DOMESTIC CAT (Felis catus )
PREDATION OF BIRDS IN AN URBAN ENVIRONMENT
Carol A. Fiore and Karen Brown Sullivan
2000
ABSTRACT
To quantify the effects of urban domestic cat predation on birds in Wichita,
Kansas, a city of approximately 300,000 residents, we collected birds killed by study
cats, analyzed fecal material, and tracked cats using radio collars to estimate mean
numbers of birds killed per cat. A random survey and information from local veterinarians
were used to calculate pet cat density. The results indicate that the average urban cat in
Wichita kills 4.2 birds per year. Additionally, we found that the majority of cats (83%)
kill birds, and the greatest risk to all birds occurred during the months of May and June (43%).
House Wrens (Troglodytes aedon ) appear to be at increased risk (9%)
when compared with other avian species in the Wichita area.
Given the current climate of increasing concern over declining songbird populations
(Franzreb and Phillips 1995), the need for better documentation of the impact of domestic
cats on birds is warranted. Much of the information on the hunting abilities of domestic
cats (Felis catus), an introduced predator in the United States, is anecdotal; few
studies have been done to assess the impact that free-roaming domestic house cats have on
native bird populations. The negative impact of cats on island birds has been well documented
(Fitzgerald and Karl 1979; Ludwig 1994; Turner and Bateson 1988; Heidemann and Vauk 1970).
Since the fauna on most islands did not evolve in the presence of mammals, many insular birds
are ill-equipped to deal with the predatory domestic cat.
Several different approaches have been taken to determine the relative contribution of
birds to the diet of domestic cats. Hubbs (1951) examined stomach contents of feral cats and
found birds in 74 of 184 stomachs examined; when present, bird remains comprised 25.2%
of the total bulk. Nilsson (1940, cited in Fitzgerald and Karl 1979) recorded 26 birds in the
stomachs of 86 house cats in Oregon. Liberg (1984), using scat analysis to study diets of
domestic cats in southern Sweden, determined that birds were the fourth important prey item
after rabbits, field voles, and hares. All of Liberg’s study cats were fed by their human caretakers
yet continued to hunt prey year round. Fitzgerald and Karl (1979) examined scat of feral
house cats and found evidence of bird consumption in 12% of cats. Davis (1957) indicated
that 24-25% of scats from study farm cats near Baltimore contained feathers and argued
this estimate is conservative as it did not reflect the vast quantity of pigeons the study cats were
seen to consume. Patton compiled statistics on prey taken by cats in Australia by asking owners
to fill out questionnaires (Potter 1991). Results indicated 50 - 60% of cats preyed on birds.
Mitchell and Beck (1992) collected prey items from one rural cat and four urban cats in Virginia.
During the period from January - November 1990, the rural cat captured 25 birds and the
urban cats caught 12. It was noted that the rural cat preyed almost exclusively on songbirds
during the winter. While much evidence exists to document that cats do prey on birds, few studies
have quantified cats’ impact on wild bird populations. Those few that have measured impact
have concentrated on rural or village cats.
Churcher and Lawton (1987, 1989) enlisted the help of all but one of the cat owners in
the small English village of Bedfordshire, England, and for one year all prey items observed by
owners were collected and turned over to the researchers. Fecal material was not analyzed
nor were cats tracked and observed by the researchers. Their estimate of 20 million birds
per year killed in England by house cats was a very conservative one because it did not
account for kills not brought to the owners. The proportion of unobserved kills may be
significant. For example, a researcher in southern Illinois speculated that his three house cats,
which he followed for six years, brought home only about 50% of killed prey individuals
(George 1974). A study which quantified the amount of prey killed by cats but not brought
home would be of value. Coleman and Temple (1993,1994, 1996) estimated cat density
in rural Wisconsin through the use of an elaborate eight page survey, and although they
tracked cats to determine ranges, they did not collect or attempt to estimate numbers
of prey items killed by cats. Their figure of 39 million bird kills per year in Wisconsin is
based on average bird kills per cat determined by other researchers (e.g., Errington 1936;
Eberhard 1954; Fitzgerald 1988; Mitchell and Beck 1992).
