Predation of wildlife by domestic cats in Great Britain
Written for The Mammal Society by MICHAEL WOODS, ROBBIE A. MCDONALD and STEPHEN HARRIS
To be published in Mammal Review
Domestic cats Felis catus are the most abundant carnivores in Great Britain and their numbers appear to be growing. In 1981, the national population of cats was estimated to be 6 million (UFAW, 1981). In 1993, the Cats Protection League estimated that approximately 25% of British households owned at least one cat and that the national population was approximately 7.6 million. This was predicted to increase to 8 million by 2000 (Cats Protection League, 1993). A more recent estimate supported this predicted increase and estimated that there were 7.8 million cats in 1998 (Turner & Bateson 2000). In addition, a minimum of 813 000 cats are estimated to live in a feral or semi-wild state in rural areas and a further unknown number of cats have only loose associations with domestic households in urban areas (Harris et al., 1995). It therefore seems likely that the total cat population in Britain in 2003 is in the region of nine million. By comparison to native carnivores, this is nearly 20 times the estimated pre-breeding populations of stoats and weasels and 38 times the estimated pre-breeding population of foxes Vulpes vulpes (Harris et al., 1995).
cats depend on food supplied by their owners. Therefore, their populations
are not limited by the availability of wild prey. Cats frequently kill
wild animals and the combined impact of predation by millions of cats
may have a substantial effect on wildlife. This prospect has previously
been highlighted by several authors (Churcher & Lawton, 1987; May,
1988; Barratt, 1997, 1998). The recent rapid declines seen in British
populations of many farmland and garden birds (Mead 2000), and the increasing
importance of gardens for a range of small birds (Mead 2000), has led
to renewed concern over the potential impact of cat predation on bird
populations. However, experimental evaluations of the impact of cat predation
on wildlife remain scarce and assessments of factors that might mitigate
any such impact often lead to public controversy, arising from concern
about the welfare of cats (May, 1988; Proulx, 1988; Fitzgerald, 1990;
& Lawton (1987) calculated that in a single English village, cats
were responsible for up to 30% of mortality in a house sparrow population
and concluded that domestic cats were a major predator in a typical English
village. They found that the average cat caught and brought home approximately
14 prey items over the 12 months of their survey. May (1988) extrapolated
from this figure and suggested that about 100 million wild birds and small
mammals could be killed by 6 million cats every year in Britain. Mead
(1982) ascribed 31% of recoveries of ringed robins and dunnocks to cat
predation, but believed that there was no evidence that cats affected
the overall populations of these species. Sharing this view, Fitzgerald
(1988) and Fitzgerald & Turner (2000) asserted that on continental
landmasses, wildlife had co-evolved with cats for hundreds of generations
and that any species that were susceptible to predation would be "long
extinct". It has also been argued that in many situations cats may
limit populations of other predators, such as rats Rattus spp. that could
have a more pronounced effect on wildlife (Fitzgerald, 1990; Fitzgerald,
Karl & Veitch, 1991). However, while cats may eat Rattus spp., larger
individuals of one of the most damaging rat species Rattus norvegicus
are often avoided since they are hard to handle (Childs, 1986). Most authors
do agree, however, that wildlife on oceanic islands is likely to be particularly
susceptible to the impact of predation by cats. A particularly well-known
example is that of the Stephens Island wren Xenicus lyalli, the entire
population of which was eliminated by a single cat belonging to the island's
lighthouse keeper (Oliver, 1955). Likewise, the Socorro dove Zenaida graysoni
has also been driven to extinction primarily by cats (Jehl & Parks,
1983). Similarly, mammalian species such as hutias Geocapromys spp. have
been exterminated from several Caribbean islands as the result of predation
by cats (Fitzgerald, 1988).
investigators of predation of wildlife by cats have frequently taken advantage
of their habit of bringing back their prey to their owners' houses. Owners
have been asked to record the species caught and the date it was retrieved
or have retained the prey animal and given it to investigators for identification
(Borkenhagen, 1978; Howes, 1982; Churcher & Lawton, 1987; Barratt,
1997, 1998). Similarly, but working at a smaller scale, Carss (1995) recorded
the captures of his own two domestic cats over two periods of two years.
