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A feral cat is a domesticated cat that has returned to the wild, or the descendants of such an animal. It is distinguished from a stray cat, which is a pet cat that has been lost or abandoned, while feral cats are born in the wild. The offspring of a stray cat can be considered feral if born in the wild.
In many parts of the world, feral cats are descendants of domestic cats that were left behind by travelers. Cats introduced into areas in which they are not indigenous often cause harm to local environments by preying on local species. This is particularly true on islands, where feral cats have sometimes had a substantial and deleterious effect on the local fauna. Cats have been blamed for the extinction of 33 species.
The term "feral" is sometimes used to refer to an animal that does not appear friendly when approached by humans, but the term can apply to any domesticated animal without human contact. Hissing and growling are self-defense behaviors, which, over time, may change as the animal (whether "feral" or "stray") begins to trust humans that provide food, water, and care.
Feral cats that are born and living outdoors, without any human contact or care, have been shown to be adoptable and can be tamed by humans, provided they are removed from a wild environment before truly feral behaviors are established. Such behaviors are established while it is still a kitten being raised by its mother.
The lifespan of feral cats is hard to determine accurately, although one study reported a median age of 4.7 years, with a range between 0 and 8.3 years, while another paper referenced a mean life span of 2–8 years. By contrast, in captivity, an average life expectancy for male indoor cats at birth is 12 to 14 years, with females usually living a year or two longer.
During the Age of Discovery, ships released rabbits onto islands to provide a future food source for other travelers. They eventually multiplied out of control and cats were introduced to keep their numbers, and that of mice and rats, down. The cats tended to favor local species as they were ecologically naive and easier to hunt. Their numbers, too, increased dramatically and soon they colonised many areas and were seen as pests. Cats were introduced to Tasmania in 1804 and had become feral by the 1840s. Feral cats were reported on mainland Australia around Sydney in 1820. It has been suggested that feral cats could have been introduced accidentally to the north-western coast in the 17th century from the wrecks of Dutch ships; alternatively, they could have arrived earlier, possibly around the fifteenth century, via mariners from Indonesia.
Domestic and feral cats have generally been found to eat a very broad range of vertebrate and invertebrate prey. Preferred prey usually are small mammals, birds and lizards, especially those with body weights under 100g. Feral cats in Australia prey on a variety of wildlife. In arid and semi-arid environments they eat mostly introduced European rabbits and house mice. In arid environments where rabbits do not occur, native rodents are taken. In forests and urbanised areas, they eat mostly native marsupial, birds and reptiles. On Macaronesian islands, cats prey mainly on introduced mammals but also on birds and reptiles.
Feral cats may be apex predators in some local ecosystems. In others, they may be preyed on by feral dogs, dingoes, coyotes, wolves, bears, cougars, leopards, bobcats, lynx, hyenas, fishers, crocodiles, snakes, foxes and birds of prey.
A recent study by the Smithsonian Conservation Biology Institute suggests that cats are the top threat to US wildlife as they were found to be responsible for the deaths of up to 3.7 billion birds and 20.7 billion mammals annually, with feral and stray cats being the worst offenders. These figures were much higher than previous studies suggested as they found cats had killed more than four times the number of birds as had been previously estimated. In the US, the American Robin along with shrews, voles, mice, squirrels and rabbits were most at risk from cat predation.
The impact of domestic cats on wildlife is a century-old debate between passionate cat lovers and those of conservation and scientific beliefs. In a 1916 report for the Massachusetts State Board of Agriculture titled The Domestic Cat: Bird Killer, Mouser and Destroyer of Wildlife, noted ornithologist Edward Howe Forbush stated in the preface:
Questions regarding the value or inutility of the domestic cat, and problems connected with limiting its more or less unwelcome outdoor activities, are causing much dissension. The discussion has reached an acute stage. Medical men, game protectors and bird lovers call on legislators to enact restrictive laws. Then ardent cat lovers rouse themselves for combat. In the excitement of partisanship many loose and ill-considered statements are made.
The report cited Extinct Birds, published in 1905 by zoologist Walter Rothschild, who stated, "man and his satellites, cats, rats, dogs, and pigs are the worst and in fact the only important agents of destruction of the native avifaunas wherever they go." Rothschild gave several examples of cats causing the extermination of some bird species on islands.
Some farmers and gamekeepers see feral cats as vermin. Feral cats catch and eat ground-nesting birds such as pheasants and partridge. To protect their birds, some gamekeepers set traps and shoot feral cats as part of pest control.