Patronek (1998), in a critique of published studies focused on cat predation,
pointed out several common methodological problems. For example, extrapolation of
data from the England study (Churcher and Lawton 1987, 1989) to other ecological
areas with different habitats and bird species is problematic. Coleman and Temple (1993)
may have overestimated cat density populations by polling residents when kittens were likely
to be most abundant. The use of small non-random data sets of cats, as in Mitchell and
Beck (1992), and a lack of physical evidence when polls are taken (e.g., Potter 1991)
were all noted. Patronek also claims that cats hunt birds during the day and tend to bring prey home.
A lack of studies exists on urban cats in general, and the methodology used in much
of the research to date is limited in its application. The purpose of our study was to collect
information necessary to estimate reliably the impact of domestic cats on birds in an urban
environment. To date, no other research has attempted to estimate average bird kills per
cat by combining information from bird collection, scat analysis, and radio tracking of cats.
Additionally, we estimated pet cat density in the city and collected information regarding
public perceptions and opinions about cat predation and regulation of cats, an undertaking
not attempted by other researchers.
MATERIALS AND METHODS
The Study Site.—The 41 cats in the study all resided with their owners in
Wichita, Kansas. The city has a population of 329,211 (U.S. Census Bureau estimate as of
July 1, 1998) within an area of 140 square miles (Wichita City Planning Department
estimate as of January 1, 2000). Residences of the 27 cat owners who participated in the
study were distributed inside the city limits and included single family homes, duplexes,
mobile homes, and apartments, all with different types and amounts of vegetation
on and around the property. A few owners resided on the edge of town bordering grassland areas.
The Study Cats.— Cats ranged in age from one year to 13 years,
both sexes were well represented (23 males and 18 females), and all cats were fed at least
once per day by their owners. All cats had been spayed or neutered and seven cats were
declawed. Every cat volunteered for the study was accepted provided the cat spent at least
part of the time outside. Some owners also owned a cat that was never allowed outside access,
and several households registered more than one cat for the study. Over half of the cats in the
study did not have access to a litter box. Each cat was given a unique identification number (1-46);
a few numbers were unused.
Determination of Average Bird Kills per Cat.— To determine the average
number of birds killed per cat per year, three different methodologies were used: collection of
birds killed by cats, scat analysis, and tracking of cats by radio telemetry. Volunteers bagged
dead birds or remains brought to them by their cat(s) or taken from their cat. Most of the volunteers
started collection in May 1998 and continued for one year. Cat owners were provided with
marked plastic bags and a pager number to call for rapid pickup of bird remains.
The second methodology used to calculate average number of bird kills per cat was to
examine fecal material of the study cats for the presence of bird parts, primarily feathers.
Persuading volunteers to turn over scat was difficult for several reasons. More than half of the
volunteers did not own a litter box and some volunteers were reluctant to participate in this
particular activity. A total of 14 households collected scat at least once during the study.
Two different methods were used in analyzing the fecal material. In the first, four
volunteers separately bagged each day’s scat for five consecutive days each month. Each day
was treated as a separate analysis. After approximately 12 hours, most of the contents of a
cat’s stomach have been passed (Hubbs 1951). For this reason, a feather(s) in each day’s
scat was conservatively counted as one kill. The data indicate that this approach is valid since
we rarely found feathers on consecutive days. In the second type of collection, 10 households
bagged the entire contents of the litter box, in which case the entire collection was treated as a
single analysis. The procedure used to check for feathers in scat was similar to that used by
Day (1966), Liberg (1982, 1984), and Fitzgerald and Karl (1979). Scat from each separate
collection was soaked in water in glass beakers for at least 24 hours. The contents of each beaker
was rinsed in a 1.4 mm wire mesh sieve which was held over an opaque plastic container. The
water in the container and the material in the sieve were then screened for feathers.
Questionable items were removed with tweezers and deposited in a separate viewing
container for examination under a dissecting microscope. When feathers were found they were
placed in vials, marked, and information recorded. No attempt was made to identify the species
of bird nor to assess the proportion of bird parts to other prey items. The objective was solely
to ascertain whether the cat had ingested a bird for which the owner had not collected remains.
