A new
study by researchers with the
Smithsonian Migratory Bird Center illustrates the impacts that outdoor cats can have on the
survival of Gray Catbird young.
There is perhaps no topic in bird conservation as contentious and controversial as that of outdoor house cats and birds. This is especially evidenced by an incident that occurred in Texas that drew national media attention, and polarized bird lovers and cat lovers across the U.S.
In November of 2006 Jim Stevenson, director of the Galveston Ornithological Society and author of the “Wildlife of Galveston,” was out birding at a favorite spot near the San Luis Pass channel bridge spanning in Galveston in November of 2006. He found a group of federally endangered Piping Plovers roosting among the grassy beach dunes below the bridge. As he watched the birds, a feral cat, from a nearby cat colony, began stalking the plovers. To protect the birds, he attempted to capture the cat and failed. The next day, Stevenson returned with a rifle and shot the cat that had been stalking the plovers the day before (Barcott 2007). It was a shot that set off a powder keg of debate and legal proceedings.
A bridge toll operator had been feeding and maintaining the feral cat colony that lived below the bridge. He considered the 15-20 cats to be his pets (Williams 2008). He became outraged when Stevenson killed the cat he had nicknamed Mama Cat (Murphy 2007) and called the police. Stevenson was arrested and charged with animal cruelty, which carried a penalty of up to two years in jail and a $10,000 fine (Barcott 2007, Murphy 2007). The case went to trial and eventually charges against Stevenson were dropped because of a deadlocked jury (Williams 2008).
Shortly after his trial, Stevenson had to flee Texas for a period of time because of death threats and a reported attempt on his life (Meyers 2007). Unfortunately, this case illustrates how emotion-fueled this issue is and both sides mean well for the animals that they love.
How Many Cats? How Many Dead Birds?
The introduction of domesticated cats into North America was innocent enough. It’s thought they were brought from Europe in the early 1800’s to help control rodents in eastern seaboard cities (George 1974). However, while their intended targets were rodents cats are opportunistic and will prey upon whatever they can catch. Almost a century after their introduction North America’s cat population had grown substantially as did their impact on non-target species, like birds.
In his 1915 book, “Wild Bird Guests,” Ernest Harold Baynes began compiling some early estimates of how many outdoor cats existed in the U.S., and how many birds they killed annually. Baynes reported that Frank Chapman, a prominent ornithologist of the time, calculated that a single cat could kill as many as fifty birds in a single season, and that the estimated 25 million cats of New England could kill 500,000 birds annually. Similarly, Baynes reported that another ornithologist had estimated 70,000 farm cats in Massachusetts were killing 700,000 birds every year in that state (Baynes 1915).
In 1972 the American Humane Association estimated 31 million cats existed throughout the U.S. (Ogan and Jurek 1997). By 1990 there were an estimated 60 million cats owned by households in the U.S, according to U.S. Census data (Coleman et al. 1996). These numbers do not include feral or semi-feral cats that are not considered pets. Recent estimates by the American Bird Conservancy put the number of pet cats in the U.S. closer to 90 million. A 1997 report by the Progressive Animal Welfare Society (PAWS) estimated that 50 million cats lived outdoors, as feral animals, in urban alleys, abandoned buildings, and parks across the U.S. Conservatively, anywhere from 40 to 80 million cats may roam the outdoors and perhaps many more when we consider feral cat colonies.
A survey of landowners in southeast Michigan estimated that approximately 15-56% of landowners had outdoor cats and the total number of cats ranged from ~800 to ~3100 and killed between ~16,000 and ~47,000 birds (Lepczyk et al. 2003). Sadly, the researchers suggest this may be an underestimate of both the number of outdoor cats and the number of birds killed in the region. While rural landowners typically had more outdoor cats, urban areas had higher cat densities (cats per hectare (ha)). Additionally, over 20 species of birds were reported as prey items, with sparrows and Blue Jays being the most frequently reported prey items (Lepczyk et al. 2003).
