fledglings that have been confirmed dead (80%) displayed injuries consistent with either having been preyed
on or scavenged. Evidence of mustelid predation, or
scavenging (caching, tooth punctures of the appropriate
size and spacing, hair), was found in 13 (65%) of these
cases. Although no diagnostic mustelid sign was found
on the other remains, this cannot be ruled out since all
were missing body parts and in two cases only the
fledgling’s bloodstained transmitters were found.
Table 3
Relative incidence of different kinds of nesting failure in kaka at sites with and without predator control
Predator control (n=70)
No predator control (n=43)
Egg mortality
Female killed
Nestling mortality
Non-predation
Total
4%
46%
3%
21%
3%
12%
6%
5%
16%
84%
‘‘Female killed’’ means that the female was preyed on while nesting. Nesting attempts in which females died have been treated as a separate
category to other causes of nesting failure. ‘‘Egg mortality’’ means that eggs failed to hatch for unknown reasons, or, because they were preyed on.
‘‘Nestling mortality’’ means that nestlings were either preyed on or scavenged. ‘‘Non-predation’’ includes all causes of failure other than predation
(e.g. flooding).
Table 4
Survival of radio-tagged kaka fledglings to 1 year at sites with (bold-type), and without, predator control
Study-site
RNRP
WEA
Eglinton
Big Bush
Rotoroa
Whirinaki
No. radio-tagged
No. confirmed dead
No. not located
No. confirmed alive
No. surviving females
% Surviving
95% Confidence limits
30
12
3
15
6
50
31–69
18
7
0
11
c. 5
61
38–84
13
0
0
11
c. 5
85
65–100
5
1
2
2
c.2
40
0–84
4
1
2
1
1
25
0–68
7
4
0
3
c. 1
43
6–80
Big Bush data are from Wilson et al. (1998). Data were collected between 1996 and 2000 except for Big Bush (1984–1996). Numbers of surviving
female fledglings shown for the WEA, Eglinton, Big Bush, and Whirinaki assume a 50:50 sex ratio at fledging.
40
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
At Big Bush, the WEA, Rotoroa and Whirinaki all
fledgling mortality occurred within 10 days of fledging.
Kaka fledglings are vulnerable to predators during this
period because they are inept fliers and spend most of
their time on the ground (Moorhouse and Greene, 1995;
Wilson et al., 1998). However, in the RNRP all but two
of the 12 cases of fledgling mortality occurred at least 35
days after fledglings had left the nest, long after fledglings are capable fliers and spend virtually no time on
the ground (Moorhouse and Greene, 1995). Causes of
mortality other than predation included three cases of
apparent starvation in the WEA plus one case of disease
and one of brodifacoum poisoning in the RNRP. In the
last case, brodifacoum was detected in the fledgling’s
liver and it had suffered extensive haemorrhaging in the
alimentary tract, a characteristic indication of anticoagulant poisoning. The livers of two of the 10 fledglings
that appeared to have been killed by stoats in the RNRP
were recovered for toxicological analysis. One of these was
also found to contain sub-lethal levels of brodifacoum.
4.5. Relative importance of different predators
Considering all mortality in which predators were
implicated, evidence of stoat predation was found in
30% of cases and evidence of stoat or ferret predation in
another 15% (Table 5).
4.6. Demographic significance of the results
For predator control to be of any benefit to kaka
conservation the difference in nesting success between
managed and unmanaged sites must be demographically
as well as statistically significant; the recruitment of
young females at managed sites must exceed adult
female mortality. Since free-living female kaka can
breed at 1 year of age (K. Barlow, personal communication) we can estimate potential recruitment by estimating the survival of female fledglings to 1 year.
Because we only have a relatively reliable estimate of
female fledgling survival for the RNRP we have used
this to estimate the potential number of female recruits
at the other sites. We did this by assuming that half of
the total number of fledglings produced at each site was
female and then multiplying this number by the proportion of female young to survive to 1 year in the RNRP.
Since the RNRP had the lowest fledgling survival of
the three managed sites our estimates of potential
female recruitment for the other managed sites should
be conservative. Nonetheless, estimated potential female
recruitment significantly exceeded adult female mortality at all three managed sites but at none of the unmanaged sites (Fig. 6). In fact, the reverse was true at two
of the unmanaged sites while at the third, Whirinaki, the
difference between adult female mortality and recruitment was insignificant (Fig. 6).
This analysis does not address the minimum area over
which predator control operations need to be conducted
to benefit kaka. Virtually all the young fledged within
the WEA’s kaka study-site (1100 ha) have remained
there indicating that this area was sufficient to allow
recruitment into managed habitat. However, most
fledglings produced within the RNRP (825 ha) and the
Eglinton (13,000 ha) have dispersed into surrounding
unmanaged habitat suggesting that neither of these sites
are big enough to contain most of the young fledged
within them. This variability in the mobility of kaka
fledglings from site to site makes it impossible to provide a generalised estimate of the minimum area over
which predator control needs to be conducted. A computer model (Elliott et al., in preparation) is being
developed to estimate minimum area requirements for
specific populations.
