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ARTICLE
Received 6 Sep 2012 |Accepted 12 Dec 2012 |Published 29 Jan 2013
The impact of free-ranging domestic cats
on wildlife of the United States
Scott R. Loss1, Tom Will2& Peter P. Marra1
Anthropogenic threats, such as collisions with man-made structures, vehicles, poisoning and
predation by domestic pets, combine to kill billions of wildlife annually. Free-ranging domestic
cats have been introduced globally and have contributed to multiple wildlife extinctions on
islands. The magnitude of mortality they cause in mainland areas remains speculative, with
large-scale estimates based on non-systematic analyses and little consideration of scientific
data. Here we conduct a systematic review and quantitatively estimate mortality caused by
cats in the United States. We estimate that free-ranging domestic cats kill 1.3–4.0 billion birds
and 6.3–22.3 billion mammals annually. Un-owned cats, as opposed to owned pets, cause the
majority of this mortality. Our findings suggest that free-ranging cats cause substantially
greater wildlife mortality than previously thought and are likely the single greatest source of
anthropogenic mortality for US birds and mammals. Scientifically sound conservation and
policy intervention is needed to reduce this impact.
DOI: 10.1038/ncomms2380
1Migratory Bird Center, Smithsonian Conservation Biology Institute, National Zoological Park, P.O. Box 37012 MRC 5503, Washington, District of Columbia
20013, USA. 2U.S. Fish and Wildlife Service, Division of Migratory Birds, Midwest Regional Office, 3815 American Boulevard East, Bloomington, Minnesota
20013, USA. Correspondence and requests for materials should be addressed to S.R.L. (email: LossS@si.edu).
NATURE COMMUNICATIONS | 4:1396 | DOI: 10.1038/ncomms2380 | www.nature.com/naturecommunications 1
&2013 Macmillan Publishers Limited. All rights reserved.
Domestic cats (Felis catus) are predators that humans have
introduced globally1,2 and that have been listed among
the 100 worst non-native invasive species in the world3.
Free-ranging cats on islands have caused or contributed to 33
(14%) of the modern bird, mammal and reptile extinctions
recorded by the International Union for Conservation of Nature
(IUCN) Red List4. Mounting evidence from three continents
indicates that cats can also locally reduce mainland bird and
mammal populations5–7 and cause a substantial proportion
of total wildlife mortality8–10. Despite these harmful effects,
policies for management of free-ranging cat populations and
regulation of pet ownership behaviours are dictated by animal
welfare issues rather than ecological impacts11. Projects to
manage free-ranging cats, such as Trap-Neuter-Return (TNR)
colonies, are potentially harmful to wildlife populations, but
are implemented across the United States without widespread
public knowledge, consideration of scientific evidence or the
environmental review processes typically required for actions
with harmful environmental consequences11,12.
A major reason for the current non-scientific approach to
management of free-ranging cats is that total mortality from cat
predation is often argued to be negligible compared with other
anthropogenic threats, such as collisions with man-made
structures and habitat destruction. However, assessing the
conservation importance of a mortality source requires identifica-
tion of which species are being killed (for example, native versus
non-native invasive species and rare versus common species) in
addition to estimation of total numbers of fatalities. Estimates of
annual US bird mortality from predation by all cats, including
both owned and un-owned cats, are in the hundreds of
millions13,14 (we define un-owned cats to include farm/barn
cats, strays that are fed by humans but not granted access to
habitations, cats in subsidized colonies and cats that are
completely feral). This magnitude would place cats among the
top sources of anthropogenic bird mortality; however, window
and building collisions have been suggested to cause even greater
mortality15–17. Existing estimates of mortality from cat predation
are speculative and not based on scientific data13–16 or, at best,
are based on extrapolation of results from a single study18.In
addition, no large-scale mortality estimates exist for mammals,
which form a substantial component of cat diets.
We conducted a data-driven systematic review of studies that
estimate predation rates of owned and un-owned cats, and
estimated the magnitude of bird and mammal mortality caused
by all cats across the contiguous United States (all states excluding
Alaska and Hawaii). We estimate that free-ranging domestic cats
kill 1.3–4.0 billion birds and 6.3–22.3 billion mammals annually,
and that un-owned cats cause the majority of this mortality. This
magnitude of mortality is far greater than previous estimates of
cat predation on wildlife and may exceed all other sources of
anthropogenic mortality of US birds and mammals.