There are several qualifications for the use of this method to describe
the killing behaviour of cats, as opposed to alternatives such as analysing
faeces or gut contents (reviewed by Fitzgerald, 1988; Fitzgerald &
Turner 2000). Qualifications include the following: The prey brought home
by cats should be representative of the total and variety of prey they
actually kill. Cats belonging to households participating in the survey
should exhibit behaviour representative of the killing behaviour of cats
in general. Cat owners should be competent and honest recorders of the
items their cats bring home. Unfortunately, for logistical reasons, these
qualifications have usually remained untested. Nevertheless, the participation
of cat owners is the only logistically feasible method of investigating
cat predation of a range of species at a large scale.
applied during this survey were based on the work of Churcher & Lawton
(1987) and Barratt (1997, 1998). A survey was conducted by The Mammal
Society between 1st April and 31st August 1997. Participants were recruited
through Society membership and appeals through the national media, including
radio, newspapers and magazines. Forms were sent out to approximately
1400 households. Cat owners were asked to record in as much detail as
possible the prey items brought home by each cat in their ownership. Where
several cats lived in the same house, it was not always possible to separate
the items brought home by individual cats. In these cases, records were
included in our basic description of prey items, but were omitted from
statistical analysis, which was based on households that could assign
all retrieved items to individual cats. In common with previous investigators,
we have assumed that the numbers of prey brought home by the cats was
closely related to the numbers and variety of prey they actually kill.
We have also assumed that the cat owners participating in the survey are
representative, competent and honest recorders of the items their cats
brought home. The validity and consequences of these assumptions are discussed
were grouped by taxonomic Class and Order where possible. In some cases,
householders were unable to identify the prey remains brought home by
their cat at all and these items were omitted from statistical analysis
and were included only in total counts of prey items brought home (Appendix
1). Statistical analyses were conducted first on the numbers of birds,
mammals and herpetofauna (reptiles and amphibians) and then on the numbers
of bird and mammal species brought home by each cat. Counts of prey and
prey species were log10 (n +1) transformed and cat ages were square root
(n + 0.375) transformed to improve their approximation to normality (Zar,
1996). Four analyses of covariance (ANCOVA) were conducted. The first
investigated the factors associated with the household that influenced
the numbers of animals brought home by cats to the house. Region, residence
type and the provision of bird food were factors and the total number
of cats in the household was a covariate. The second ANCOVA investigated
the factors associated with individual cats that influenced the numbers
of animals they brought home. This analysis considered only the cats that
lived in households with no other cats. This was in order to avoid possible
pseudoreplication (Hurlbert, 1984) arising from similar conditions being
applied to several cats living in one household. Sex, allowing the cat
out at night and equipping the cat with a bell were factors and the age
and condition of the cat were covariates. Both of these analyses included
cats that did not bring home any animals, i.e. zero counts. These analyses
were then repeated, but with the number of bird and mammal species brought
home as the dependent variables and the total number of birds and mammals
brought home as an additional covariate, to control for the effect of
predation rate on species range. In this case, since the identification
of species clearly required at least one animal to have been brought home,
these analyses included only cats that had brought home at least one bird
or mammal. Few herpetofauna were identified to species and the range of
species was small, hence analysis of the number of species was not undertaken.
Interaction terms were initially included in both analyses but were removed
if found not to be significant and the analyses were rerun. No statistical
analysis was made of other prey groups. Residuals from the ANCOVAs were
checked for normality. Adjusted group means, corrected for covariates,
of significant factors from ANCOVAs were compared using the Bryant-Paulson-Tukey
test with Kramer's modification for unequal sample sizes (Bryant &
Paulson, 1976; Day & Quinn, 1989).
the order of magnitude of the total numbers of animals that may be brought
home by cats in Britain, we first estimated the population of cats likely
to bring home prey of each group. This was achieved by multiplying the
estimated cat population, here taken to be 9 million, by the proportion
of this sample that brought home at least one of each prey type (Table
3). Then, for all the cats that did bring home at least one of each prey
category, we calculated the mean and 95% confidence intervals of the log-transformed
data. The number of prey animals brought home by all cats in Britain was
then estimated by multiplying the estimated population of cats that retrieved
that prey type by the back-transformed means and the associated, asymmetric
95% confidence limits. In order to be sure of the confidence intervals
for this estimate, it was derived from the records from households where
all prey items were assigned to individual cats, rather than the returns
from households where the kills of several cats could not be distinguished.
