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Reintroducing threatened falcons into vineyards reduces bird-damage to wine grapes

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Background/Question/Methods Land use intensification is driving range reduction and even extinction of many iconic species, despite the potential ecosystem services provided by these species. Although it is well documented that conserving natural enemies of insect pests may provide direct biological control benefits, comparatively little research has examined the benefits of protecting natural enemies of vertebrate pests. In vineyards, pest birds directly reduce yield by feeding on grapes, and reduce wine quality through increased fungal infection on pecked bunches. In order to assess a joint conservation/ pest management project that reintroduced the threatened New Zealand falcon, Falco novaeseelandiae, into vineyards, we estimated pest bird abundance and quantified grape damage in vineyards containing resident falcons and vineyards without falcons. Results/Conclusions We found that falcon presence significantly decreased the number of grape-removing introduced European pest species, and hence the incidence of grape removal and overall pest bird damage. Falcons did not affect the number of native silvereyes, Zosterops lateralis, which peck grapes and cause fungal infection, but falcons did reduce the amount of pecking found on grapes. Our results indicate that reintroducing native birds of prey into vineyards can reduce both pest bird abundance and grape damage, resulting in considerable savings for the vineyards while protecting an endemic species.
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Conservation Practice and Policy
Effects of Introducing Threatened Falcons
into Vineyards on Abundance of Passeriformes
and Bird Damage to Grapes
SARA M. KROSS,JASON M. TYLIANAKIS, AND XIMENA J. NELSON
School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
Abstract: Agricultural landscapes are becoming an important focus of animal conservation, although ini-
tiatives to conserve predators to date have rarely provided economic benefits to agricultural producers. We
examined whether introduction to vineyards of the New Zealand Falcon (Falco novaeseelandiae),aspecies
listed as threatened by the New Zealand Department of Conservation, affected the abundance of 4 species of
Passeriformes that are considered vineyard pests or affected the amount of economic loss due to grape (Vitis
vinifera) damage. Three of the species were introduced and remove whole grapes from bunches (Blackbird
[Turdus merula], Song Thrush [Turdus philomelos], and Starling [Sturnus vulgaris]), whereas the one native
species (Silvereye [Zosterops lateralis]) pecks holes in grapes. The introduction of falcons to vineyards was
associated with a significant decrease in the abundance of introduced passerines and with a 95% reduction
in the number of grapes removed relative to vineyards without falcons. Falcon presence was not associated
with a change in the number of Silvereyes, but there was a 55% reduction in the number of grapes pecked in
vineyards with falcons. Our results indicate that, relative to damage in vineyards without falcons, the presence
of a falcon could potentially result in savings of US$234/ha for the Sauvignon Blanc variety of grapes and
$326/ha for Pinot Noir variety of grapes.
Keywords: biological control, ecosystem service, IPM, pest management, raptors, threatened species
Efectos de la Introducci´
on de Halcones Amenazados en Vi˜
nedos Sobre la Abundancia de Passeriformes y el Da˜
no
de Aves a Uvas
Resumen: Los paisajes agr´
ıcolas se est´
an convirtiendo en un objetivo importante para la conservaci´
on de
animales, aunque a la fecha las iniciativas para conservar depredadores raramente proporcionan beneficios
econ´
omicos a los productores agr´
ıcolas. Examinamos si la introducci´
on en vi˜
nedos de Falco novaeseelandiae,
una especie enlistada como amenazada por el Departamento de Conservaci´
on de Nueva Zelanda, afect´
ola
abundancia de 4 especies de Passeriformes que son consideradas plagas en los vi˜
nedos o afect´
o la cantidad de
p´
erdida econ´
omica debido a da˜
nos a la uva (Vitis vinifera). Tres de las especies son introducidas y remueven
uvas enteras (Turdus merula, Turdus philomelos ySturnus vulgaris, mientras que una especie nativa, Zosterops
lateralis, picotea los frutos. La introducci´
on de halcones en los vi˜
nedos se asoci´
o con una disminuci´
on signi-
ficativa de la abundancia de las especies introducidas y con una reducci´
on de 95% en el n´
umero de uvas
removidas en comparaci´
on con vi˜
nedos sin halcones. La presencia de halcones no se asoci´
o con cambios en
el n´
umero de Zosterops lateralis, pero en los vi˜
nedos con halcones hubo una reducci´
on de 55% en el n´
umero
de uvas picoteadas. Nuestros resultados indican que, en relaci´
on con el da˜
no en vi˜
nedos sin halcones, que la
presencia de un halc´
on potencialmente resulta en ahorros de US$233/ha para la variedad de uvas Sauvignon
Blanc y de $326/ha para la variedad Pinot Noir.