5. Discussion
Although further work is required to establish the
minimum area over which predator control should be
conducted, our results indicate that poison bait-station
grids, either alone or in combination with FennTM
trap-lines, can potentially reverse the decline of kaka
populations.
Since stoats are both ubiquitous (King, 1990) and an
important predator of kaka, the high nesting success
observed in the WEA in the absence of stoat-trapping
seems anomalous. However, King et al. (1996a,b) found
that stoats were relatively rare in the WEA before the
Table 5
Relative frequency of predation by different predators on kaka clutches, nesting females, broods and fledglings
Suspected predator
Clutches
Females
Broods
Stoat
Stoat or ferret
Possum
Rat
Unknown
1
0
2
1
19
8
0
2
0
0
5
0
1
0
1
Total
23
10
7
Fledglings
Total
%
4
9
0
0
7
18
9
5
1
27
30
15
8
2
45
20
60
–
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
beginning of predator control and that they did not
display the marked seasonal peaks of abundance recorded in beech forest. There is also considerable evidence
that stoats can be effectively controlled (100% mortality
of resident radio-tagged animals) by secondary poisoning following 1080TM or brodifacoum operations for
possums or rats (Alterio et al., 1997; Brown et al., 1998;
Gillies and Pierce 1999; Murphy et al., 1999; Alterio,
2000). Since one of these toxins has always been in use
in the WEA this could also have contributed to the low
incidence of stoat predation on kaka there.
5.1. Generality
Although our results came from study-sites that were
both far apart and located in different forest types, the
low number of replicates limits their generality. This is
particularly so with respect to podocarp forest which
comprised only two of the six study-sites. The evidence
for the effectiveness of predator control in beech forest
is better, both because of the greater number of replicates and the ‘‘before’’ and ‘‘after’’ comparison
provided by the Big Bush and RNRP data. The proximity and similarity of these sites provides good evidence
that predator control can improve kaka breeding
success in beech forest.
41
5.2. Predator abundance
With the exception of possums in the WEA, the
available data on predator abundance are insufficient to
establish if predator numbers at managed sites were
lower than at unmanaged sites. However, we think such
bias to be unlikely. With respect to stoats, the close
proximity of Big Bush and the RNRP together with the
relatively long periods during which kaka breeding success was monitored at both sites argues against the
possibility of significant differences in stoat abundance
between them. Futhermore, the stoat abundance data
from the RNRP and Eglinton show the order of magnitude increases in abundance that typically occur in
beech forests in the year following a beech-seed year
(King, 1983). As one of these peaks coincided with a
kaka breeding season our estimates of breeding success
for these sites include at least one season during which
stoat numbers were relatively high.
Since we have no data on stoat abundance for the two
podocarp forest sites and these are relatively far apart, it
is possible that the WEA had significantly fewer stoats
than Whirinaki. King et al. (1996a) found that stoat
densities in North Island podocarp forest appeared to
be lower and to display much less marked seasonal
fluctuations than in beech forest. This may explain the
Fig. 6. Estimated female recruitment and estimated adult female mortality at kaka study-sites with (bold type) and without predator control.
Confidence limits are 77% since this gives a 0.05 probability of overlap between recruitment and mortality estimates. Data were collected between
1996 and 2000 except for Big Bush (1984–1996). RNRP=Rotoiti Nature Recovery Project, WEA=Waipapa Ecological Area, EGTN=Eglinton,
BB=Big Bush, RTR=Rotoroa, WNKI=Whirinaki. Numbers in parentheses indicate the number of nesting attempts monitored at each site.
42
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
relatively high nesting success of Whirinaki relative to
the other unmanaged sites (Table 2).
5.3. The evidence for predation as the main cause of nest
failure at unmanaged sites
Although we cannot exclude the possibility that some
clutches or broods may have been abandoned and then
scavenged, the fact that not a single failed clutch or
brood was found intact suggests that predation was the
primary cause of nesting failure. With respect to the
mortality of nesting females observed in this study, the
absence of a single instance of such mortality in 35
nesting attempts on stoat and possum-free Kapiti island
(Moorhouse, 1991) strongly suggests that introduced
predators killed these birds.
Causes of mortality other than predation were relatively unimportant. However, the fact that one of the
three dead fledglings in the RNRP whose livers were
available for analysis had almost certainly died of brodifacoum poisoning while another had ingested sublethal levels of brodifacoum, raises the possibility that
fledglings may have been scavenged after dying of brodifacoum poisoning, or have been rendered more
vulnerable to stoats after eating this toxin.
Since kaka fledglings do not begin to forage for
themselves until about 3 months after fledging (Moorhouse and Greene, 1995), this could explain why fledglings tended to die later in the RNRP relative to other
study-sites. The discovery that kaka fledglings are vulnerable to brodifacoum poisoning obviously has serious
implications for pest management in New Zealand.
