When Owls go to Town: the Diet of Urban Barred Owls

Abstract

We investigated the diet of Barred Owls (Strix varia) inhabiting urban environments in the Lower Fraser Valley of southwestern British Columbia, Canada. Our objective was to use the diet information to gain insight into the pathways of exposure to anticoagulant rodenticides (ARs) previously documented in this owl species. In particular, we examined whether such exposure is driven by the consumption of rodents commonly targeted during AR application, Norway rats (Rattus norvegicus), black rats (Rattus rattus) and house mice (Mus musculus), or if the secondary exposure is via consumption of native non-target rodents feeding at outdoor bait-stations. We identified 688 prey items from eight urban nest/roost sites. Rats (54.5%) were by far the most common prey, followed by field voles (Microtus townsendii; 19.3%), and deer mice (Peromyscus maniculatus; 5.2%). The consumption of rats was positively correlated with the degree of urban development within Barred Owl home ranges (rp = 0.70, r2 = 0.48, P < 0.05, one-tailed). Barred Owls consumed predominantly younger rats, as the average rat weight was 103 ± 51.7 grams (n = 164). Surprisingly, no house mice were found in the prey remains, supporting the assumption that house mice seldom venture outdoors and therefore are not a likely vector of ARs to owls. If we assume more intensive AR usage in urban environments, then the higher consumption of rats in urban areas implicates rats as the likely pathway for secondary AR exposure to Barred Owls in urban landscapes.

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When Owls go to Town: the Diet of Urban Barred Owls
Source: Journal of Raptor Research, 49(1):66-74.
Published By: The Raptor Research Foundation
DOI: http://dx.doi.org/10.3356/jrr-14-00012.1
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WHEN OWLS GO TO TOWN: THE DIET OF URBAN BARRED OWLS
SOFI HINDMARCH
1
AND JOHN E. ELLIOTT
Environment Canada, Science and Technology Branch, 5421 Robertson Road, Delta, British Columbia, Canada
ABSTRACT.—We investigated the diet of Barred Owls (Strix varia) inhabiting urban environments in the
Lower Fraser Valley of southwestern British Columbia, Canada. Our objective was to use the diet informa-
tion to gain insight into the pathways of exposure to anticoagulant rodenticides (ARs) previously docu-
mented in this owl species. In particular, we examined whether such exposure is driven by the consumption
of rodents commonly targeted during AR application, Norway rats (Rattus norvegicus), black rats (Rattus
rattus) and house mice (Mus musculus), or if the secondary exposure is via consumption of native non-target
rodents feeding at outdoor bait-stations. We identified 688 prey items from eight urban nest/roost sites.
Rats (54.5%) were by far the most common prey, followed by field voles (Microtus townsendii; 19.3%), and
deer mice (Peromyscus maniculatus; 5.2%). The consumption of rats was positively correlated with the degree
of urban development within Barred Owl home ranges (r
p
5 0.70, r
2
5 0.48, P , 0.05, one-tailed). Barred
Owls consumed predominantly younger rats, as the average rat weight was 103 6 51.7 grams (n 5 164).
Surprisingly, no house mice were found in the prey remains, supporting the assumption that house mice
seldom venture outdoors and therefore are not a likely vector of ARs to owls. If we assume more intensive
AR usage in urban environments, then the higher consumption of rats in urban areas implicates rats as the
likely pathway for secondary AR exposure to Barred Owls in urban landscapes.
K
EY WORDS: Barred Owl; Strix varia; diet; rats; rodenticide; urban.