email saramaekross@gmail.com
Paper submitted November 4, 2010; revised manuscript accepted June 21, 2011.
142
Conservation Biology, Volume 26, No. 1, 142–149
C
2011 Society for Conservation Biology
DOI: 10.1111/j.1523-1739.2011.01756.x
Kross et al. 143
Palabras Clave: control biol´
ogico, especies amenazadas, manejo de plagas, MIP, rapaces, servicios del
ecosistema
Introduction
Conservation has been viewed traditionally as an en-
deavor separate from agriculture (Green et al. 2005;
Perrings et al. 2006). However, recently ecologists have
examined whether biological control of pests may pro-
vide an incentive for the conservation of certain species
within agricultural systems (Daily et al. 2000; Tilman et al.
2002). Conservation of predators can be a successful
and sustainable approach for the control of many insects
considered to be pests. Therefore, substantial research
has focused on the management of habitats of predatory
arthropods that reduce the abundance of arthropod pests
(Chiverton & Sotherton 1991; Landis et al. 2000). Efforts
to augment natural populations of predatory arthropods
often represent additional costs to landowners because
land is taken out of production or yields are reduced (Fo-
ley et al. 2005; Green et al. 2005), and these predators
are seldom classified as threatened by conservation orga-
nizations. The ability of vertebrates to control arthropod
agricultural pests has received much less attention, de-
spite evidence that predators such as birds and lizards can
effectively reduce damage to agricultural crops caused
by their prey (Borkhataria et al. 2006; Kellermann et al.
2008). Moreover, when the agricultural pests themselves
are vertebrates, control methods rarely focus on the
preservation of natural predators because the predators
of vertebrates tend to be large carnivorous species that
are difficult to contain and rarely specialize on a single
prey species (Hoddle 1999).
Vineyards are particularly vulnerable to predation by
Passeriformes because ripening grapes (Vitis vinifera)
represent an abundant food source for these birds in
late summer and autumn (Somers & Morris 2002; Tracey
& Saunders 2003; Saxton et al. 2004). For example,
in Marlborough, New Zealand’s largest wine-growing
region, 3 species of introduced European birds remove
whole grapes from bunches and 1 native species pecks
holes in grapes to drink the juice from within, which ex-
poses the grapes to fungal infection (Tracey & Saunders
2003; Saxton et al. 2004). The 3 introduced species are
also known as dispersers of non-native, invasive, fruiting
plants (Williams & Karl 1996).
To mitigate grape damage, viticulturalists use acoustic
and physical bird deterrents and kill birds. However, com-
mercial deterrents are often expensive, their efficacy may
be exaggerated by advertisers (Fukuda et al. 2008), and
some methods may even increase the amount of dam-
age to grapes (Bomford & Sinclair 2002). Furthermore,
even when physical or acoustic deterrents are used, birds
can damage up to 83% of a vineyard’s crop (Tracey &
Saunders 2003).
Trained falcons are sometimes used to remove avian
pests from areas such as airports and landfills (Baxter &
Allan 2006; Soldatini et al. 2008), and artificial perches
(Wolff et al. 1999) and nest boxes (Meyrom et al. 2009)
have been used to attract wild birds of prey into some agri-
cultural areas to reduce the abundance of rodents. These
efforts demonstrate that captive or wild birds of prey can
reduce pest abundance. Using the New Zealand Falcon
(Falco novaeseelandiae) as a case study, we compared
the abundance of birds considered to be pests and the
levels of grape damage in vineyards with resident falcons
(introduced for conservation) with vineyards without
falcons.