Results
The magnitude of bird mortality caused by cat predation. After
excluding studies that did not meet a priori inclusion criteria
designed to increase the accuracy of our analysis, we developed
probability distributions of predation rates on birds and mam-
mals. We combined predation rate distributions with literature-
derived probability distributions for US cat population sizes, and
we also accounted for the proportion of owned cats allowed
outdoors, the proportion of owned and un-owned cats that hunt,
and imperfect detection of owned cats’ prey items.
We generated an estimated range of bird and mammal mor-
tality caused by cat predation by incorporating the above
distributions—including separate predation rate distributions for
owned and un-owned cats—and running 10,000 calculation
iterations. We augmented US predation data by incorporating
predation rate estimates from other temperate regions
(Supplementary Table S1). For birds, we generated three US
mortality estimates based on predation data from studies in:
(1) the United States, (2) the United States and Europe and
(3) the United States, Europe, and other temperate regions (pri-
marily Australia and New Zealand). Owing to a lack of US studies
of un-owned cat predation on mammals, we estimated mammal
mortality using data groupings 2 and 3. We based all other
probability distributions on US studies (distribution details in
Table 1; data in Supplementary Table S2).
The three estimates of bird mortality varied moderately, with a
19% difference among median estimates (Table 2). We focus
interpretation on the estimate generated using US and European
predation data because it is the lowest value. Furthermore, this
estimate is more likely to be representative of the US than the
estimate based on incorporation of data from Australia and New
Zealand, where the wildlife fauna and climate are less similar
to the United States. We estimate that cats in the contiguous
United States annually kill between 1.3 and 4.0 billion birds
(median ¼2.4 billion) (Fig. 1a), with B69% of this mortality
caused by un-owned cats. The predation estimate for un-owned
cats was higher primarily due to predation rates by this group
averaging three times greater than rates for owned cats.
The magnitude of mammal mortality caused by cat predation.
Our estimate of mammal mortality was robust to the choice of
predation data as evidenced by a 1.6% difference between the two
median estimates (Table 2). We focus interpretation on the lower
estimate, which was based on United States and European pre-
dation data and US values of other parameters. We estimate
annual mammal mortality in the contiguous United States at
between 6.3 and 22.3 billion (median ¼12.3 billion) (Fig. 1b) with
89% of this mortality caused by un-owned cats. The estimate that
incorporated European data (but not data from Australia and
New Zealand) may be slightly lower because wildlife across much
of Europe were historically exposed to predation by a similarly-
sized wild cat (Felis sylvestris) and, therefore, may be less naive to
predation by domestic cats. However, it is unlikely that European
wildlife have fully adapted to the unusually high densities of
domestic cats in much of this continent9.
Factors explaining estimate uncertainty. For both birds and
mammals, sensitivity analyses indicated that un-owned cat
parameters explained the greatest variation in total mortality
estimates (Fig. 2). Un-owned cat population size explained the
greatest variation in mortality estimates (42% for birds and 51%
for mammals), and the un-owned cat predation rate explained the
second greatest variation (24% for birds and 40% for mammals).
The only other parameters that explained 45% of variation in
mortality estimates were the owned cat predation rate on birds
(16%) and the correction factor for imperfect detection of owned
cats’ prey items (8%).
Discussion
Our estimate of bird mortality far exceeds any previously
estimated US figure for cats13,14,16, as well as estimates for any
other direct source of anthropogenic mortality, including
collisions with windows, buildings, communication towers,
vehicles and pesticide poisoning13,15–21. Systematic reviews like
ours, which includes protocol formulation, a data search strategy,
data inclusion criteria, data extraction and formal quantitative
analyses22, are scarce for other anthropogenic mortality sources.21
Increased rigour of mortality estimates should be a high priority
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2380
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&2013 Macmillan Publishers Limited. All rights reserved.
Table 2 | Median estimates of annual wildlife mortality caused by cat predation in the contiguous United States.