Since estimated female recruitment exceeded estimated
adult female mortality in the RNRP and no evidence of
kaka fledglings being poisoned by brodifacoum (or any
other toxin) was found at the other managed sites,
the benefits of brodifacoum would appear to outweigh
the risks. Nonetheless, it would be preferable to
investigate alternative forms of pest control, kaka-proof
bait stations, or baits that are unattractive to kaka
fledglings.
5.4. Relative importance of different predators
Although little evidence was found to identify the
predators of eggs, stoats were implicated in most cases
of predation on nesting females and nestlings, and
stoats and ferrets were implicated in most cases of
fledgling mortality. Population modelling (Seal et al.,
1993) suggests that predation of adult females is the
most damaging to kaka populations. Consequently, the
significantly lower incidence of such predation at sites
with predator control is an important result. Although
possums are known to prey on other native bird species
(Brown et al., 1993; Innes et al., 1999), the evidence
found in this study that possums preyed on kaka eggs,
nestlings and nesting females is, to the best of our
knowledge, the first. Although the sample size is too
small to reliably estimate the relative importance of
such predation, since possums are now both widespread
and abundant (Cowan, 1990) it could be significant.
The single instance of poaching recorded during this
study occurred at Whirinaki where there is an isolated
human settlement in which many of the residents have a
long tradition of hunting forest birds for food. We suspect that the poaching of kaka is a relatively rare
occurrence and is confined to such localities.
6. Conclusion
6.1. Implications for management
Although a variety of predator control methods were
involved in this study it is possible to make some general recommendations. Poison baits, either 1080TM or
brodifacoum, were used to control possums and rats at
all managed sites and may also have controlled stoats
through secondary poisoning. Consequently, despite the
apparent risk to kaka fledglings from brodifacoum, we
recommend the use of these, or similar toxins, to control
pest species in kaka habitat. Poison baits should be dispensed from bait stations spaced at 100–150 m intervals.
The fact that most fledglings dispersed from the
RNRP suggests that a managed area of 825 ha is insufficient for recruitment into managed habitat in homogenous beech forest. The dispersal of juvenile kaka from
the Eglinton suggests that even 13,000 ha may not be
big enough in more heterogeneous beech forest where
kaka travel long distances to localised food sources. In
the absence of other information we can only recommend that predator control in homogenous beech forest
be conducted over an area of at least 1600 ha as suggested by computer simulation of the RNRP population
(Elliott et al., in preparation). With respect to podocarp
forest, we recommend that predator control be conducted over an area at least as large as the WEA’s kaka
study-site (1100 ha) since this was evidently large
enough to contain a self-recruiting population.
We would also recommend the use of FennTM traplines to control stoats in beech forest. These can be
deployed as a perimeter ring with additional internal
lines as in the RNRP, or as a single transect along a
valley floor as in the Eglinton. Since kaka are most at
risk of predation when nesting, if kaka are the sole focus
of conservation management predator control need only
be done in breeding years. However, it is presently only
possible to reliably predict breeding years in beech forest. Since kaka are long-lived, once populations have
recovered it should also be possible to suspend predator
management for several years. This kind of ‘‘pulsed
management’’ appears to be a cost-effective way of
R. Moorhouse et al. / Biological Conservation 110 (2003) 33–44
conserving a population of kokako (Calleas cinerea;
Basse et al., in press).
6.2. Further research
Because kaka are not in imminent danger of extinction (there are strong populations on several offshore
islands and captive bred birds have been successfully
reintroduced to the wild; K. Barlow, personal
communication), there is scope for additional research
to identify exactly what kinds of predator management
are most effective. Apart from determining the minimum area over which predator control operations
should be carried out, it is also important to establish if
either poison baits or trapping alone might be sufficient
to conserve kaka in beech forest. Alternatives to brodifacoum or safer ways of dispensing this toxin should
also be investigated. The recent discontinuation of
1080TM and brodifacoum use in the RNRP should help
answer these questions.
Acknowledgements
We thank the New Zealand Department of Conservation’s St Arnaud and Murupara Area Offices, and
Pureora and Te Anau field centres, for logistic support.
Joe Hayes, Mike North, Jacqueline van Haal, Kirsty
Moran, Andy Blick, Karin Ludwig and Keryn Squires
assisted with fieldwork. John Bristow, Capral New
Zealand Ltd., donated the aluminium used to protect
kaka nest-trees. We are grateful to: Graeme Elliott for
statistical advice and Graeme Elliott, David Butler,
Peter Wilson, Jaqueline Beggs and Kath Walker for
valuable discussion on many aspects of this work;
Elaine Murphy for identifying predator hairs; Craig
Gillies for identifying predator sign on kaka corpses and
Chris Edkins for drafting the figures. Finally, we thank
Don Newman, Brenda Greene, Kate McInnes and two
anonymous referees for constructive comments on
earlier drafts of this manuscript.
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