Methods
Falcon Introduction
The New Zealand falcon is the country’s only remain-
ing endemic bird of prey. The population size and dis-
tribution of falcons decreased considerably after the ar-
rival of human settlers (Fox 1977; Gaze & Hutzler 2004),
and the species is now classified as threatened by the
New Zealand Department of Conservation (Miskelly et al.
2008). The Falcons For Grapes project has been relocat-
ing wild New Zealand Falcon chicks from their nests
in the mountains to the vineyards of Marlborough since
2005 (Fox 2005). No falcons, or other passerine-hunting
raptors, occurred in this region prior to the relocations.
Relocated falcons are provided with supplementary food
and their nests are protected from mammalian predators.
There are no a priori criteria for selection of vineyards
into which falcons are introduced.
We selected 6 vineyards in which falcons had been
introduced and 6 vineyards in which falcons had not
been introduced (controls). Treated and control vine-
yards were interspersed spatially, and edges of vineyards
were a minimum of 4 km apart (Supporting Information).
All vineyards were managed using common commercial
(not organic) methods for spatially extensive viticulture
that were approved by Sustainable Winegrowing New
Zealand. We trained workers in control vineyards in fal-
con identification and asked them to report any falcon
sightings over the 2 years of the study.
Vineyard Characteristics
As of 2009, 23,600 ha were planted in wine grapes in the
Marlborough region, with a mean vineyard size of 31 ha
Conservation Biology
Volume 26, No. 1, 2012
144 Falcons as Bird Control in Vineyards
(SE 7.35) (MAF 2009) and individual vineyards growing
between 1 and 5 grape varieties in single-variety blocks.
Sauvignon Blanc is the dominant variety of wine grape
(mean =21.5 ha/vineyard [SE 4.43]), and Pinot Noir is
the second-most common (3.3 ha/vineyard [4.52]) (MAF
2009). The vineyards we studied had 49.3 ha (10.4) of
Sauvignon Blanc grapes and 13.0 ha (4.8) of Pinot Noir
grapes. There was no significant difference in the area of
each grape variety grown in vineyards with falcons and
control vineyards (Supporting Information).
Birds feed on grapes primarily at vineyard edges, near-
est to vegetation or structures that provide passerines
with shelter from potential predators. Feeding decreases
toward the center of the vineyard (Somers & Morris
2002). However, Starlings in Europe and Australia have
been observed to feed toward the center of agricultural
fields, in areas farther away from shelter, because the
open space may better accommodate their antipreda-
tor behavior (Whitehead et al. 1995; Tracey & Saunders
2003). Grapevines do not provide shelter for birds be-
cause the birds are known to flee from vineyards when
approached by potential predators (Laiolo 2005). We
therefore classified sampled vines as either edge or in-
terior to account for differences in distance to vegetation
(from long grasses to dense trees) in which passerines
could take shelter from predators (bird shelter). We con-
sidered 50 m the threshold between interior and edge
because in discussions with experienced vineyard man-
agers before the onset of data collection we learned that
the majority of damage in the previous 5 years occurred
within 50 m of bird shelter. The mean percentage of edge
compared with interior of the sampled area was 27.3%
(SE 4.2) for Sauvignon Blanc and 33.0% (6.2) for Pinot
Noir.