The survey received prey records from 618 of the 1400 (44%) households (Table 1). A total of 14370 prey items were brought home by 986 cats (Table 2).
A minimum of 20 species of wild mammal, 44 species of wild bird, four species of reptile and three species of amphibian were recorded (Appendix). The numbers of prey items brought home was recorded for 696 individual cats living in 506 households (Table 3).
The frequency distribution of the numbers of animals brought home by each cat was markedly skewed in both cases (Figure 1). For 467 cats that brought home mammals that could be identified to species, the mean number of species brought home was 2.4 (95% confidence interval 2.2-2.5, range 1-9). For 475 cats that brought home birds that could be identified to species, the mean number of species brought home was also 2.4 (95% CI 2.3-2.6, range 1-10). There was a significant but weak relationship between the number of mammal species and the number of bird species brought home by cats that brought home at least one identifiable species of mammal or bird (rs = 0.121, n = 277, P < 0.05). The number of species brought home was a function of the number of animals brought home (Number of bird species = 0.484 * Number of birds + 0.174, r2 = 0.61, F1,473 = 734.9, P < 0.001: Number of mammal species = 0.306 * Number of mammals + 0.225, r2 = 0.52, F1,465 = 508.8, P < 0.001).
Full details of all factors and covariates relating to households were collated for 396 households owning 555 cats. The number of cats varied between households: 297 (75%) households had only one cat, 65 (16%) had two cats, 20 (5%) had three cats, 9 (2%) had four cats. Five, six, seven and eight cats were owned by one, two, one and one households respectively. The mean number of cats per household was 1.4 (95% CI 1.3-1.5), though the median was 1 cat per household. Food was provided for birds by 265 (67%) households. The number of herpetofauna brought home per household was positively related to the total number of cats living in the household, but no effect was observed for mammals or birds (Table 4).
Region significantly affected the numbers of herpetofauna brought home, but had no significant effect on the numbers of birds or mammals. There were, however, no significant pairwise differences between regions in the numbers of herpetofauna brought home, probably due to limited sample sizes in certain regions. Nonetheless, there was a general trend of decreasing numbers of herpetofauna brought home with increasing latitude (Figure 2).
The numbers of both birds and mammals, but not herpetofauna, brought home were significantly affected by residence type (Figure 3). Greater numbers of birds were brought home by cats living in bungalows than in terraced houses or flats. Greater numbers of mammals were brought home by cats living in detached houses than in semi-detached and terraced houses.
The mean number of birds and herpetofauna brought home per cat was significantly lower in households that provided food for birds but no significant effect of providing bird food was observed for mammals (Figure 4). In contrast, the number of bird species brought home was significantly greater in households that provided food for birds than in those that did not (F1,363 = 4.11, P< 0.05) (Figure 4). No other factors relating to households significantly affected the numbers of bird or mammal species brought home.
Full details of all factors and covariates relating to individual cats were collated for 282 cats living in households with no other cats. The sex ratio of these cats (147M:135F) was not significantly different from even (G test with Yates' correction for continuity G = 0.43, d.f. = 1, P > 0.05). Bells were worn by 92 (33%) cats. At night, 90 (32%) cats were kept indoors. There was no significant association between bell-wearing and being kept indoors at nights (2 test with Yates' correction for continuity: ?2 = 1.96, d.f. = 1, P > 0.05). The mean age of the cats was 5.5 years (95% CI 5.1-5.9) and the mean condition was 2.0 (95% CI 1.9-2.1). There was a significant positive correlation between the numbers of mammals and birds brought home (r = 0.23, d.f. = 280, P < 0.001), though 94% of the variation in the numbers of birds brought home remained unexplained by the number of mammals. The sex of the cat did not significantly affect either the numbers of mammals or birds brought home (Table 5).