Scat data were used in two different ways. When feathers were found during examination of scat,
an additional kill was recorded for that cat’s kill record if there had been no remains collected at
that time. If more than one cat was using the litter box, kills were alternated between cats
(starting with the lowest numbered cat), but only one cat in the household was credited with a kill.
Only one kill was counted per analysis regardless of the quantity of feathers recovered from the
scat. Since the scat data set was based on a small number of households, a second method was
developed to quantify the data. The following formula was used for each cat:
total number of times feathers were found (without owner knowledge)
total number of analyses
An average of all values was used to arrive at the percentage of time a cat could be expected
to have ingested a bird with no owner knowledge. This value was used to adjust the total number
of birds killed per cat per year.
The third method used was to track cats using radio collars. Although tracking
provided important supplemental information about cat activities and ranges, cat/owner
relationships, and owner knowledge of cat location and habits, the information gained was
qualitative. The equipment consisted of a TRX-3S receiver, a large directional antennae,
and an SOC-2270 transmitter collar (Wildlife Materials, Carbondale, Illinois). Binoculars
were used to watch the cat once it was located. The waterproof transmitter was mounted
on a 1.59 cm wide nylon collar. The antennae was hidden between the fabric on the collar,
and the signal had a range of about 0.62 - 0.93 km. Preliminary observations suggested that
the collar did not affect behavior. Habituation was achieved by fitting the collar to the cat
several days before tracking started.
The majority of the cats were not available for tracking due to owner or neighbor
objections, physical barriers (e.g., privacy fences), and trespassing issues. Although tracking
proved difficult in an urban setting, we observed eight cats during daylight hours for a total of 57 hours.
Cat Density Estimation.— Bird collection data, scat analysis, and tracking
enable a good estimate of mean number of bird kills, but without an estimate of cat density it is
difficult to judge the total effect of cat predation on bird populations in an urban setting. Since the
city of Wichita does not require registration or reporting of pet vaccinations, nor is there a leash
or nuisance law, cat density was not known at the start of this study. Thus, we used a twofold
approach to estimate density of pet cats in the city. All of the veterinarians in Wichita and the
surrounding areas (N = 61) were contacted by mail to determine the number of rabies vaccines
that were given to city cats for the year 1997. The data collected gave an estimate of currently
vaccinated pet cats in the city, not an estimate of the total number of pet cats nor an estimate
of ferals or strays.
The second approach utilized to determine cat density used the results of a random
telephone survey. Written by Dr. Ellie Shore of Wichita State University (WSU), with a
grant from the Edith J.Goode Residuary Trust, this Pet Ownership Survey was conducted
in January 1999 by the Elliot School of Communication at WSU and was aimed at
understanding pet ownership trends. For this study, only data from city residents
(N = 622) were used and relevant responses included those concerning observations
of feral and stray cats, current vaccines of pet cats, and questions regarding feelings
about cat predation and regulation. These data allowed calculation of two independent
estimates of pet cat density. In the first, the total number of cats vaccinated by
veterinarians was divided by the percentage of cats that survey respondents claimed
to have been vaccinated (e.g., if 500 cats received vaccinations and respondents
indicated that 50% of pet cats had been vaccinated, 1000 cats would be the expected
density). In the second method, the average number of cats per residence, as calculated
from the survey responses, was multiplied by the total number of residences in Wichita,
obtained from the City Planning Department.
RESULTS
Bird Kills.—A total of 113 birds, representing at least 23 species,
was collected during the course of the study (Table 1). Due to the poor condition of some collected
remains, it was not possible to identify 17 of the birds, some of which may have represented
additional species. The most birds collected were European Starlings (14%) followed
by House Sparrows (12%). The birds collected comprised adults (69%),
immatures (24.8%), and nestlings (6.2%). The protected bird species collected
in greatest numbers was the House Wren (9%). "Protected bird species" are granted
protection under the federal Migratory Bird Treaty Act of 1918. The data suggest that
ground-feeding and ground-nesting birds may be at higher risk (Table 1)
than those that nest and
feed above ground level. Cats in this study preyed on 7.3% of the total bird species
reported in the city although this total includes a large proportion of locally rare sightings.