Abundance of Passeriformes
We established 1 edge and 1 interior transect within each
of 4 vineyards with falcons (falcon vineyards) and 4 con-
trol vineyards. Transects were 500 m long and a mini-
mum of 150 m away from center transect lines to avoid
sampling the same individuals within both interior and
edge transects. Because the edge transect ran alongside
bird shelter, we used one-sided transect methods (Bibby
et al. 2000). We walked each transect in one direction for
20 minutes and, using field binoculars (15 ×50 IS), iden-
tified all Passeriformes seen within 50 m of the transect
line, including the 4 focus pest species (introduced Black-
birds [Turdus merula], Song Thrushes [Turdus philome-
los],Starlings [Sturnus vulgaris], and the native Silver-
eye [Zosterops lateralis]) and 11 other common nonpest
species. We ignored birds flying overhead. All transects
ran along the edges of blocks, perpendicular to the rows
of grapes. The number of individuals of each bird species
observed during each survey of a transect was recorded;
we refer to these counts as abundance data. We collected
abundance data at 6 of the vineyards (3 falcon and 3 con-
trol) once a week starting the week of 23 November
2008 and at 2 of the vineyards (1 falcon and 1 control)
once a week starting the week of 1 January 2009. We
surveyed until the week of 18 March 2009. We analyzed
each sample (transect survey) separately, but time (week)
was included as a factor to control for temporal effects
and vineyard was included to control for nonindepen-
dence of samples from multiple visits (see Analyses). We
collected abundance data along transects between 06:00
and 10:00 and did not collect data when winds were high,
temperatures were hot, or rain was moderate to heavy. If
conditions precluded sampling, we sampled the transect
during the same week under better conditions.
Grape Damage
In 2009 we measured grape damage in the 8 vineyards
in which we conducted bird abundance surveys. In 2010
we again sampled 3 of the vineyards with falcons and 3 of
the control vineyards from 2009, with the addition of 2
recently established falcon vineyards and 2 new control
vineyards. One vineyard contained falcons in 2009 but
not in 2010, so we treated it as a falcon vineyard in 2009
and as a control in 2010.
We sampled grape damage in the weeks of 18 March
2009 and 22 March 2010, which were immediately prior
to the onset of harvest. Therefore, we used damage
recorded during this period to estimate economic loss.
We split vineyards into a grid of 50 ×50 m sampling
plots and randomly selected a minimum of 10 edge and
10 interior plots for sampling in each vineyard (Support-
ing Information). Each plot contained only one variety of
grapes. The few rows that were covered in bird-exclusion
netting were not sampled. We sampled 1 grape bunch
from each of 10 vines within each plot. We sampled 5
vines on each side of a row approximately midway along
the plot at the edge nearest bird shelter. Sampling vines
on both sides of the row controlled for differences in
sunlight exposure.
We selected grape bunches for sampling with a method
we adapted from Saxton (2006) that ensures a random se-
lection of bunches from locations within and outside the
vine canopy (Supporting Information). We estimated, to
the nearest 5 grapes, the total number of grapes that had
notbeenpeckedorremoved(undamagedgrapes),the
number of grapes that had been pecked, and the number
of stems (pedicels) indicating grapes had been removed.
We sampled 750 and 1490 bunches in control vineyards
and 850 and 1050 bunches in vineyards with falcons in
2009 and 2010, respectively. We measured the cardinal
direction each bunch faced and visually estimated the
level of canopy cover for each bunch: 0, bunches with no
canopy cover; 1, bunches with >0–50% canopy cover; 2,
bunches with >50–100% canopy cover. From the edge
of each plot, we measured the distance to the nearest
Conservation Biology
Volume 26, No. 1, 2012
Kross et al. 145
bird shelter. If that bird shelter was located within 50 m
of the sampled plot, we also characterized the type of
shelter according to the presence or absence of grasses,
shrubs, small trees (<3 m in height), large trees (3m
in height), buildings, and water. We used a scale from 0
to 5 to quantify the bird-scaring methods applied to each
vineyard each year: 0, no bird-scaring methods; 1, only
static nonacoustic methods (e.g., kites, balloons, ribbon);
2, static acoustic methods (gas cannons or avian alarm
calls) set to go off every 5–10 minutes between dawn
and dusk and rarely (1–2 times per week) deployed mo-
bile acoustic methods (workers on 4-wheeled bikes or
in vehicles honking horns or activating mobile gas can-
nons); 3, moderately deployed (once per day) mobile
acoustic or lethal (workers with shotguns) methods; 4,
often-deployed static nonacoustic methods and mobile
acoustic and lethal methods (3–4 times daily); and 5, con-
tinuous mobile acoustic and lethal methods throughout
daylight hours.