Predation data used Mortality estimate (millions)
Owned cats Un-owned cats Total
Bird US 1,053 (104–3,039)* 1,792 (861–3,276) 2,967 (1,358–5,324)
US and Europe 684 (221–1,682)w1,652 (803–2,955)2,407 (1,306–3,992)
All temperate 508 (172–1,226) 1,876 (983–3,124) 2,437 (1,378–3,814)
Mammal US and Europe 1,249 (512–2,862)10,903 (4,991–20,874)12,269 (6,259–22,257)
All temperate 958 (397–2,117) 11,426 (5,874–19,451) 12,473 (6,874–20,421)
Both Taxa 1,933 (733–4,544) 12,555 (5,794–23,829) 14,676 (7,565–26,249)
*Values in parentheses indicate central 95% of estimates.
wBold face indicates estimates from which inference is drawn in the text.
Table 1 | Probability distributions used for parameters in cat predation model.
Parameter Number of studies used* Distribution type Distribution parameters
Owned cats
Number of owned cats in contiguous United States 2 Normal Mean ¼84 M, s.d. ¼2.5 M
Proportion of owned cats with outdoor access 8 Uniform Min ¼0.4, max ¼0.7
Proportion of outdoor owned cats that hunt 3 Uniform Min ¼0.5, max ¼0.8
Correction for owned cats not returning prey 3 Uniform Min ¼1.2, max ¼3.3
BIRD return rate per cat per year
US studies 4 Uniform Min ¼1.0, max ¼34.1
US and Europe studies 11 Uniform Min ¼4.2, max ¼18.3
All temperate studies 17 Uniform Min ¼3.4, max ¼13.2
MAMMAL return rate per cat per year
US studies 1 NAwNA
US and Europe studies 7 Uniform Min ¼11.1, max ¼29.5
All temperate studies 13 Uniform Min¼8.7, max ¼21.8
REPTILE return rate per cat per year
US studies 0 NA NA
US and Europe studies 1 NA NA
All temperate studies 8 Uniform Min ¼0.4, max ¼2.21
AMPHIBIAN return rate per cat per year
US studies 0 NA NA
US and Europe studies 1 NA NA
All temperate studies 5 Uniform Min ¼0.05, max ¼0.5
Un-owned cats
Number of un-owned cats in contiguous United States 5 Uniform Min ¼30 M, max ¼80 M
Proportion of un-owned cats that hunt 2 Uniform Min ¼0.8, max ¼1.0
BIRD predation rate per cat per year
US studies 8 Uniform Min ¼24.4, max ¼51.4
US and Europe studies 11 Uniform Min ¼23.2, max ¼46.2
All temperate studies 19 Uniform Min ¼30.0, max ¼47.6
MAMMAL predation rate per cat per year
US studies 6 Uniform Min ¼162.3, max ¼354.9
US and Europe studies 7 Uniform Min ¼139.4, max ¼328.6
All temperate studies 13 Uniform Min ¼177.3, max ¼299.5
REPTILE predation rate per cat per year
US studies 1 NA NA
US and Europe studies 2 NA NA
All temperate studies 10 Uniform Min ¼4.2, max ¼12.4
AMPHIBIAN predation rate per cat per year
US studies 0 NA NA
US and Europe studies 0 NA NA
All temperate studies 3 Uniform Min ¼1.9, max ¼4.7
*Number of studies found that include an estimate of the model parameter.
wNo calculation was conducted for this data grouping because of limited data.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2380 ARTICLE
NATURE COMMUNICATIONS | 4:1396 | DOI: 10.1038/ncomms2380 | www.nature.com/naturecommunications 3
&2013 Macmillan Publishers Limited. All rights reserved.
and will allow increased comparability of mortality sources23.
Nonetheless, no estimates of any other anthropogenic mortality
source approach the value we calculated for cat predation, and
our estimate is the first for cats to be based on rigorous data-
driven methods. Notably, we excluded high local predation rates
and used assumptions that led to minimum predation rate
estimates for un-owned cats; therefore, actual numbers of birds
killed may be even greater than our estimates.