While no threatened or endangered birds were taken, a Dickcissel, a bird listed by the
National Fish and Wildlife Foundation Partners In Flight Program as a species of
conservation concern, was brought into the home of the owner of cat 18 in May 1998.
This male cat also captured the largest number of birds (17 birds). The largest number
of birds taken by one household was from the residence of cats 9, 10, and 11, who
together killed 21 birds (Table 2); inclusion of scat data brings the total to 23 birds.
Our data indicated that 72% of female cats caught birds as compared to 91%
of males. The data also showed that 87% of cats out at night caught birds, and
although only 50% of daytime hunters took birds, two of these took a large number.
There were seven declawed cats in the study and all but one took birds; the top predator
cat was declawed. The owner of the top predator cat lived on an older small suburban
lot on the north edge of the city, and a review of the birds killed by this cat suggest the
animal had a large range. The greatest number of bird collections occurred during the
months of May and June, with secondary peaks during the months of April and July
(Figure 1).
Bird kills were counted only when actual physical remains were recovered (or feathers
found in scat). Verbal reports from volunteers were never counted as kills; the 113 birds
represent actual remains collected from volunteers (scat analysis added additional
birds to this total). Taking into account the actual number of days each volunteer
participated in the study and the number of bird kills recorded, we calculated a mean
of 3.44 ± 0.01 SE bird kills/cat/year.
Out of 215 separate scat analyses, each of which could have composed
several beakers of fecal material, feathers were found a total of 28 times. In only one
instance, however, did the owner know that a bird had been killed and/or consumed.
When scat data and bird collection data were combined, the number of kills recorded
for cat 13 totaled 17 birds (all but three were credited with scat information), which
puts this cat alongside cat 18 for the highest number of kills in the study
(Table 2).
A mean value of the percentage of time a cat could be expected to ingest a bird with
no owner knowledge was calculated to be 21% and includes four values of zero
for volunteers who provided scat just once during the study. Combining scat data
with bird collection data resulted in a final value of 4.2 bird kills/cat/year.
Radio Tracking.—Although tracking data revealed several trends
and patterns, the information was difficult to quantify. A total of eight different study
cats (4 males and 4 females) and several strays were tracked on and off at various times
of the day throughout the year of data collection for approximately 57 daylight hours.
Cats were observed hunting during the day, and cats confined indoors at night were
much more active during the day than cats allowed outdoors at all times. The cats
varied greatly in their daily routines and, although some appeared to take shelter during
unfavorable weather conditions, several cats were observed stalking during rainy times.
Without exception, owners had inaccurate knowledge about the range of their cat(s).
Tracking revealed that cats had no trouble scaling eight foot privacy fences, climbing
and jumping onto roofs, trees, and other structures (e.g., declawed cat 23 was
seen climbing trees on several occasions).
Cat Density.—Wichita veterinarians indicated a total of
27,776 rabies vaccines were administered to cats residing in the city limits in 1997.
Seven veterinary offices failed to respond and the number of rabies vaccines
attributed to these offices was estimated from the mean of all reporting offices
(514.4 ± 99.4 SE, N = 54), yielding a final total of 31,377 cat rabies vaccines
given in Wichita. Information from the Pet Ownership Survey indicates that 87.8%
of respondents claimed to have vaccinated their cat(s). Using this value (i.e., 31,377
divided by 0.878) yields a total of 35,737. Additional data provided by the City of
Wichita Animal Shelter and the local Wichita Humane Society in 1997 indicate an
additional 7,762 cats were picked up or dropped off, bringing the total to 43,499.
This number gives an estimate, although an extremely conservative one, of the number
of pet cats in the city of Wichita. An alternative method for determining densities of
pet cats is to estimate the average number of cats per home surveyed and then
multiply it by the number of homes in Wichita. For cat owners living inside the
city limits, average cat density per house was estimated to be 1.52 ± 0.07 SE
by the Pet Ownership Survey. Cat owners represent 19% of total residents
surveyed (N = 622, cat owners 117) suggesting that approximately 26,866 homes in
Wichita have cats. Thus, the total Wichita population is equal to 40,836 pet cats.