Analyses
Passeriformes abundance was the number of individuals
of each focal species counted in each vineyard per week.
We used generalized linear mixed models with a Poisson
error, the most appropriate distribution for count data,
and a log-link function to analyze the associations among
abundance of focal species, weekly variability of focal
species’ presence, falcon presence, and location of the
transect (interior vs. edge). We used the lme4 package
(Bates et al. 2008) in R (version 2.7.2) for the gener-
alized linear mixed models (R Core Development Team
2008). Mixed-effects models allow the inclusion of group-
ing (random) factors to account for nonindependence of
data in nested and split-plot designs. We included vine-
yard as a random effect (so that multiple samples within
one vineyard were not treated as independent) and week,
falcon presence, and transect location as fixed factors,
with interaction terms included among all 3 fixed fac-
tors in the maximal models. The generalized linear mixed
models incorporated our hierarchical design and tested
the effect of falcon presence over an error term, with de-
grees of freedom derived from the number of vineyards.
For transect location (edge vs. interior) error degrees of
freedom were derived from the number of transects (but
blocked according to vineyards). We simplified the max-
imal models by removing interactions then main effects
until no further reduction in residual deviance (measured
using Akaike’s information criterion) was obtained.
For the grape-damage data, we used a principal com-
ponents analysis (PCA) to reduce the number of vari-
ables characterizing bird shelter to the 4 orthogonal axes
that each explained more than 10% of the variance and
cumulatively explained 86.7% of the variance in these
variables (Supporting Information). We then used gen-
eralized linear mixed models with a binomial error and
a logit link function to test whether falcon presence,
canopy cover, grape variety, vineyard bird-scaring effort,
distance from nearest bird shelter, cardinal direction the
bunch faced, the 4 PCA axes, and plot location were
significantly associated with the proportion of grapes
per bunch (bunches being the unit of replication) that
were damaged. We used separate models to test each of
our damage categories: proportion of grapes per bunch
removed but not pecked (removed) and proportion of
grapes per bunch pecked but not removed (pecked). We
included vineyard, plot, and year as random effects; plot
nested within vineyard accounted for nonindependence
of bunches within plots and of plots within vineyards and
potential variation in damage across years. We initially in-
cluded up to as many as 4 interaction terms between
combinations of all available predictor variables and then
reduced the maximal model to the minimum adequate
model with the procedure outlined above. We tested all
models for evidence of overdispersion (on the basis of
the ratio of residual deviance to degrees of freedom) and
reanalyzed overdispersed models with generalized linear
mixed models fitted with penalized quasi-likelihood (the
glmmPQL function) in the MASS package (Venables &
Ripley 2002) in R (version 2.7.2). We used the parameter
estimates from each model (after applying an appropriate
inverse-link function) to estimate the actual abundance
of birds or proportion of grapes removed or pecked. To
compare the abundance of birds per vineyard with the
amount of each type of damage, we used Spearman cor-
relations (Supporting Information).
Economic Effect
We estimated the economic effect of falcon presence in
vineyards by combining a model of overall grape dam-
age (the sum of pecked and removed grapes) in vine-
yards with falcons and control vineyards (Supporting In-
formation) with the average value of grapes harvested per
hectare. In the overall grape-damage model, we used the
same analysis methods as above for each damage class.
In 2009, the average gross purchase price of grapes in
Marlborough was US$13,790/ha (SE 430) for Sauvignon
Blanc (assuming a conversion rate of 1NZ$ =US$0.718)
and $13,951/ha (738) for Pinot Noir grapes (MAF 2009).