Free-roaming cats in the United States may also have a
substantial impact on reptiles and amphibians. However, US
studies of cat predation on these taxa are scarce. To generate a
first approximation of US predation rates on reptiles and
amphibians, we used the same model of cat predation along
with estimates of cat predation rates on these taxa from studies in
Europe, Australia and New Zealand. We estimate that between
228 to 871 million reptiles (median ¼478 million) and between
86 and 320 million amphibians (median ¼173 million) could be
killed by cats in the contiguous United States each year. Reptile
and amphibian populations, and, therefore, cat predation rates,
may differ between the regions where we gathered predation data
for these taxa and the United States. Furthermore, reptiles and
amphibians are unavailable as prey during winter across much of
the United States. Additional research is needed to clarify impacts
of cats on US herpetofauna, especially given numerous anthro-
pogenic stressors that threaten their populations (for example,
climate change, habitat loss and infectious diseases) and
documented extinctions of reptiles and amphibians due to cat
predation in other regions4,24.
The exceptionally high estimate of mammal mortality from cat
predation is supported by individual US studies that illustrate
high annual predation rates by individual un-owned cats in excess
of 200 mammals per year6,25–28 and the consistent finding that
cats preferentially depredate mammals over other taxa
(Supplementary Table S1). Even with a lower yearly predation
rate of 100 mammals per cat, annual mortality would range from
3–8 billion mammals just for un-owned cats, based on a
population estimate of between 30 and 80 million un-owned
cats. This estimated level of mortality could exceed any other
direct source of anthropogenic mortality for small mammals;
however, we are unaware of studies that have systematically
quantified direct anthropogenic mortality of small terrestrial
mammals across large scales.
Native species make up the majority of the birds preyed upon
by cats. On average, only 33% of bird prey items identified to
species were non-native species in 10 studies with 438 specimens
of 58 species (Supplementary Table S3). For mammals, patterns
of predation on native and non-native species are less clear and
appear to vary by landscape type. In densely populated urban
areas where native small mammals are less common, non-native
species of rats and mice can make up a substantial component of
mammalian prey29. However, studies of mammals in suburban
and rural areas found that 75–100% of mammalian prey were
native mice, shrews, voles, squirrels and rabbits26,30,31. Further
research of mammals is needed to clarify patterns of predation by
both owned and un-owned cats on native and non-native
mammals, and across different landscape types.
Sensitivity analyses indicate that additional research of
un-owned cats will continue to improve precision of mortality
estimates. Our finding that un-owned cat population size and
Median= 2.4 billion
600
a
b
500
400
300
Estimate frequency
200
100
0
600
500
400
300
Estimate frequency
200
100
0
0 1,000 2,000 3,000 4,000 5,000
05,000 10,000 15,000
Mortality (millions)
20,000 25,000
Median= 12.3 billion
Figure 1 | Estimates of cat predation on US birds and mammals.
(a) Probability distribution of estimated bird mortality caused by all free-
ranging cats in mainland areas of the contiguous United States.
(b) Probability distribution of estimated mammal mortality caused by all
free-ranging cats in mainland areas of the contiguous United States.
Mammal
Bird
Number of owned pet cats
Proportion of pet cats outdoors
Proportion of outdoor pet cats
hunting
Outdoor pet cat predation rate
Detectability correction for pet
cat prey items
Number of un-owned cats
Proportion of un-owned cats
hunting
Un-owned cat predation rate
0 % 10 % 20 % 30% 40 % 50 % 60%
Figure 2 | Factors explaining uncertainty in estimates of wildlife mortality from cat predation. Amount of variation in estimates of wildlife mortality in
the contiguous United States contributed by each parameter in the cat predation model (percentages represent adjusted R2values from multiple regression
models).
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2380
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&2013 Macmillan Publishers Limited. All rights reserved.
predation rate explained the greatest variation in mortality
estimates reflects the current lack of knowledge about un-owned
cats. No precise estimate of the un-owned cat population exists
for the United States because obtaining such an estimate is cost
prohibitive, and feral un-owned cats are wary of humans and tend
to be solitary outside of urban areas. In addition, human
subsidized colonies of un-owned cats are maintained without
widespread public knowledge. For example, in Washington DC
alone there are 4300 managed colonies of un-owned cats and an
unknown number of unmanaged colonies. Population size
estimates can be improved by incorporating observations of
free-ranging cats into a wildlife mortality reporting database23.