CUANDO LOS BU
´
HOS VAN A LA CIUDAD: LA DIETA URBANA DE STRIX VARIA
R
ESUMEN.—Investigamos la dieta de individuos de Strix varia que habitan ambientes urbanos en Lower
Fraser Valley, al suroeste de la Columbia Brita´nica, Canada´. Nuestro objetivo fue utilizar la informacio´n de
la dieta para conocer las vı´as de exposicio´ n a rodenticidas anticoagulantes (RAs) que han sido documen-
tados previamente en esta especie. En particular, examinamos si dicha exposicio´n es provocada por el
consumo de roedores que comu´nmente son el blanco de las aplicaciones con RA, Rattus norvegicus, Rattus
rattus y Mus musculus, o si existe una exposicio´n secundaria por el consumo de roedores que no son el
blanco de RAs, pero que se alimentan en estaciones de cebo al aire libre. Identificamos 688 restos de presas
en ocho a´reas urbanas usadas para la crı´a/posadero. Las ratas (54.5%) fueron con creces la presa ma´s
comu´n, seguidas por Microtus townsendii (19.3%) y Peromyscus maniculatus (5.2%). El consumo de ratas
estuvo positivamente correlacionado con el grado de desarrollo urbano dentro del a´mbito de hogar de
S. varia (r
p
5 0.70, r
2
5 0.48, P , 0.05, una cola). S. varia consumio´ preferentemente ratas jo´venes ya que el
peso promedio de las ratas fue de 103 6 51.7 gramos (n 5 164). Sorprendentemente, ningu´n individuo de
M. musculus fue encontrado en los restos de presas, lo que apoya la suposicio´ n de que M. musculus
raramente se aventura hacia las a´reas rurales y por lo tanto es improbable que sea un vector de RAs para
los bu´ hos. Si asumimos que existe un uso ma´s intensivo de RAs en ambientes urbanos, entonces un mayor
consumo de ratas en a´reas urbanas implica que las ratas son probablemente la vı´a de exposicio´n secundaria
de S. varia a RA en paisajes urbanos.
[Traduccio´n del equipo editorial]
Human settlements, including the highest density
urban areas, are commonly found in locations that
are also the best habitats for other biota. Those in-
clude estuaries, productive valley bottoms, and lake
and riverine shorelines, and thus, increasing urbani-
zation is considered an important threat to biodiver-
sity (McKinney 2002). However, some wildlife spe-
cies, including raptors, survive and reproduce in
urban environments. Cooper’s Hawks (Accipiter coo-
perii), Sharp-Shinned Hawks (Accipiter striatus), and
European Northern Goshawks (Accipiter gentilis) prey
on songbirds at urban backyard feeders (Powers
1996, Boal and Mannan 1998, Rutz 2008); Peregrine
Falcons (Falco peregrinus) nest on high-rise buildings
or under city bridges (Bird et al. 1996), and Eastern
1
Email address: sofi.hindmarch@gmail.com; present ad-
dress: 76 210 Street, Langley, British Columbia, Canada
J. Raptor Res. 49(1):66–74
E
2015 The Raptor Research Foundation, Inc.
66

Screech-Owls (Megascops asio) nest successfully in var-
ious human-made structures in suburban landscapes
in central Texas (Gehlbach 2008).
The Barred Owl (Strix varia) is a medium-sized
raptor that has also found niche spaces in urban
ecosystems across North America (Bird et al. 1996,
Mazur and James 2000, Harrold 2003, Dykstra et al.
2012). Inhabiting urban landscapes is associated
with an increase in anthropogenic threats, such as
trauma from collisions with vehicles and buildings
(Hager 2009). Exposure to chemical contaminants
such as lead, persistent organic pollutants, and poly-
cyclic aromatic hydrocarbons can also be greater in
urban environments (Van Metre et al. 2000, Li et al.
2001, Newsome et al. 2010, Henny et al. 2011).
High-density human developments also attract tra-
ditional commensal pest species such as rats (Rattus
sp.) and house mice (Mus musculus; Feng and Hims-
worth 2013). Consequently, the need for pest con-
trol, particularly for rats, is greater in urban settings
(Riley et al. 2007, McMillin et al. 2008). The primary
method for controlling commensal rodent infesta-
tions worldwide is the use of anticoagulant rodenti-
cides (ARs; Corrigan 2001). Second generation ARs
in particular are persistent, bioaccumulative, and
highly toxic compounds (Parmar et al. 1987, U.S.
