Pateke population trends and the impact of
predator control, Great Barrier Island
by John Ogden
In 2000, following
evidence of a numerical decline in pateke on Great Barrier Island,
the Department of Conservation instigated an audit of the brown teal
recovery programme, which recommended intensified management and
increased predator control1. A recent review of pateke
monitoring compares results before and after increased predator
control (c. 2000) on Great Barrier, and at Mimiwhangata in
Northland, giving numerical trends in controlled and uncontrolled
sites in both these areas2. The aim was to determine
whether changes in population trends could be linked to the predator
control established in the two areas. A secondary aim was to
establish the causes of pateke mortality.
In this article, I
review the data presented by Watts et.al, examine the trends in the
two pateke populations, and ask why the population on Great Barrier
has been recovering so slowly despite ongoing predator control at
Great Barrier Island data
The data presented
in the paper are averages of replicate counts at flock sites in
February or March, providing a comparable index of numbers. The data
presents some challenges for analyses due to varying count
techniques, different authors, different numbers of replicates in
different years, additions and deletions of flock counting sites as
monitoring has progressed, and the use of averages. The authors are
cognisant of these difficulties and have avoided questionable
statistical comparisons. The results are presented as simple graphs
of numbers versus time with fitted linear regressions for pre- and
The most notable
feature of the results is that the trapped (predator controlled)
area in Northland clearly benefited from predator control (pateke
numbers increased 162% compared with no change in un-trapped areas3,
which was not the case on Great Barrier. Trapping for cats and
periodic pukeko and rabbit reduction at Okiwi achieved only a modest
increase in numbers (36% over 15 years). This increase was matched
by a similar trend in the un-trapped Great Barrier sites, which
showed a 48% increase over the same period. These numerical trends
are probably not statistically different. They also do not allow a
conclusion that the slight post-2000 increase in pateke at Okiwi is
due to predator control, despite clearer results supporting such a
has halted – but why?
The positive result
for Great Barrier is that the pre-2000 decline, monitored by Dumbell4
and dismally ‘predicted’ by other authors5, has
apparently halted (Figure 1). This trend was also demonstrated by
the analyses in the 2010 State of the Environment Report6.
So, if predator control cannot be invoked as the cause for the
sudden change in teal demography about 2000, what was it that
reduced mortality everywhere on the island at that time?
When looking for a
factor equally affecting both trapped and un-trapped populations
(not statistically differently), the first suspect must be that
something changed in the data acquisition process in that year.
Figure 1. Annual
pateke flock counts at Great Barrier Island from 1985–1987 and
1994–2015 at; left, all flock sites; right, historical flock sites7.
Flock counts and regression trend lines at trapped sites are denoted
by • and solid lines, for untrapped sites by ∆ and dashed lines.
Trapping was implemented after the 2000 flock count (vertical line).
From Watts et al. 2016.
slowed the decline in pateke numbers before 2000, and something has
slowed recovery since.
personnel changed; was this a factor? Some factor may have reached a
critical limit, but, as the authors of this paper conclude, it’s
hard to pinpoint what that might have been, except for a change in
predation pressure – a factor elsewhere.
One difference in
predator control methods at Mimiwhangata and Okiwi is given little
attention. At Mimiwhangata, ground-application of brodifacoum and
sodium fluoroacetate (1080) was carried out (in five separate
years), while at Okiwi no toxins were used. At Okiwi, cat trapping,
pukeko and rabbit culls were rather irregular, responding to pest
outbreaks and dependent on staff availability – euphemistically
referred to as “maximum practical predator control”. Thus, although
some degree of pest control was maintained in both locations, the
‘trapped’ populations were treated rather differently. This
difference – in particular rat control (using toxins) at
Mimiwhangata – could explain the difference in predator control
results between the areas.
How effective is
Okiwi predator control?
One approach to
assessing the effectiveness of the control measures implemented by
the Department of Conservation at Okiwi is to examine correlations
between teal numbers and numbers of pukeko, cats and rabbits culled
each year. We might predict that years in which few predators were
trapped would be followed by declines in teal survival in that or
the following year. Conversely, if large numbers of cats and pukeko
were removed, an increase in pateke should occur. The cat and pukeko
data8 show no such correlations.
correlation is evident between deviations from averages in cat and
pukeko numbers, suggesting that years control measures had a big
impact on cats coincided with a similar impact on pukeko. Although
the results are strongly influenced by particular years, there is no
evidence from these figures that culling cats or pukeko resulted in
more teal surviving (Figure 2).