A 1996 study from Wisconsin suggests that the 1.4 to 2 million estimated free-ranging outdoor cats in that state may kill anywhere from 8 to 219 millions birds every year. If we assume that other states have approximately the same number of free-ranging outdoor cats that kill the same estimated number of birds, a rough calculation would find that there are approximately 70 to 100 million outdoor cats in the U.S. that kill anywhere from 400 million to 11 billion birds annually. Another estimate suggests there are at least 120 million free-roaming cats that kill an estimated 500 million to 3 billion birds annually (Dauphiné 2008). If either estimate is accurate, then the annual avian mortality caused by outdoor cats is potentially comparable or greater to mortality resulting from collisions. It’s also scary to think that birds make up only an estimated 20% of the prey items of outdoor cats. Small mammals make up an additional 70%, with the remaining 10% being other animals including reptiles and amphibians (Coleman et al. 1996).
Ecological interactions between birds and cats
As bird-lovers, we might be concerned that providing bird-feeders in our backyard might increase the number of birds that are preyed upon by cats, especially given that a 1994 study based on Project FeederWatch data suggests that cats account for 29% of the predation of birds at feeders (Dunn and Tessaglia 1994). A survey study in Michigan found that the number and density of bird feeders in a landscape was not correlated with the number of birds killed by outdoor cats (Lepczyk et al 2003). This means that it doesn’t matter if you have one feeder or dozens of feeders in your backyard, cats will potentially kill the same number of birds in your yard.
A study in Georgia found that 28 outdoor cats visited a yard over the course of a two year period. 26 of those cats were considered to be feral cats. Two were domesticated cats that were allowed to roam outdoors. The number of cats preying upon birds in the yard was enough to result in a decreased abundance of birds in the yard (Dauphiné and Cooper 2008). As bird lovers we have to be aware that creating habitat for birds in our backyards may expose birds to higher levels of predation from cats, if there are a lot of outdoor cats roaming our neighborhoods. Being aware of this is important because it allows us to take measures to help reduce predation pressures from cats.
While direct predation on birds is what we think about when we think about bird-cat ecological interactions, we must remember that cats can affect birds in other ways as well. For example, as an efficient predator, cats are a potential competitor for predatory birds; competing for rodent prey.
One study found that six cats were capable of removing 4200 mice from a 35 acre study plot in just eight months (Pearson 1964).
The predation behavior of three cats was observed over the course of five years to measure what kind of impact they could make in the potential prey items of raptors in a 20 acre area. Between 1967 and 1971 the three cats caught almost 484 prey items with 42% of those prey items being Prairie Voles (Microtus ochrogaster). Young cottontail rabbits (Sylvilagus floridanus) made up the greatest volume (40%) of prey items (George 1974). Both species are major prey items of a variety of raptor species including Red-tailed Hawks (Preston and Beane 1993), American Kestrel (Smallwood and Bird 2002), and especially the winter diet of Northern Harriers (Macwhirter and Bildstein 1996).
Beyond predation or competition outdoor cats may also cause stress on birds that might affect their survivability and their ability to reproduce (Dauphiné 2008).
Trap and Release ProgramsOne of the biggest problems with outdoor cats is that they are capable of being prolific breeders, and because outdoor cats are often subsidized by well-meaning humans who feed them (Patronek 1998), the off-spring of outdoor cats possess a greater likelihood of surviving to adulthood than many natural predators might have. As a result, outdoor cat populations can become disproportionately large and have a greater impact on native wildlife populations. While it may not sound pleasant, unfortunately the most effective solution is trapping and euthanizing outdoor cats (Andersen et al. 2004).
Some well-meaning cat-lovers have promoted an alternative remedy to this problem, in lieu of euthanasia, by promoting “Trap, Test, Vaccinate, Neuter, and Release” (TTVNR) programs. The idea behind these programs is that by trapping outdoor cats, testing them for diseases, and neutering them before releasing them back into the outdoors, it reduces the capability of outdoor cats to increase their populations which lessens the number of cats preying on birds and other animals.