The numbers of mammals brought home was significantly lower if the cat wore a bell, but no similar effect was observed for birds or herpetofauna (Figure 5).
The numbers of mammals brought home were significantly lower and numbers of herpetofauna were significantly higher numbers if the cat was kept indoors at night, but no effect was observed for birds (Figure 6).
The age and
condition of the cat were both negatively related to the numbers of birds
and herpetofauna brought home, but not to the numbers of mammals. No factors
relating to individual cats were found significantly to affect the number
of bird or mammal species brought home.
By virtue of their abundance in many ecosystems, domestic and feral cats are a major predator of wild animals in Great Britain. While their impact as predators and the necessity of measures to mitigate this are a controversial topic, there is a clear need for analysis of the numbers of animals affected and of the factors influencing kill rates. It was our aim to report on the numbers of animals brought home by cats at a national scale and to establish hypotheses for some of the factors affecting this. This descriptive survey was not intended to gauge the impact of predation by cats on the population dynamics of their prey, and so no data on prey populations were collected.
Surveys that require the participation of the public should be considered in the light of several limitations. It was not possible to determine how representative this sample is of cats in general, nor how representative the behaviour of the cats surveyed was. A major caveat is the likely bias of this survey towards cat owners who have a particular interest in wildlife biology and conservation (Fitzgerald & Turner 2000). Equally there is a likely bias towards cats that have predilections for killing wild animals and for bringing home their prey. Approximately 20% of cats brought home either no birds or no mammals during this survey while cats that brought home no prey items at all made up 8.9% of the sample. In their exhaustive survey of all the cats living in one village, Churcher & Lawton (1987) found that 6 of approximately 70 cats (8.6%) did not bring home any prey during the year of their study. This similar figure encourages the conclusion that the cats in this survey were comparable to cats in at least one other detailed study. Cats that do not bring home prey may either not be killing wild animals at all or they could be killing them but not then bringing them home. A failure to consider greater numbers of these two "non-predatory" and "non-retrieving" types of cat would lead to a considerable difference between our measures of the total numbers of wild animals brought home and the actual number of wild animals killed by cats in general. The fact that "predatory" cats will not bring home all the prey they kill and that our estimates are therefore a minimum number of prey killed, may compensate for this bias to some extent.
Clearly, cats will not bring home all the prey they kill. Some smaller prey will be completely eaten while other items too large to transport large distances may be abandoned or eaten in situ. Nonetheless, the number of prey items brought home and recorded by householders is an index representing the minimum number of animals killed by cats. The quality of this index has not been investigated in detail here or elsewhere, though George (1974) found that three farm cats never ate or deposited their prey where it was caught, but always carried it to his house or lawn. Our main aim was to determine which factors influence the prey capture rates of domestic cats. Therefore, an index of the number of animals killed is adequate, if we can assume that it is the cat's ability or inclination to capture prey that is influenced by the factors being investigated and not its inclination to bring prey home, i.e. it seems safe to assume that by wearing a bell a cat's ability to catch a mouse may be affected, but not the cat's tendency then to bring the mouse home.
We have confirmed
the findings of numerous previous investigators that there is great variation
between individual cats in the numbers of wild animals they bring home
(Churcher & Lawton, 1987; Barratt, 1998). The simple average (total
items / n cats) number of animals brought home in this five month survey
(16.6) was rather higher than other studies; Over 12 months Churcher &
Lawton (1987) recorded an average of "about 14" items per cat,
though this average included only vertebrate prey. Barratt (1998) recorded
a 12 month average of 10.2 vertebrate prey items and Borkenhagen recorded
a mean of 5.7 items per cat. Howes (1982) found that young cats between
18 months and 2 years old caught an average of 33 vertebrate prey items
a year, though this declined to 12 items a year in older cats. Our survey
took place during the period April-August when most animals were brought
home by cats in the other surveys (Churcher & Lawton, 1987). Nonetheless,
ours is still rather a high average and further studies would be required
to account for this high figure. It should be noted that in all of the
above cases the simple average number of animals brought home, is not
a useful measure of central tendency because of the skewed frequency distribution
of the numbers of prey items brought home (Figure 1). Hence our consideration
of these figures is limited to between study comparisons. In our estimates
of the total animals killed by cats, we used the back-transformed mean
and confidence intervals of the log-transformed data.