EPA 2004), and their spread as contaminants to
raptors has been documented worldwide (Merson
et al. 1984, Stone et al. 1999, Lambert et al. 2007,
Walker et al. 2008, Albert et al. 2010, Lima and
Salmon 2010, Murray 2011, Thomas et al. 2011).
In southwestern British Columbia, Barred Owls
are particularly affected by AR usage. Barred Owl
carcasses from 1993–2003 had substantial exposure
to ARs, with 92% (n 5 23) of carcasses testing pos-
itive for one or more ARs (liver samples; Albert et al.
2010), and 12% diagnosed as having died from AR
poisoning. Among samples collected between 2005
and 2011, 100% of the owl carcasses (n 5 66) tested
positive for one or more ARs (J. Elliott unpubl. data).
Elliott et al. (2014) tested for AR residues in small
mammals in and around agricultural buildings and
found AR residues almost exclusively in rats. With the
exception of one field vole (Microtus townsendii), no
non-target rodents contained AR residues. This sup-
ports the hypothesis that rats may be an important, if
not the primary, vector responsible for secondary
exposure of non-target predators in landscapes with
intensive AR usage (McMillin et al. 2008).
The Barred Owl historically inhabited the forests of
eastern North America, but has over the last century
expanded its range northwest throughout the boreal
forest and all the way to the Pacific Northwest (Mazur
and James 2000). In 1943, the Barred Owl was first
documented in British Columbia (Campbell et al.
1990). Subsequently, in 1966 the first sighting in
southwestern British Columbia was documented in
the municipality of Surrey (Campbell et al. 1990),
and since then they have become widespread in the
region (Audubon 2011).
Barred Owls are capable of capturing a wide
range of prey species and are very efficient at sub-
sisting on temporarily or seasonally abundant prey
species, such as beetles, amphibians, songbirds (in-
cluding nestlings), young rabbits (Sylvilagus florida-
nus), and worms (Livezey 2007, Streby et al. 2008).
Nevertheless, their diet typically is dominated by
one or two species, presumably those that are most
abundant and locally profitable (Wilson 1938, Ma-
zur and James 2000 and references therein, Livezey
2007 and references therein). In the Pacific North-
west, rodents are the most important prey group
(Hamer et al. 2001, Livezey 2007, Wiens et al. 2014).
Several researchers have investigated the diet of
Barred Owls in a variety of regions (Mazur and James
2000, Livezey 2007); however, most studies were con-
ducted in forest, grassland, and agricultural settings,
and few have been in more urban areas. Such infor-
mation would be of particular interest for rapidly
urbanizing regions, such as the Lower Fraser Valley
of British Columbia, where the loss of forests and
agricultural lands surrounding urban centers may
push owl and other wildlife into the remaining green
spaces, including parks, suburban woodlots, and
fragments of undeveloped land.
The goal of our study was to document the diet of
Barred Owls inhabiting suburban and urban land-
scapes in southwestern British Columbia. Because of
the high incidence and concentrations of AR resi-
dues in Barred Owls (Albert et al. 2010, J. Elliott
unpubl. data), we also investigated whether there
was a relationship between the degree of urbaniza-
tion surrounding nest/roost sites, and the propor-
tion of commensal rodents (Rattus norvegicus, R. rat-
tus, Mus musculus) in their diet.
METHODS
Study Area. We conducted surveys for Barred Owl
nest and roost sites from December 2010 to June
2013 throughout the Lower Fraser Valley in the mu-
nicipalities of Richmond, Vancouver, Burnaby, Del-
ta, Surrey and New Westminster (847 km
2
), in
southwestern British Columbia, Canada (49u8.09N,
122u18.09W; Fig. 1). The area includes some of the
M
ARCH 2015 THE DIET OF URBAN BARRED OWLS 67

main stopover sites for birds migrating on the Paci-
fic flyway, and encompasses important areas of wild-
life habitat such as the Alaksen National Wildlife
Area, Burns Bog, Boundary Bay, as well as large ur-
ban green spaces such as Stanley Park and Pacific
Spirit Park. Prior to European settlement, the low-
lying floodplains were dominated by grassland and
low shrub vegetation, whereas higher elevations
were covered primarily by coniferous forest (North
and Teversham 1984). Today, the landscape ranges
from agricultural land to suburban and urban, with
the remaining lower-elevation grassland and
forested habitats facing ongoing development pres-
sure as the projected human population in the re-
gion is expected to increase 50% by 2036 (Storen
2011).