Context for the population impact of a mortality source depends
on comparing mortality estimates to estimates of population
abundance of individual species. However, continental-scale
estimates of wildlife population abundance are uncertain due to
spatio-temporal variation in numbers. For mammals, clarification
of the population impacts of cat predation is hindered by the
absence of nationwide population estimates. For all North
American land birds, the group of species most susceptible to
mainland cat predation (Supplementary Table S3), existing
estimates range from 10–20 billion individuals in North
America32. A lack of detail about relative proportions of
different bird species killed by cats and spatio-temporal variation
of these proportions makes it difficult to identify the species and
populations that are most vulnerable. The magnitude of our
mortality estimates suggest that cats are likely causing population
declines for some species and in some regions. Threatened and
endangered wildlife species on islands are most susceptible to the
effects of cat predation, and this may also be true for vulnerable
species in localized mainland areas5because small numbers of
fatalities could cause significant population declines. Threatened
species in close proximity to cat colonies—including managed
TNR colonies11,12—face an especially high level of risk; therefore,
cat colonies in such locations comprise a wildlife management
priority. Claims that TNR colonies are effective in reducing cat
populations, and, therefore, wildlife mortality, are not supported
by peer-reviewed scientific studies11.
Our estimates should alert policy makers and the general
public about the large magnitude of wildlife mortality caused by
free-ranging cats. Structured decisions about actions to reduce
wildlife mortality require a quantitative evidence base. We
provide evidence of large-scale cat predation impacts based on
systematic analysis of multiple data sources. Future specific
management decisions, both in the United States and globally,
must be further informed by fine scale research that allows
analysis of population responses to cats and assessment of the
success of particular management actions. We are not suggesting
that other anthropogenic threats that kill fewer individuals are
biologically unimportant. Virtually nothing is known about the
cumulative population impacts of multiple mortality sources.
Furthermore, comparison of total mortality numbers has limited
use for prioritization of risks and development of conservation
objectives. Combining per species estimates of mortality with
population size estimates will provide the greatest information
about the risk of population-level impacts of cat predation.
Although our results suggest that owned cats have relatively less
impact than un-owned cats, owned cats still cause substantial
wildlife mortality (Table 2); simple solutions to reduce mortality
caused by pets, such as limiting or preventing outdoor access,
should be pursued. Efforts to better quantify and minimize
mortality from all anthropogenic threats are needed to increase
sustainability of wildlife populations.
The magnitude of wildlife mortality caused by cats that we
report here far exceeds all prior estimates. Available evidence
suggests that mortality from cat predation is likely to be
substantial in all parts of the world where free-ranging cats
occur. This mortality is of particular concern within the context
of steadily increasing populations of owned cats, the potential for
increasing populations of un-owned cats12, and an increasing
abundance of direct and indirect mortality sources that threaten
wildlife in the United States and globally.
Methods
Literature search. We searched JSTOR, Google Scholar, and the Web of Science
database (formerly ISI Web of Science) within the Web of Knowledge search
engine published by Thomson Reuters to identify studies that document cat pre-
dation on birds and mammals. We initially focused this search on US studies, but
due to a limited sample of these studies, we expanded the search to include pre-
dation research from other temperate regions. We also searched for studies pro-
viding estimates of cat population sizes at the scale of the contiguous United States
and for US studies that estimate the proportion of owned cats with outdoor access
and the proportion of cats that hunt wildlife. The search terms we used included:
‘domestic cat’ in combination with ‘predation,’ ‘prey,’ ‘diet,’ ‘food item’ and
‘mortality’; all previous terms with ‘domestic cat’ replaced by ‘Felis catus,’ ‘feral,’
‘stray,’ ‘farm,’ ‘free-ranging,’ and ‘pet’; ‘trap-neuter-return colony’; ‘TNR colony’;
and ‘cat predation’ in combination with ‘wildlife,’ ‘bird,’ ‘mammal,’ and ‘rodent’.
We checked reference lists of articles to identify additional relevant studies. Lead
authors of three studies were also contacted to enquire whether they knew of
ongoing or completed unpublished studies of cat predation in the United States.