Results
Bird Abundance
Vineyard workers in vineyards without falcons reported
no falcon sightings over the 2 years of our study. After
controlling for differences through time, falcon presence
in falcon vineyards was associated with a 78.4% reduc-
tion in the abundance of Song Thrushes (Z=−3.17, p
<0.01), an 82.5% reduction in the abundance of Black-
birds (Z=−2.44, p=0.02), and a 79.2% (nonsignificant)
Conservation Biology
Volume 26, No. 1, 2012
146 Falcons as Bird Control in Vineyards
reduction in the abundance of Starlings (Z=−1.85, p=
0.06) (Fig. 1) relative to control vineyards. Falcon pres-
ence did not explain significant variation in Silvereye
abundance (Z=−1.03, p=0.30) (Fig. 1), so we re-
moved this variable from the Silvereye model; the lack of
effect may have been due to low power.
Interior vines were associated with 70.5% fewer Song
Thrushes (Z=−7.66, p<0.001), 95.2% fewer Silvereyes
(Z=−10.71, p<0.001), and 44.4% fewer Blackbirds
(Z=−1.83, p=0.07) relative to edge vines. Conversely,
interior vines were associated with a 57.7% increase in
Starlings (Z=3.33, p=0.001) relative to edge vines.
Grape Damage
There was significantly less grape damage in vineyards
with falcons than in control vineyards for edge and in-
terior Sauvignon Blanc and Pinot Noir bunches (Fig. 2).
Results of the generalized linear mixed models showed
that in vineyards with falcons present there were signifi-
cantly fewer grapes removed from bunches (p<0.001)
and fewer grapes pecked on bunches (p<0.01) (Sup-
porting Information). With all other variables held con-
stant, the model intercept (inverse linked) indicated that
in control vineyards an average of 0.6% of the edge Sauvi-
gnon Blanc grapes were removed and 2.3% were pecked
(Table 1). In contrast, vineyards with falcons had an av-
erage of 0.03% of edge Sauvignon Blanc grapes removed
and 1.0% pecked (Table 1).
Canopy cover was associated with observed damage
to grapes. In control vineyards, bunches with >0–50%
canopy cover had 43.3% fewer grapes removed (p<
0.001) than bunches with no canopy cover, whereas
grape bunches with >50–100% cover had 89.4% fewer
grapes removed and 47.0% fewer pecked grapes (both
p<0.001) (Supporting Information) than bunches with
no canopy cover. However, in vineyards in which fal-
cons were present, 32.5% more grapes were removed
(p<0.001) in bunches with >0–50% cover than in
bunches with no canopy cover (Supporting Information).
Distance from bird shelter had a significant negative
association with the number of grapes pecked (0.02%
damage, p=0.04), but this variable was taken out of
the removed-damage model (Supporting Information).
Bunches within the vineyard interior had significantly
fewer grapes removed (0.03%, p<0.001) and pecked
grapes (1.0%, p=0.02) than bunches at the edge of the
vineyard (Supporting Information). Principal component
axis 1, which was negatively correlated with the pres-
ence of natural features such as shrubs (variable loading =
0.54), small trees (0.46), large trees (0.43), and wa-
ter (0.44) (Supporting Information), had a significant
negative association with both damage categories (both
p<0.001) (Supporting Information). Principal compo-
nent axis 2 was positively correlated with the presence
of buildings (0.95) (Supporting Information) and had a
Figure1. Theeffectsoffalconpresenceonthe
abundance of (a) Song Thrushes ( p<0.01),
(b) Blackbirds ( p=0.02), (c) Starlings ( p=0.06),
and (d) Silvereyes ( p=0.30, removed from model).
Lines graphs show the mean (SE) number of
individuals observed in each of 17 weeks (beginning
the week of 23 November 2008 and ending the week
of 18 March 2009) along edge and interior transects
combined at 4 vineyards with resident falcons present
(Falcon) and 4 vineyards with falcons absent (No
falcon). Grape ripening began at approximately week
12.