Owl Surveys and Pellet Collection. Suitable areas
to survey for nest/roost sites and subsequent pellet
and prey remains were found with the help of local
naturalist clubs and by surveying urban parks and
green spaces for any signs of owl presence such as
excreta, feathers, and pellets. To confirm the pres-
ence of Barred Owls, and to determine specific
locations to search for pellets, we used call-playbacks
at dusk, following the nocturnal owl survey guide-
lines for North America (Takats et al. 2001). If we
found pellets or prey remains, and we were able to
verify that they were from Barred Owls, either by
observing Barred Owls or their molted feathers,
we revisited the site every 2–3 mo and collected
additional pellets and prey remains. Collection of
pellets and prey remains at nest/roost sites is a
widely used and accepted method for examining
owl diets (see Marti et al. 2007 for a review of sam-
pling methodologies for raptor diet studies). Inci-
dental observations of predation at these sites were
also included in our data set. We included sites only
if the total number of prey found was $50 prey
items, and prey from different years were pooled
so that each site provided only one sample over
Figure 1. Study area in the Lower Fraser Valley, British Columbia, Canada. Barred Owl diet was assessed at the marked
nest and roost site locations.
68 HINDMARCH AND ELLIOTT VOL. 49, NO.1

the entire duration of the study. We determined
whether breeding had occurred by listening at dusk
to survey for nestling Barred Owls between April
and July. The nestlings were quite vocal at 4–5 wk
of age and actively followed the parents, begging for
food. We also searched for evidence of rodenticide
use within parks or green spaces, specifically black
bait stations placed alongside the perimeter of
buildings or other structures within the park, and
measured the distance from the bait stations to the
nearest nest/roost.
Pellet Analysis. We dissected pellets and identified
mammalian prey items from bone remnants and fur
using Nagorsen (1996) and Nagorsen (2005). Avian
prey remains were identified and categorized based
on bones, feathers, gizzard sac, bill, or feet. We de-
termined the number of individuals of any species
within each pellet by pairing each skull with the cor-
rect number of ischia, left and right mandibles, ti-
biae/fibulae, or in the case of birds, each skull with
sternum, gizzard sac, and feet. We assembled the
remaining bones contained within the pellet to de-
termine the minimum number of additional indivi-
duals. For smaller prey items (,100 g), we assumed
the remains of each prey were contained within a
single pellet, as it is rare for bones from one small
prey item to be parcelled into two successive pellets
(Raczynski and Ruprecht 1974). We estimated rat
weights following Morris (1973). We classified uni-
dentified songbird (Passeriformes sp.) remains as
either (1) small songbirds (,30 g) or (2) medium
songbirds (30–80 g). We considered all exoskeleton
remains as order Coleoptera.
Land Use and Spatial Analysis. We quantified the
amount of urban land within a 1-km radius (3 km
2
,
300 ha) of each nest/roost site from digitized data
layers using Geographic Information System (GIS)
software (ArcMap 10). Urban land consisted of re-
sidential and industrial categories, and industrial
included commercial and institutional lands. We
used a 1-km radius as this approximates the average
home range for Barred Owls, which was estimated
to be approximately 3 km
2
(Nicholls and Warner
1972, Mazur and James 2000, Johnsgard 2002).
Land use within the circular plots was derived from
a 2006 Vancouver Regional District land-use layer
map, which categorizes land parcels based on zon-
ing (Metro Vancouver 2008). We compared the
2006 land-use layer map with 2010 Bing Ortho pho-
tos (Bing Maps 2010) to correct for any recent
changes in land use or discrepancies in current land
use and zoning.