Classification of cat ranging behaviour. We grouped studies based on the ran-
ging behaviour of cats investigated. We defined owned cats to include owned cats in
both rural and urban areas that spend at least some time indoors and are also
granted outdoor access. We defined un-owned cats to include all un-owned cats
that spend all of their time outdoors. The un-owned cat group includes semi-feral
cats that are sometimes considered pets (for example, farm/barn cats and strays
that are fed by humans but not granted access to habitations), cats in subsidized
(including TNR) colonies, and cats that are completely feral (that is, completely
independent and rarely interacting with humans). We did not classify cats by
landscape type or whether they receive food from humans because the amount of
time cats spend outdoors is a major determinant of predation rates33,34 and
because predation is independent of whether cats are fed by humans6,34,35.
Study inclusion criteria. Studies were only included if: (1) they clearly reported cat
ranging behaviour (that is, a description of whether cats were owned or un-owned
and whether they were outdoor cats or indoor-outdoor cats), and (2) the group of
cats investigated fit exclusively into one of the two groups we defined above (that is,
we excluded studies that lumped owned and un-owned cats in a single predation
rate estimate). For some studies, we extracted a portion of data that met these
criteria but excluded other data from cats with unknown ranging behaviour. We
only included mainland and large island (New Zealand and United Kingdom)
predation studies, because cat predation on small islands is often exceptionally
high36,37 and focused on colony nesting seabirds38. We excluded studies from
outside temperate regions and those with predation rate estimates based on fewer
than 10 cats, o1 month of sampling, or on cats that were experimentally
manipulated (for example, by fitting them with bells or behaviour altering bibs).
We included studies that used cat owners’ records of prey returns, but we excluded
those that asked owners to estimate past prey returns because such questionnaires
may lead to bias in estimation of predation rates39. (For a list of all included and
excluded studies, see Supplementary Table S1).
Data extraction and standardization of predation rates. Most studies report an
estimate of cat predation rate (that is, daily, monthly or annual prey killed per cat)
or present data that allowed us to calculate this rate. When studies only reported
predation rate estimates for all wildlife combined, we calculated separate predation
rates by extracting taxa-specific prey counts from tables or figures and multiplying
the total predation rate by the proportion of prey items in each taxon. If taxa-
specific counts were not provided, we directly contacted authors to obtain this
information. For studies that presented low, medium and high estimates or low and
high estimates, we used the medium and average values, respectively. For studies
that presented more than one predation estimate for cats with similar ranging
behaviour (for example, owned cats in rural and urban areas), we calculated the
average predation rate.
Nearly all studies of un-owned cats report numbers or frequencies of occurrence
of different taxa in stomachs and/or scats. For studies reporting numbers of prey
items, we estimated annual predation rates by assuming one stomach or scat
sample represented a cat’s average daily prey intake (for example, an average of one
prey item per stomach or scat ¼365 prey per cat per year). This assumption likely
resulted in conservative estimates because cats generally digest prey within 12 h
(ref. 28) and can produce two or more scats each day29. For studies reporting
occurrence frequencies of prey items, we assumed this proportion represented a
cat’s average daily prey intake (for example, a 10% bird occurrence rate ¼0.1 bird
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per stomach or scat ¼36.5 birds per cat per year). This assumption results in coarse
predation rate estimates, but estimates from this approach are even more
conservative than those from the first assumption because many stomachs and
scats undoubtedly included more than one bird or mammal.
Predation rate estimates from many studies were based on continuous year-
round sampling or multiple sampling occasions covering all seasons. However,
seasonal coverage of some studies was incomplete. To generate full-year predation
rate estimates in these cases, we adjusted partial-year predation estimates according
to the average proportion of prey taken in each month as determined from year-
round studies reporting monthly data (birds and mammals8,33, birds only7,40). For
partial-year estimates from the northern hemisphere, we offset monthly estimates
from southern hemisphere studies by 6 months. The final annual predation rate
estimates for all studies are presented in Supplementary Table S1. The year-round
studies we used represent different geographical regions (for birds—England,
Kansas (US), Australia and New Zealand; for mammals—England and Australia)
with varying climates and slightly varying seasonal patterns of predation. For both
birds and mammals, averaging across full-year studies resulted in higher
proportions of predation in the spring and summer compared with fall and winter,
an expected pattern for much of the United States. The reference studies we used,
therefore, provide a reasonable baseline for correcting to full-year mortality
estimates. This approach greatly improves upon the assumption that mortality is
negligible during the period of the year not covered by sampling.