Perhaps not surprisingly, given differences in appropriate hunting techniques and prey availability, our results have suggested that the factors influencing the rate of prey capture differ between prey groups. The regional effect on numbers of herpetofauna brought home likely reflects the distribution of these species, several of which are found mainly in southern regions of Britain. Curiously, cats living in households that provided food for birds brought home fewer birds and fewer herpetofauna than cats living in houses that did not provide bird food. When the effect of bird capture rate on species range was controlled for, cats living in households where birds were fed brought home a wider range of species. It may have been expected that feeding birds attracted both a greater number of birds and a wider range of species to the garden and placed them within easy reach of predators. This appears to be the case at least in terms of the range of bird species brought home. Dunn & Tessaglia (1994) asserted that predation rates in gardens where birds were fed were similar to predation rates in other areas. In support of our findings, however, several authors have suggested that feeding birds may reduce susceptibility to predation in two ways. A greater number of birds may enhance group vigilance behaviour and thus warn against predator presence (Siegfried & Underhill, 1975; Waite, 1987; Popp, 1988). Alternatively, extra food supply may reduce foraging time and thus the time at which the birds are at risk of being captured by cats (Jansson et al., 1981). The range of species would not necessarily be affected in this way, since foraging responses to predation risk vary between species (Giesbrecht & Ankney, 1998) and such mechanisms may be more significant in common, flocking species. While such biological explanations are appealing, it remains a possibility that households that owned cats they knew to be prodigious killers of birds or herpetofauna chose not to feed the birds. This alternative explanation also seems plausible for the observed pattern in herpetofauna and for the pattern in species range, since owners are likely to decide not to feed birds on the basis of numbers rather than diversity of birds brought home.
We found that younger or thinner cats brought home more birds and herpetofauna. As cats get older or fatter, the rates of retrieval of these groups declined. Fewer mammals were brought home by cats that were equipped with bells and by those that were kept in at nights. These results were intuitive. Bells may serve as a warning to mammals of a predator's approach. The capture rates of neither birds nor herpetofauna were affected by equipping cats with bells. It is possible to speculate that this may be because birds rely largely on visual cues in predator avoidance behaviour, or because the acoustic qualities of cat bells may not lend themselves to warning birds or herpetofauna. Alternatively, a cat may have been equipped with a bell because without a bell it was a prodigious killer. Cats that were kept in at night brought home fewer mammals and greater numbers of herpetofauna than cats that were allowed at night. Wild mammals are predominantly nocturnal and so keeping cats indoors at night would reduce their access to mammalian prey. Producing a similar pattern, some cat owners may encourage their cats to be outdoors at night, in order that they "keep down vermin" and so these cats bring back higher numbers of mammalian prey. It is not clear why patterns of retrieval of herpetofauna should differ in relation to being kept in at nights, though it is possible that cats that are let out early in the morning and are keen to hunt are able to catch reptiles and amphibians while they are still cold and inactive. The same would not apply to birds. Clearly, it is essential that an experimental approach be taken to identify which of the factors we have identified as influencing prey capture rates are causally related to the numbers of wild animals killed. This aim has recently been realised by the experimental work of Ruxton et al. (2002), who have confirmed that the numbers of wild animals killed and brought home by cats can be reduced by equipping them with bells.