We conducted a one-way correlation analysis to
assess whether the consumption of commensal ro-
dents increased with the amount of urban land
within the circular plots. We used IBM SPSS 19
(IBM SPSS, IBM Inc., Armonk, New York, U.S.A.)
for all statistical analyses.
RESULTS
We found eight suburban and urban nest/roost
sites regularly occupied by Barred Owls throughout
the year. All sites were in parks situated in residen-
tial neighborhoods, and the amount of urban land
within the circular plots (3 km
2
) ranged from 53.6%
to 89.5% (Fig. 1, Table 1). We documented bait sta-
tions along the perimeter of buildings and outbuild-
ings (e.g., sheds and garages) in four parks where
Barred Owls were nesting. In one of the four parks,
a bait station was fastened next to a stream approxi-
mately 100 m from the nearest building. In these
parks, the distance from the roost/nest site and the
location of the AR usage ranged from 50 to 250 m.
All the bait stations contained products with the
active ingredient bromadiolone, which is the only
second generation AR now registered for outdoor
use in Canada (Health Canada 2012).
Pellets and prey remains were found underneath
nest/roost trees. As a result, some of the pellets
were weathered considerably, and it was impossible
to determine the exact number of pellets collected,
but in total, we identified 16 different species from
688 prey remains. Rats were the primary prey item
recorded (54.5% for all prey remains combined; X
–
5 56.4 6 16.3% [mean 6SD, n 5 8 sites]), followed
by field voles (Microtus townsendii; 19.2%, X
–
5 17.6 6
18.0%), and there was an inverse relationship be-
tween the proportions of the two species (r
p
5
20.95, r
2
5 0.89, P , 0.01). The proportion of rats
in the diet was positively correlated with the amount
of urban land within the circular plots (r
p
5 0.70, r
2
5 0.48, P , 0.05, one-tailed, Fig. 2).
Barred Owls consumed predominantly smaller,
most likely younger adult rats, with an average mass
of 103 6 51.7 grams (n 5 164), most commonly in
the 120-g category (Fig. 3). Other species were only
marginally represented in the diet, with each con-
tributing ,15% of the total at each site (Table 1).
Finally, we documented no house mice in the diet.
DISCUSSION
In North America, Barred Owls are recognized as a
generalist predator traditionally associated with for-
ested ecosystems, where small mammals are often the
M
ARCH 2015 THE DIET OF URBAN BARRED OWLS 69

dominant prey group in their diet (Mazur and
James 2000, Johnsgard 2002, Livezey 2007, Wiens et
al. 2014). We found that rats, field voles, and deer
mice (Peromyscus maniculatus) constituted the pri-
mary prey for Barred Owls living in suburban-to-
urban landscapes in the Lower Fraser Valley. We docu-
mented a variety of other species, but their
contribution to the overall diet was minimal. The
prevalence of rats in the diet was positively correlated
with the amount of urban development surrounding
each nest/roost site, although sample size was small.
The higher consumption of rats in urban environ-
ments demonstrated the opportunistic behavior of
the Barred Owl, as individuals from two rural Barred
Owl nest sites consumed predominantly field voles
(50% and 87%, respectively), and only 11% and
1% rats (S. Hindmarch unpubl. data; concurrent
study in rural areas of Delta and Surrey, B.C., Ca-
nada). In addition, we found an inverse relationship
between the proportion of rats and field voles in the
urban diet, most likely reflecting the availability of
these prey species in the urban landscape.
Higher rat consumption in urban environments
has been previously documented for Great Horned
Owls (Bubo virginianus), such as those nesting in ur-
ban parks in Seattle, Washington (Lambert 1981). In
addition, the Eurasian Eagle-Owl (Bubo bubo) con-
sumed more rats when nesting in suburban land-
scapes in Trento, Italy, and Bras¸ov, Romania, where
rats were an abundant year-round food source but
rabbits (Oryctolagus cuniculus; the main prey of ea-
gle-owls in southern Europe) were scarce (Marchesi
et al. 2002, Sandor and Ionesco 2009). Overwintering
Long-eared Owls (Asio otus) in the city of Milan, Italy,
Table 1. Prey species and/or taxon (%) found in Barred Owl pellets collected in suburban and urban landscapes from
December 2010 to June 2013 in the Lower Fraser Valley, British Columbia, Canada. Sites: F.R. 5 Ferry Road, F.V.5
Fraserview, M.P.5 Musquieam Park, C.P.5 Crescent Park, B.C.P.5 Bear Creek Park, C.P.B.5 Central Park Burnaby,
K.P.5 Kits Park, V.D.5 Van Dusen.