While the idea sounds good in theory, it is extremely flawed. Outdoor cats, regardless of whether they have been neutered or not, still prey upon birds. Secondly, TTVNR cats often are managed in cat colonies by individuals or groups of volunteers from animal welfare organizations. These cat colonies increase the density of predatory cats in a given area, where they have the potential of having a greater impact on local wildlife populations.
In the summer the beaches of Cape May, New Jersey host federally threatened Piping Plovers, a small migratory shorebird related to American Killdeer. The beaches are also home to a TTVNR cat colony very near to the plover nesting beaches (AP 2007). The cats pose a threat to the threatened plovers; preying upon the adult plovers while on their nests, their eggs, and the young plovers that are born flightless. In order to protect the Piping Plovers Cape May’s City Council implemented a plan to move feral cat colonies at least 1000 feet away from beaches that host Piping Plover nest colonies. This was decided only after federal agencies threatened to withhold necessary funds that would enable Cape May to replenish its beaches. The 1000 foot buffer was a compromise between what cat lovers wanted and what US Fish and Wildlife Service (USFWS) officials had a promoted—a one mile buffer (AP 2008). While the 1000 foot buffer may have fulfilled Cape May’s commitment to the USFWS to receive the necessary federal funding for its beaches, it may do little to protect Piping Plover nest sites as cats can easily cover the 1000 foot distance while hunting.
Cape May is not alone with regard to conflicts between outdoor cat colonies, their advocates, bird nesting colonies, and the people who watch and protect birds. Feral cat colonies have been established on Long Island’s South Shore beach alongside Piping Plover nest colonies (Kilgannon 2006). Florida in particular has potentially some of the largest feral cat colonies, because cats are regularly abandoned by people who stay in Florida in the winter but travel north in the spring and summer. Florida also has some of the most emotionally charged battles over endangered species and feral cat colonies (Gorman 2003).
In some cases feral cat colonies may pose a threat to human health in a different way. The Port Authority of New York and New Jersey has had to take action to round up feral cats at J.F.K. International Airport due to the potential threat the cats pose to planes on runways. The action to capture the feral cats was mandated by the Federation Aviation Authority, which regulates how wildlife and other animals are managed around airports. The action has met with opposition from the Humane Society and other animal activist groups (Lee 2008).
In an attempt to curb TTVNR efforts many groups are stepping forward to make the problems with these programs known. The Association of Wildlife Veterinarians and the National Association of State Public Health Veterinarians, through written statements, have publicly opposed TTVNR programs (Burton and Doblar 2004). The American Bird Conservancy launched the Cats Indoors! in 1997 in an effort to keep both cats and birds safe by teaching cat and bird lovers alike about the importance of keeping cats indoors. These programs, while important, have yet to be shown to truly effective at reducing the problem of outdoor cat colonies and their impacts on birds.
Safety of Outdoor Cats
Beyond the threat that outdoor cats pose to birds and other wildlife they may also be a threat to themselves and to people.
Outdoor cats are susceptible to any number of environmental stressors like inclement weather conditions and cold temperatures.
Outdoor cats are also vulnerable to larger predators, even in urban settings that might seem relatively sheltered from wildlife. Recent studies by researchers at Ohio State University have found that feral cats composed at least 1% of the diet of urban coyotes in Chicago, but that coyotes in urban settings may also kill outdoor cats as a way of removing potential competitors for prey items (Gehrt 2007). In southern California the presence of coyotes in habitat fragments had a positive influence on bird populations in the fragments. Coyotes preyed upon cats in the fragments, which helped keep cat populations in check. As a result, scrub-breeding birds in habitat fragments with coyotes had greater bird diversity. 21% of coyote scat samples collected during the study contained the remains of cats that had been preyed upon by the coyotes. Additionally, 25% of radio-collared cats in the study were preyed upon (Crooks and Soulé 1999).