Conservation Biology
Volume 26, No. 1, 2012
Kross et al. 147
Figure 2. Mean (SE) percent overall damage to grapes
(removed and pecked combined), grapes removed,
and pecked grapes in falcon and control vineyards for
(a) edge Sauvignon Blanc (b) interior Sauvignon
Blanc, (c) edge Pinot Noir, and (d) interior Pinot Noir.
Table 1. Mean percent damage per bunch to vineyard grapes due to
passerine foraging in control vineyards and in vineyards containing
resident falcons, calculated from inverse-linked parameter estimates
from generalized linear mixed models for removed and pecked
grapes.
Damage, grape
type, locationControl (%
damage)
Falcon (%
damage)
Relative
change
in
damage
with
falcon
presence
(%)
Grapes removed
Sauvignon Blanc
edge 0.62 0.03 95.8
interior 0.03 0.00 99.0
whole vine yard 0.19 0.01 95.4
Pinot Noir
edge 1.12 0.05 95.8
interior 0.06 0.00 95.8
whole vine yard 0.35 0.01 95.8
Grapes pecked
Sauvignon Blanc
edge 2.26 1.00 55.6
interior 1.05 0.46 55.9
whole vine yard 1.38 0.61 56.0
Pinot Noir
edge 3.50 1.57 55.3
interior 1.65 0.73 55.8
whole vine-vyard 2.15 0.95 55.6
Whole vineyard damage was calculated as the amount of edge dam-
age multiplied by the proportion of control and treatment vineyards
that consisted of edge vines (Sauvignon Blanc =27%, Pinot Noir =
33%) plus the amount of interior damage multiplied by the propor-
tion of our control and treatment vineyards that consisted of interior
vines.
nonsignificant positive association with pecked damage
(p=0.06), but we took this variable out of the removed-
damage model during simplification (Supporting Infor-
mation). Principal component axis 3 was negatively cor-
related with the presence of water (0.53, Supporting
Information) and had a significant positive association
with removed and pecked damage (both p<0.01) (Sup-
porting Information). We removed PCA 4 during simpli-
fication of both damage models.
We removed level of bird scaring in vineyards from
the removed- and pecked-damage models during sim-
plification. The cumulative number of Blackbirds, Song
Thrushes, and Starlings in vineyards in the final 5 weeks of
grape ripening was correlated with the number of grapes
removed in vineyards (rho =0.54, p=0.03), whereas
the number of grapes pecked was highly correlated with
the cumulative number of Silvereyes (rho =0.72, p<
0.01) (Supporting Information).
Economic Effect
For the combined average overall damage from both
pecked and removed damage, 2.4% of the Sauvignon
Conservation Biology
Volume 26, No. 1, 2012
148 Falcons as Bird Control in Vineyards
Blanc crop and 3.4% of the Pinot Noir crop in control
vineyards were damaged by birds (Supporting Informa-
tion), equivalent to losses of $338/ha and $481/ha, re-
spectively. A mean of 0.8% of the Sauvignon Blanc crop
and 1.1% of the Pinot Noir crop was damaged in vine-
yards with falcons (Supporting Information), equivalent
to losses of $104/ha and $155/ha, respectively.
Discussion
Our results show that relative to vineyards without fal-
cons, vineyards with falcons were associated with sig-
nificantly fewer non-native focal passerines and signif-
icantly fewer pecked and removed grapes. Relative to
vineyards without falcons, falcon presence was associ-
ated with $234/ha less crop damage for Sauvignon Blanc
and $326/ha less damage for Pinot Noir. Because these
are rough calculations derived from model estimates, the
values should not be treated as exact.