S
ITES
(PERCENT URBAN LAND WITHIN PLOT CIRCLES)
P
REY TYPES
F.R.
(53.6)
F.V.
(57.1)
M.P.
(58.3)
C.P.
(64.5)
B.C.P.
(65.0)
C.P.B.
(69.7)
K.P.
(75.8)
V.D.
(89.5) SUM
Order Rodentia
Microtus townsendii 26.0 33.8 55.0 4.2 10.0 1.2 9.6 - 19.2
Microtus oregoni ----1.1---0.1
Peromyscus maniculatus 4.0 2.6 2.3 13.5 3.3 14.6 - 1.1 5.2
Rattus sp. 64.0 36.4 23.7 51.0 66.3 59.8 54.8 83.1 52.8
Rattus norvegicus - - 3.1 1.0 - - - - 0.7
Rattus rattus - - 1.5 - - - 6.8 - 1.0
Sciuridae sp. - - - 1.1 2.5 - - - 0.5
Order Lagomorpha
Sylvilagus floridanus - - - 3.1 - - 2.7 - 0.7
Order Soricomorpha
Scapanus orarius - 2.6 0.8 2.1 12.2 3.7 6.8 3.4 3.9
Neurotrichus gibbsii - - - 3.1 1.1 - - - 0.6
Sorex sp. - 13.0 9.2 2.1 - 1.2 - - 3.5
Order Passeriformes
Passeriformes (30–80 g)
1
4.0 7.8 2.3 4.2 1.1 7.3 4.1 6.7 4.5
Passeriformes (,30 g)
1
2.0 3.9 - 9.4 2.2 1.2 - - 2.3
Corvus brachyrhynchos 1.3 - 1.5 2.1 - - 1.4 - 0.7
Order Anura
Rana catesbeiana - - - - - 2.4 - - 0.3
Order Coleoptera - - 0.8 3.1 - 8.5 13.7 5.6 3.8
# Prey 50 77 131 96 90 82 73 89 688
1
Unidentified songbirds were allocated into two categories based on mass (g).
70 HINDMARCH AND ELLIOTT VOL. 49, NO.1

also ate primarily rats (65.2% of the consumed bio-
mass; Pirovano et al. 2000). Urban Tawny Owls (Strix
aluco), also in northern Italy, had a seasonally diverse
diet, with rodents (primarily rats) being the main
prey in the autumn and winter, and then birds, in
particular sparrows (Passeridae), the most important
component during spring and summer (Galeotti et
al. 1991). In our study, the rats consumed by the
Figure 2. Relationship between the proportion of urban development within circular plots surrounding nests/roosts
(3 km
2
) and the proportion of rats in the diet of Barred Owls (r
p
5 0.70, r
2
5 0.48, P , 0.05, one-tailed).
Figure 3. Mass classes of rats (610 g) consumed by Barred Owls in urban/suburban Lower Fraser Valley, British
Columbia, Canada. Average mass: 103 6 51.7 g, n 5 164. Mass (g) of rats estimated by measuring the lower jaw
(mandible) of rat prey remains, and following Morris’s (1979) suggested conversion table of jaw length to body weight.
MARCH 2015 THE DIET OF URBAN BARRED OWLS 71

urban Barred Owls were smaller (103 grams) than
the typical Norway rats (142 grams) trapped in Van-
couver (Himsworth et al. 2014). Younger, smaller
rats are likely more naive and less risky to capture
than larger adult rats.