One important finding of the southern California study found that cat owners around the habitat fragments were surveyed and reported that 42% had lost a cat to coyote predation. Additionally, when coyotes were thought to be present in an area 46% of cat owners restricted their cats’ outdoor activities (Crooks and Soulé 1999).
Disease can also be a major source of mortality and injury for outdoor cats, and these diseases pose a threat to people as well.
A study of animal bites in El Paso, Texas in 1995 found that a majority of cat bites (89%) resulted from provoking cats, and women and adults in general were more likely to be bitten. The disturbing finding of the study, however, was that 92% of cat bites were from cats that had not been vaccinated against rabies (Patrick and O’Rourke 1998). In 2002, a major advocate of outdoor cat colonies was bitten while feeding the feral cats in the colony on Singer Island in Florida. The cat was rabid, and as a result the cats in the colony were destroyed by the county for public health reasons (Gorman 2003).
Feline Leukemia is another disease that outdoor cats are susceptible of contracting. According to the Cornell Feline Health Center (2006) 2-3% of all cats in the U.S. are infected with the Feline Leukemia virus, but that infection rates rise significantly to 13% or greater in cats that are in high risk of infection. Outdoor cats, especially those in feral cat colonies, are especially susceptible to risk of infection because they are exposed to other cats of unknown infection status and because they have a higher risk of being bitten by an infected cat. Feline Leukemia is a common source of cancer in cats, but can also weaken their immune system sufficiently enough to make them susceptible to a variety of other diseases. Fortunately, tests have shown that cats may not be able to pass the disease to humans, however the other diseases that they may be susceptible to, from a weakened immune system, may be transmitted to people.
One disease in particular that deserves more attention due to its potential impacts on humans is Toxoplasmosis. Toxoplasmosis is a disease caused by a parasitic microorganism named
Toxoplasma gondii.According to the Center for Disease Control (CDC) (2008) 60 million American are infected with Toxoplasmosis. Once infected with the microorganism you’re infected for life. Fortunately, most healthy people don’t realize they are infected because
T. gondii establishes a balance between itself and the host’s immune system. However, pregnant women, children, the elderly and others with compromised immune systems are more susceptible to the effects of
T. gondii.
In most healthy individuals, an infection by
T. gondii may produce flu-like symptoms until the parasite is established in the immune system of its host (Zimmer 2006). However in pregnant women, the microorganism can result in miscarriage, a stillborn child, and the birth of children with abnormally enlarged or smaller heads (CDC 2008).
In some cases
T. gondii infection can result in lesions of the eyes, though typically only occurs in
T. gondii uses a body’s dendritic cells to quickly travel throughout a body. Dendritic cells are commonly found in the spleen and lymph nodes and help regulate a body’s immune system. When
T. gondii infects a body it hijacks dendritic cells and directs these cells to move throughout the body, which enables the microorganism to travel into places it would not normally be able to including our brains (Zimmer 2006).
Some scientists suspect there may be a connection between schizophrenia and Toxoplasmosis infection in humans, though this hasn’t been well established yet. Researchers at Johns Hopkins University found that soldiers diagnosed with schizophrenia were twice as likely to have blood samples exhibiting Toxoplasma infection than soldiers not diagnosed with schizophrenia (Zimmer 2006).
Right now you might be asking yourself what does human schizophrenia and Toxoplasmosis have to do with feral cats. Well, cats are a carrier and distributor of the
T. gondii microorganism.
TO BE CONTINUED…Literature CitedAndersen, M.C., B.J. Martin, and G.W. Roemer. 2004. Use of matrix population models to estimate the efficacy of euthanasia versus trap-neuter-return for management of free-roaming cats. Journal of the American Veterinarian Medical Association. 225:1871-1876.