Relative to vineyards without falcons, the presence of
falcons was associated with a lower abundance of non-
native focal species and less grape damage associated
with non-native and native focal species (Supporting In-
formation). All 4 species are part of the diet of New
Zealand Falcons (Fox 1977; Seaton et al. 2008). Our find-
ings are likely a result of the combined effects of direct
predation and increased predation risk. Direct predation
reduces pest bird populations, whereas high predation
risk increases antipredator behavior (e.g., avoidance and
vigilance relative to time spent foraging) and may cause
birds to forage in suboptimal locations that offer better
protection from predators (Lima & Dill 1990; Fern´
andez-
Juricic & Teller´
ıa 2000; Devereux et al. 2006).
The relation between grape damage levels and the
bird-scaring strategies employed by vineyards was not
significant. Birds easily become habituated to common
deterrent methods, especially if the same methods are
used throughout the grape-ripening season (Bomford &
Sinclair 2002; Fukuda et al. 2008). Understanding pest
bird foraging behavior may allow better coordination of
deterrent methods (Tracey & Saunders 2003). For ex-
ample, knowing that fewer Starlings will forage at the
vineyard interior if a falcon is present could encourage
more efficient use of deterrent methods at the vineyard
edge.
Our sample size was low because of the small num-
bers of falcons available for introduction into vineyards.
Despite this low power, we found significant associa-
tions between falcon presence, passerine abundance,
and grape damage. We do not think these correlations
were spurious because falcons were introduced to these
vineyards. In addition, a lower percentage of grapes in
the vineyard in which falcons were present in 2009 and
absent in 2010 were damaged in 2009 (mean [SE] dam-
age =2.0% [0.5]) than 2010 (5.2% [1.0]), whereas the
remaining vineyards showed no significant between-year
difference (Supporting Information). We believe that this
finding means the falcon effects were not spurious. Nev-
ertheless, we assumed that the effect of falcon presence
was equal within and across the vineyards in which they
were present, even though they may not visit all areas of
a vineyard with the same frequency, and we did not in-
clude the few vines that were covered in bird-exclusion
netting in our analyses. Thus, there may be some variation
in falcon effectiveness within a given vineyard.
Mitigation of conflicts between humans and wildlife
has become a key facet of predator conservation (Treves
& Karanth 2003). Agriculture continues to intensify and
to expand (Perrings et al. 2006) into areas inhabited by
raptors, and when raptors hunt for valuable domestic
or game species, they are sometimes killed by humans
(Thirgood & Redpath 2008). Our results suggest that
threatened falcons can reduce both the number of pest
birds and the amount of damage that pest birds cause to
wine grapes and that in this instance the goals of agricul-
ture and predator conservation can converge.
Acknowledgments
We are grateful to E. Fleishman, R. Knight, M. McCarthy,
and 2 anonymous reviewers for helpful comments on
previous drafts of the manuscript. N. Fox, C. Wynn, and
V. Saxton gave advice on methodology. B. Halkett as-
sisted with data collection. Pernod Ricard New Zealand,
Marisco Vineyards, Winegrowers of Ara, Cloudy Bay
Vineyards, Johnson Estate Vineyard, Stembridge Vine-
yards, Kersely Vineyards, Redwood Pass Vineyards,
Yealands Estate, Koura Bay Estate Wines, and Dumgree
Estate provided access and vineyard information. This re-
search was supported by a University of Canterbury Doc-
toral Scholarship (to S.M.K.) and grants from the Brian-
Mason Scientific & Technical Trust (to S.M.K. and X.J.N.)
and Canon New Zealand (to S.M.K.).
Supporting Information
Further information on the sampling design
(Appendix S1), correlations between the original
bird-shelter habitat variables and the first 4 axes from a
PCA (Appendix S2), methods and results used to com-
pare the abundance of focal species with the amount of
each type of damage (Appendix S3), summary of overall
damage-model results (Appendix S4), summary of the
variables retained in our grape-damage models (Ap-
pendix S5), and a figure showing the changes in damage
in a vineyard with falcons in 2009 and without falcons in
2010 (Appendix S6) are available online. The authors are
solely responsible for the content and functionality of
Conservation Biology
Volume 26, No. 1, 2012
Kross et al. 149
these materials. Queries (other than absence of material)
should be directed to the corresponding author.
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