Interestingly, in an extensive urban-vs.-rural study
of Barred Owl diet conducted during the breeding
season in Mecklenburg County, North Carolina,
birds were the main prey of urban Barred Owls
(Cauble 2008). Apparently, there were a great num-
ber of bird attractors, such as bird feeders and baths
in residential neighborhoods, which resulted in a
substantial urban bird population. Further, Cauble
(2008) argued that the typical open understory in
parks and residential gardens made birds easily
available for Barred Owls. Urban Tawny Owls in Po-
land also consumed predominantly birds during the
breeding season, and the proportion of the differ-
ent bird species in the diet reflected their availabil-
ity in the urban landscape (Zalewski 1994). In both
studies, the rural Barred Owls and the suburban
Tawny Owls had significantly different diets com-
pared to their urban counterparts, which demon-
strated the capacity of both species to establish ter-
ritories in a variety of habitat types, and adapt to
hunting locally profitable prey species.
Previous analyses of rodenticide concentrations in
Great Horned Owl, Barred Owl, and Barn Owl (Tyto
alba) carcasses collected between 1988 and 2003 in
southwestern British Columbia showed that Barred
Owls were most frequently exposed to ARs (92%),
had the highest concentrations of the more toxic
and persistent second generation anticoagulant ro-
denticides (SGARs), and the most incidences of
death caused by SGAR poisoning (12%). Great
Horned Owls were also highly exposed at 70%, fol-
lowed by Barn Owls at 62% (Albert et al. 2010). More
recently, all the Great Horned and Barred owl car-
casses sampled between 2005 and 2011 tested posi-
tive, and 75% of Barn Owls had residues of one or
more SGARs in their livers (J. Elliott unpubl. data).
Our results, documenting the year-round presence
of Barred Owls in urban landscapes, often where
SGAR bait stations are used within 1 km of nest/roost
sites, suggest that Barred Owls are at high risk of
exposure and accidental poisoning by rodenticides.
The diet in the urban environment implicates rats as
the main pathway for that exposure, as demonstrated
by Elliott et al. (2014).
Rats and house mice are the main targets during
AR application, but non-target species such as squir-
rels (Sciuridae spp.; Stone et al. 1999, U.S. EPA 2004),
voles, and sparrows (Passer and Melospiza spp.) also
directly consume bait (Elliott et al. 2014). Surpris-
ingly, we found no house mice in the prey remains,
supporting the assumption that house mice seldom
venture outdoors, and are therefore not a likely vec-
tor of ARs to owls. Likewise, the total consumption of
birds was low (7.5%) and squirrel remains (n 5 4)
were documented only for three Barred Owl pairs
nesting in two urban parks. The low presence of
squirrels and birds in the diet suggests that they are
not a vector of ARs to owls.
In summary, the wide range of prey species docu-
mented in this study confirmed that Barred Owls are
highly opportunistic predators, as shown elsewhere
(Mazur and James 2000, Johnsgard 2002, Cauble
2008). Yet our diet data also showed that urban
Barred Owls in southwestern British Columbia fed
primarily on two rodent taxa, rats and voles, and
there was an inverse relationship between the two.
Further, the proportion of rats in the diet was posi-
tively correlated with the amount of urban develop-
ment within the landscape surrounding the nests/
roosts, resulting in rats being the primary prey con-
sumed at six of the eight sites. The high proportion
of rats in the diet points to rats as the main pathway
for secondary AR exposure to Barred Owls. Finally,
the use of SGARs in urban parks in conjunction with
the year-round presence of Barred Owls in these ar-
eas provides further evidence of the risk of secondary
AR poisoning to these owls, as reported by Albert et
al. (2010) and Thomas et al. (2011).
ACKNOWLEDGMENTS
We thank Paul Levesque and Alexa and Jamie Coote, for
their great help in looking for owl pellets. In many instanc-
es their efforts consisted of crawling around in the rough
understory of urban parks. Also, we thank the two inde-
pendent reviewers for their knowledgeable insights and
efforts in reviewing an earlier version of this report.
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Received 22 January 2014; accepted 9 May 2014
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