Barcott, B. 2007. Kill the cat that kills the bird? New York Times (12.2.07)
Baynes, E.H. 1915. Wild bird guests: how to entertain them. E.P. Dutton & Company. New York.
Burton, D.L. and K.A. Doblar. 2004. Morbidity and mortality of urban wildlife in
the midwestern United States. Pages 171-181 in Shaw et al., (eds.), Proceedings 4th International Urban Wildlife Symposium.
Coleman, J., S. Temple and S. Craven. 1996. A conservation dilemma—The Free-ranging Domestic Cat. The Probe. 127:1-2, 5.
Cornell Feline Health Center. 2006. Feline Leukemia Virus. Prepared by the American Association of Feline Practitioners and the Cornell Feline Health Center, Cornell University, College of Veterinary Medicine, Ithaca, New York 14853-6401.
www.vet.cornell.edu/fhc/brochures/felv.html
Crooks, K.R. and M.E. Soulé. 1999. Mesopredator release and avifaunal extinction in a fragmented system. Nature. 400: 563-566.
Dauphiné, N. 2008. Impacts of domestic cats on birds in North America (Abstract). Proceedings of the 4th International Partners in Flight Conference.
Dauphiné, N. and R.J. Cooper. 2008. Conservation meets the cat lady: Protecting bird species of continental importance in our own backyards (Abstract). Proceedings of 4th International Partners in Flight Conference.
Dunn, E.H., Tessaglia, D.L., 1994. Predation of birds at feeders in winter. Journal of Field Ornithology. 65:8-16.
Gehrt, S.D. 2007. Urban Coyote Ecology and Management: The Cook County, Illinois, Coyote Project. Ohio State University Extension Bulletin no. 929.
George, W.G. 1974. Domestic cats as predators and factors in winter shortages of raptor prey. Wilson Bulletin. 86: 384-396.
Gorman, J. 2003. Bird lovers hope to keep cats on a very short leash. New York Times (03.18.2003)
Kilgannon, C. 2006. On Long Island, cats and birds clash, and people take sides. New York Times (03.20.06)
Lee, J.S. 2008. Airport cat roundups resume, as do protests. New York Times (06.03.08)
Lepczyk, C.A., A.G. Mertig and J. Liu. 2003. Landowners and cat predation across rural-to-urban landscapes. Biological Conservation. 115:191-201.
Meyers, R. 2007. Cat shooter Stevenson flies the coop. The Daily News: Galveston County (11.30.07)
Murphy, K. 2007. Birder admits killing cat, but was it animal cruelty? New York Times (11.14.07)
Patrick, G.R. and K.M. O’Rourke. 1998. Dog and cat bites: epidemiologic analyses suggest different prevention strategies. Public Health Reports. 113:252-257.
Patronek, G.J. 1998. Free-roaming and feral cats – their impact on wildlife and human beings. Journal of American Veterinary Medical Association 212:218-226.
Pearson, O.P. 1964. Carnivore-mouse predation: an example of its intensity and
bioenergetics. Journal of Mammalogy. 45:177-178.
Williams, S.E. 2008. Bridge worker gets citation for too many cats. The Daily News: Galveston County (01.08.08)
Ogan, C.V.; Jurek, R.M. 1997. Biology and Ecology of feral, free-roaming, and stray cats Pages 87-92 in J.E. Harris, and C.V. Ogan, (eds.), Mesocarnivores of northern California: biology, management, and survey techniques, workshop manual. August 12-15, 1997, Humboldt State University, Arcata, CA. The Wildlife Society, California North Coast Chapter, Arcata, CA 127 p.
Unknown. 2007. Government may step in to save birds from cats in Cape May. The Associate Press (08.05.07)
Unknown. 2008. Cape May's feral cats to be moved to protect birds. The Associate Press in South Jersey News Online (03.05.08)
Zimmer, C. 2006. A common parasite reveals its strongest asset: stealth. New York Times (06.20.2006)
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