2. Cats, pukeko and pateke numbers at Okiwi, recorded between 2002 and
Causes of Pateke
Adult mortality may
be more important than that of eggs or juveniles in controlling
population growth rates9. Predators such as hawks, pukeko
and rats, probably predate nests and young birds more than adults.
The causes of death of radio-tracked pateke adults were reviewed by
Watts et al., who point out that the Great Barrier birds tend to be
lighter than those elsewhere; 16% of mortalities were associated
with low body fat, suggesting starvation. Compared to Mimiwhangata,
relatively few deaths were due to mammalian predation (5% vs 42%),
although this was expected due to the absence of mustelids on Great
the decline in pateke numbers before 2000, and something has slowed
harriers was hard to separate from other causes of mortality (where
birds were subsequently scavenged).
Overall though, the
data suggest that harrier predation could be significant. If so, and
as the authors suggest, the best approach would be to reduce rabbit
numbers as primary prey, and this would probably control harrier
Why only ‘slight
increases’ in pateke since 2000?
conclude that despite predator control efforts, the pateke
population on Great Barrier Island has shown only a slight increase
since 2000. This trend contrasts with the success of predator
control elsewhere, suggesting that predators – or rather – those
predators actually targeted – are not the problem on the island.
on Whangapoua Estuary. Rakitu in the distance. The estuary and
surrounding Okiwi basin is one of the strongholds for pateke in New
Could rats, have both
caused the decline, and be hindering the recovery? A better explanation
is hard to identify. No data of relevance exists on rat populations from
Okiwi, although we know rats have devastating effects on many bird
populations. Given this, surely rats should be given greater prominence
in the research (and predator control) programme?
The 2016 pateke count
was very low – down by about 200 birds on the previous year – despite
very good vegetation growth. Rats increased strongly throughout Great
Barrier, with anecdotal evidence being supported by data from Windy Hill10.
The authors also
conclude that non-predation factors related to habitat and food supply
for teal may be a factor. This conclusion is only weakly supported by
the data, although, again it’s hard to see what changed around 2000 when
mortality must have declined, or survivorship started a slow increase.
Overall, in view of the
evidence from elsewhere, I believe that predation is likely to be the
main factor on Great Barrier Island – and I am not convinced that the
current predator control regime is having the desired effect. Are
Northland, Coromandel and other Hauraki Gulf island’s pateke getting
more attention (pest control) than the original source population on
Great Barrier Island?
With the reduction in
Department of Conservation expertise on Great Barrier Island, this ‘gap’
in current control measures may now be a crucial factor in ensuring the
long-term survival of this population.
Photo: K. Waterhouse
Issue 36 Winter 2016
et al. 2000. Audit of brown teal recovery programme. Unpublished DOC
J., Maloney R., Keedwell R., Holzapfel A., Neill E., Pierce R., Sim J.,
Browne T., Miller N., Moore S. 2016. Pateke (Anas chlorosis)
population trends in response to predator control on Great Barrier
Island and Northland New Zealand. New Zealand Journal of Zoology.
Percentages are derived from the data in the Abstract of the Watts et
Dumbell, G. S. 1987. The ecology, behaviour and management of New
Zealand brown teal or pateke. Unpubl. PhD thesis. University of
Ferreira, S.M. & Taylor, S. 2003. Population decline of brown teal (Anas
chlorosis) on Great Barrier Island. Notornis 50: 141-147.
Barrier Island Charitable Trust 2010. State of Environment Report.
See Chapter 10, p.6 figure 3.
‘Historical flock sites’ are those which have been counted throughout,
thus giving directly comparable results.
and pukeko data for Okiwi was made available by Louise Mack.
Ferriera et al. [Endnote 6] conclude that population growth is more
sensitive to adult female mortality than it is to juvenile mortality.
Their results also demonstrate that for an average clutch of 5.31 eggs
(1999 Okiwi data) only 0.93 actually survive past fledging. So most
nesting pairs produce on average one or fewer young, while adults
themselves have little more than 50-65% probability of surviving for a
year, although of course some live much longer.
Personal communication. Judy
Gilbert, June 2016.