This is somewhat higher than the figure of 35,737 calculated using data from rabies
vaccines. Combination of bird collection, scat data, and pet density data reveal
pet cats will kill at least 171,511 birds each year in Wichita. Addition of feral and
stray cats would greatly increase this value.
The Pet Ownership Survey indicates a total of 35.2% of households
reported seeing stray cats, with most reporting a total of two or more strays sighted.
Since home ranges of feral cats were not estimated, it is difficult to know, based on
the replies from the phone survey, how many feral cats are present in the city. The Pet
Ownership Survey indicated that 43% of cat owners surveyed claim to keep their
cats indoors; research from this study, however, indicates that cat owners often give
inaccurate information about activities of their pets, and disappearances of supposedly
indoor cats, especially at night, are common. An indoor cat as defined in this study
meant a cat that never goes outdoors. People who interpreted this question to mean
that because the cat was inside "most of the time" it was an indoor cat, would be incorrect
for purposes of this study. However, assuming respondents were accurate in their replies,
the number of birds killed by pet cats each year in Wichita might be reduced to 73,750.
Results of Survey Concerning Cat Regulation.—The Pet Ownership
Survey also asked cat and/or dog owners how they felt about having cats licensed and
confined to the owner’s property. A total of 44% of cat owners said they were at
least somewhat in favor of regulation whereas 69% of dog owners were in favor; over
50% of owners of both cats and dogs were in favor. When dog and/or cat owners
not in favor of cat regulation were asked if they would change their opinion if it were
found that cats were having a negative impact on wildlife, 32% replied affirmatively.
DISCUSSION
Any study that utilizes human volunteers, especially for such a long period of time,
is prone to problems. This study was no exception; the many sources of bias all tended to
underestimate the numbers of birds killed (i.e., missed birds, missing cats, absence and
relocation of owners and schedule conflicts, remains thrown away, lack of active participation).
Volunteers reported bird escapes and although the extent of the injuries was unknown,
it is likely that most of these birds later died. Several birds were retrieved alive from
cats, but all the birds died subsequently as a result of the injuries. Many wildlife
rehabilitators cite poor survival rates for cat-caught birds (Dowling et al. 1994;
Ludwig 1994; Potter 1991).
Although birds are of only secondary importance in the diet of domestic cats,
small mammals such as mice and young rabbits being the primary prey taken
(George 1974; Churcher and Lawton 1987, 1989), our scat and bird collection
data demonstrate that most domestic cats kill birds (83% of our study cats).
In all but one case, when feathers were found in scat, the owner was unaware
that the cat had ingested a bird. This seems to refute Patronek’s (1998) claim
that "cats tend to bring prey home". In fact, the majority of the volunteers reported
their cat(s) did not bring prey to them, rather the owners observed the cat(s)
with the bird or found remains in the house or in favored cat locations. Sources
of error biasing numbers towards low values include missed feathers (researcher error),
bird killed but cat failed to ingest bird, plucking the bird well before consuming, and
ingestion of nestlings (no feather remains to detect). Often, cats kill but do not eat
their prey, and other researchers have shown that hunting and hunger are independent
(George 1974; Adamec 1976; Morris 1986). In fact, our scat analysis suggests
that a far greater number of birds are consumed than was previously thought.
We questioned volunteers about feather cat toys and presence of pet birds in the
home; neither was deemed to be a source of error. Additionally, a few of the
homes owned indoor cats not included in the study and scat analysis may have
included droppings from these cats. It is possible samples from these litter boxes
may have failed to include any droppings from study cats.