Quantification of annual mortality from cat predation. We estimated wildlife
mortality in the contiguous United States by multiplying data-derived probability
distributions of predation rates by distributions of estimated cat abundance,
following41. Quantification was conducted separately for owned and un-owned cats
and for birds and mammals. As there was a relatively small sample of US studies
that estimated predation rates (n¼14 and 10 for birds and mammals, respectively),
we repeated calculations using predation rate distributions that were augmented
with predation rates from Europe and all temperate zones. However, we only used
studies from the contiguous United States to construct all other probability
distributions (listed below).
We estimated mortality using the following model of cat predation:
Annual mortality from owned cats mpðÞ¼npcpodpphpprcor ð1Þ
Annual mortality from unowned cats mfðÞ¼nfcpfhfpr ð2Þ
Total annual mortality from all cats ¼mp þmf ð3Þ
where npc is the number of owned cats in the contiguous United States, pod is the
proportion of owned cats granted outdoor access, pph is the proportion of outdoor
owned cats that hunt wildlife, ppr is the annual predation rate by owned cats, cor is
a correction factor to account for owned cats not returning all prey to owners, nfc
is the number of un-owned cats in the contiguous United States, pfh is the
proportion of un-owned cats that hunt wildlife, and fpr is the annual predation
rate by un-owned cats. From the probability distribution of each parameter
(see Table 1 and Supplementary Methods for details about the specific probability
distributions used), we randomly drew one value and used the above formulas to
calculate mortality. Random draws were made using distribution functions in
Programme R (rnorm and runif commands for normal and uniform distributions,
respectively). We conducted 10,000 random draws to estimate a potential
range of annual predation on each wildlife taxa. For all analyses, we report median
mortality estimates and lower and upper estimates bracketing the central 95% of
values.
Sensitivity analyses. We used multiple linear regression analysis to assess how
much variance in mortality estimates was explained by the probability distribution
for each parameter. We treated total mortality estimates as the dependent variable
(n¼10,000) and we defined a predictor variable for each parameter that consisted
of the 10,000 randomly drawn values. We used adjusted R2values to interpret the
percentage of variance explained by each parameter.
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Acknowledgements
S.R.L. was supported by a postdoctoral fellowship funded by the US Fish and Wildlife
Service through the Smithsonian Conservation Biology Institute’s Postdoctoral Fellow-
ship programme. P. Blancher provided insight for development of the model of cat
predation magnitude, and R. Kays, C. Lepczyk and Y. van Heezik provided raw data from
their publications. C. Machtans facilitated data sharing, and participants in the 2011
Society of Canadian Ornithologists’ anthropogenic mortality of birds symposium pro-
vided context and perspectives. C. Lepczyk and P. Blancher provided comments on the
manuscript. The findings and conclusions in this article are those of the authors and do
not necessarily represent the views of the Smithsonian or US Fish and Wildlife Service.
All data used for this analysis is available in the Supplementary Materials.
Author contributions
S.R.L. designed the study, collected and analysed data, and wrote the paper. T.W. and
P.P.M. designed the study and contributed to paper revisions. All authors discussed the
results and commented on the manuscript.
Additional information
Supplementary Information accompanies this paper on http://www.nature.com/
naturecommunications
Competing financial interests: The authors claim no competing financial interests
associated with this paper.
Reprints and permission information is available online at http://npg.nature.com/
reprintsandpermissions/
How to cite this article: Loss S.R. et al. The impact of free-ranging domestic cats on
wildlife of the United States. Nat. Commun. 4:1396 doi: 10.1038/ncomms2380 (2012).
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2380 ARTICLE
NATURE COMMUNICATIONS | 4:1396 | DOI: 10.1038/ncomms2380 | www.nature.com/naturecommunications 7
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