Our estimates of the total numbers of animals brought home by cats throughout Britain should be treated with requisite caution and these figures do not equate to an assessment of the impact of cats on wildlife populations. Nonetheless, they represent an early assessment of the likely order of magnitude of wild animals killed by a very large population of domestic cats. While a figure of 92 million wild animals being killed by an estimated 9 million cats over the five months April-August 1997 is doubtless striking, this amounts to the average cat bringing in only one item every two weeks. The critical element in this equation is the very large number of cats living in Britain so an accurate estimate of the British cat population is essential to improving the accuracy of these calculations. Likewise, it may not be appropriate to include feral cats in these calculations, since their killing rates may be substantially different from truly domestic cats. Our estimates took into account that sizeable proportions of the cat population are likely not to bring home any items from some prey groups (20% for mammals and birds, 80% for herpetofauna). Taking into account the fact that we may have focussed on predatory cats, if the true rates of predatory behaviour were half those we observed, there would remain the probability that the equally striking figure of 46 million wild animals are killed by the remaining "predatory" half of the population.
In Britain, most prey species have evolved under the selective pressure of predation by numerous species of wild mammalian and avian predator, albeit living at relatively low densities. Thus, these species are likely to be relatively tolerant of predation when compared to the more vulnerable fauna of oceanic islands. Nevertheless, the continuous pressure of predation by carnivores living at high densities and that are not in any way regulated by the availability of wild prey, could be considered analogous to the process of hyperpredation on oceanic islands (Smith & Quin, 1996; Courchamp et al., 1999). This is the process whereby native species are threatened by occasional predation by a large population of introduced predators that is sustained by abundant introduced prey species that are in turn well adapted to high predation pressure. A constantly renewed food source, i.e. provision of food by householders, may be compared to the ready availability of introduced rats or rabbits on oceanic islands (Courchamp et al., 1999). It is conceivable that predation by superabundant and well-fed predators such as domestic cats, could lead to the decline of continental species, if only on a local or temporary basis. Baker et al. (2003) recorded a negative relationship between numbers of wood mice and the numbers of cats visiting suburban gardens. This suggests that high levels of cat activity may deplete the numbers of otherwise common species, such as wood mice, in local areas. It is not possible directly to discern the process of hyperpredation in the data recorded here, though the occurrence of species of growing conservation concern among the prey records, such as water shrews, yellow-necked mice and harvest mice (Marsh, Poulton & Harris, 2001; Greenwood, Churchfield & Hickey, 2002; Moore, Askew & Bishop, 2003) gives additional cause for concern. Churcher & Lawton (1987) concluded that cats had a significant impact on house sparrows in the village they studied. A potential link between the frequent occurrence of sparrow predation in this and other studies and the pronounced decline in this species throughout Britain, should, therefore be considered and experimental work is called for in the light of this descriptive study. This echoes the recent suggestion of Crick, Robinson & Siriwardena (2002) that targeted studies are required to investigate the role of predation by domestic cats in the decline of house sparrows, particularly in urban areas.
In conclusion, this survey confirms that cats are major predators of wildlife in Britain. Further investigation of the extent and nature of predatory behaviour among domestic cats is clearly warranted by this initial work. In particular, detailed observation of cats in the field and description of the numbers of animals they kill and the proportion they retrieve are essential. Investigation of the response and attitude of cat owners living in a range of environments to the predatory behaviour of their cats would also be valuable (Coleman & Temple, 1993). Although this was not an experimental study, there were differences in the numbers of wild animals brought home by cats subjected to different management regimes. Experimental studies of the effects of equipping cats with bells (Ruxton et al. 2002) or other devices, keeping cats indoors at night and feeding birds will all be essential for evaluating the desirability and likely success of attempts to reduce the numbers of animals killed by growing cat populations.
This work was funded by The Mammal Society. We are grateful to Ray Littler for statistical advice, Elisabeth Pimley for assisting with data entry and for conducting follow-up surveys, Gemma Lee and Mark Jones for assisting with data entry and to the participating householders for providing detailed information about their cats. Thanks also to Mike Fitzgerald for advice and providing reprints, Jeremy Greenwood, Andrew Kitchener, Tom Langton, Graeme Ruxton and three referees for comments on previous drafts of this manuscript.
& Burton, J.A. (1978) Reptiles and Amphibians of Britain and Europe.
Mammal Society - the voice for British mammals, and the only organisation
Most recent revision March 1st 2003