Turner and Bateson (1988) remark that today’s cats are not particularly
nocturnal, but our research suggested otherwise. Goldsmith et al. (1991) tracked
cats primarily at night when they were most active. Calhoon and Haspel (1989)
found stray and feral cats avoided most humans by seeking shelter during the day
and emerging only late at night. Main activity times for cats appear to be bimodial
with peaks occurring around midnight and before sunrise (Tabor 1983; Haspel
and Calhoon 1993). Patronek (1998) has criticized cat predation studies for their
lack of direct observation of bird kills by cats but, given the secretive nature of cats,
their solitary hunting and reluctance to tolerate observers, a lack of direct documentation is not surprising. Our tracking data suggest that cats are extremely difficult to
watch and even more difficult to observe in acts of predation. This does not
suggest a lack of predation; scat analysis and bird collection data have clearly
shown otherwise.
Our estimate of pet cat density is likely to be low for several reasons.
The cat rabies data supplied by area veterinarians were from 1997 and the
population of Wichita, especially in the west part of the city, has increased over
the past two years, likely leading to a greater number of pets. Calculated density
was based on data from the Pet Ownership Survey which indicated 87.8%
of respondents claim to have vaccinated their cat(s) for rabies, an unreliable figure
according to some city officials and veterinarians. This would again yield a low number
for the pet cat population. The random survey did not question residents who did not
own either a cat or a dog concerning stray/feral cats. Since tracking data indicate that
stray and feral cats will often be present in suburban yards without a resident cat, the number
of stray cats that were actually reported from the survey could be low. Additionally, research
from this study and others indicates cats are most active at night (Haspel and Calhoon 1993),
and it is likely that residents were sleeping while cats were present on their property.
Since cats were not counted in industrial and commercial areas, parks, and more rural
areas of the city, it is likely this further biases density numbers.
The long-term ecological implications of cat predation on birds in Wichita are
far from clear. More investigation is called for, especially with regards to such sensitive birds
as the Dickcissel. Certainly there are many threats to birds besides cat predation;
habitat loss on wintering and nesting grounds may be the primary factor in the decline of
many songbirds. Cats may not be one of the primary causes of avian mortality, but they do
kill birds, and in some areas may well prove to be a cause for serious concern. The latest estimate
of the number of pet cats in the U.S. is from the 1999-2000 American Pet Products Manufacturers
Association’s Pet Owner Survey which estimates there are 64 million pet cats in the U.S. It seems
reasonable to assume that on the basis of the pet cat population alone in this country, that
if each cat killed 4.2 birds per year as did the average cat in this study, this would result
in the death of at least 269 million birds per year due to predation by pet cats alone. Further
assuming that half of these cats never go outside (e.g., the phone survey in this study indicated
that 43% of pet cats were never allowed outdoors), at least 134 million birds could be
expected to die as a result of domestic pet cats. It is likely that well under 43% of pet
cats are kept strictly inside, especially if rural cats are considered. The inclusion of feral and
stray cats would greatly increase this figure.
The common misconception that declawing cats prevents bird killing has been
proven false by this and other studies. Also, results from several studies clearly show that
bells do not prevent cats from taking prey (e.g., Potter 1991). The cat owner volunteers in
this study were asked upon its completion if they would keep their cats inside as a result of
the study, and they were told that the greatest threat to birds in Wichita occurs during the
months of May and June. Of the 26 volunteers who answered this question, 73% said
they would continue allowing their cat(s) outside exactly as they did before. However, if
even 20% of individuals are willing to restrict their cat’s activity, a significant step
towards reducing impact on native bird populations could be achieved.
ACKNOWLEDGMENTS
We thank Jerry Davis for his special inspiration, Donal O’Brien for
financial support, Bob Gress, Max Thompson, and Gene Young for help in identifying
the birds, and Linda Winter and Ron Jurek for their wonderful advice and many
documents and letters. A special thanks to Ellie Shore who designed a wonderful
survey and allowed us to incorporate our questions, and to Dr. Jon Piper for his
words of encouragement and for reading the manuscript. Free cat products, which
were used to compensate volunteers for their efforts, were provided by A&M Products,
Wal-Mart of West Wichita, and PetsMart. Without the support of Wichita State
University and our 27 cat owners, this project would not have been possible.
We would also like to thank our families, who have been incredibly supportive of our work.
No part of this article may be reproduced without the
authors' permission.
Literature Cited
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