Cats are the sole threat to some bird species, such as Townsend's Shearwater, Socorro Dove, and the Marquesan Ground Dove, or the cause of outright extinction in other cases, notably the Stephens Island Wren.
Feral cats in Australia have been linked to the decline and extinction of various native animals. They have been shown to cause a significant impact on ground nesting birds and small native mammals. Feral cats have also hampered any attempts to re-introduce threatened species back into areas where they have become extinct as the cats have simply hunted and killed the newly released animals. Numerous Australian environmentalists claim the feral cat has been an ecological disaster in Australia, inhabiting most ecosystems except dense rainforest, and being implicated in the extinction of several marsupial and placental mammal species.
The fauna of New Zealand has evolved in isolation for millions of years without the presence of mammals (apart from a few bat species). Consequently, birds dominated the niches occupied by mammals and many became flightless. The introduction of mammals after settlement by Māori from about the 12th century had a huge effect on the indigenous biodiversity. European explorers and settlers brought cats on their ships and the presence of feral cats were recorded from the latter decades of the 19th century. It is estimated that feral cats have been responsible for the extinction of six endemic bird species and over 70 localised subspecies as well as depleting bird and lizard species.
Many islands host ecologically naive animal species; that is, animals that do not have predator responses for dealing with predators such as cats. Feral cats introduced to such islands have had a devastating impact on these islands' biodiversity. They have been implicated in the extinction of several species and local extinctions, such as the hutias from the Caribbean, the Guadalupe Storm Petrel from Pacific Mexico, the Stephens Island wren; in a statistical study, they were a significant cause for the extinction of 40% of the species studied. Moors and Atkinson wrote, in 1984, "No other alien predator has had such a universally damaging effect."
Feral cats, along with rabbits, some sea birds, and sheep, form the entire large animal population of the remote Kerguelen Islands in the southern Indian Ocean. Although exotic mammals form the bulk of the diet, cat's impact on seabirds is very important.
Because of the damage cats cause in islands and some ecosystems, many conservationists working in the field of island restoration have worked to remove feral cats. (Island restoration involves the removal of introduced species and reintroducing native species). As of 2004, 48 islands have had their feral cat populations eradicated, including New Zealand's network of offshore island bird reserves, and Australia's Macquarie Island. Larger projects have also been undertaken, including their complete removal from Ascension Island. The cats, introduced in the 19th century, caused a collapse in populations of nesting seabirds. The project to remove them from the island began in 2002, and the island was cleared of cats by 2004. Since then, seven species of seabird that had not nested on the island for 100 years have returned.
In some cases, the removal of cats had unintended consequences. An example is Macquarie Island (off the coast of Tasmania), where the removal of cats caused an explosion in the number of rabbits, rats, and mice that harm native seabirds even if the eradication was positioned within an integrated pest management framework. The removal of the rats and rabbits was scheduled for 2007 and it could take up to seven years and cost $24 million.
Feral cats have interbred with wildcats to various extents throughout the world, the first reported case occurring more than 200 years ago. The significance of hybridisation is disputed. Some old books suggested that the wildcat was a separate species to the domestic cat but modern genetic analysis has shown that the domestic cat is a domesticated version of the near eastern wildcat (Felis sylvestra lybica), which is itself of the same species as the European Wildcat (felis sylvestris sylvestris). In some locations, high levels of hybridisation has led to difficulties in distinguishing a "true" wildcat from feral domestic and domestic hybrid cats, which can complicate conservation efforts. Some researchers argue that "pure" wildcats do not exist anymore, but this is disputed by others. One study in Scotland suggests that while "true" Scottish wildcats are unlikely to exist, the current wildcat population is distinct enough from domestic cats to be worth protecting. In addition to Scotland, wildcat populations notable gene introgression exist also in Italy, Hungary and Portugal. For a discussion of this issue see The Encyclopedia of Mammals, OUP, pages 656–657.
There is concern about the role of feral cat colonies, wild dog, and other native mammals, as a vector of diseases, particularly toxoplasmosis, giardiasis (esp. from beavers), rabies (e.g. raccoons), Campylobacter, Parvovirus and other diseases and parasites that can infect both humans and animals. Felids such as cougars and cats, the mammals they feed on, and undercooked meat and chicken are a source of Toxoplasma gondii, which causes toxoplasmosis.
A feral cat colony is a population of feral cats. The term is used primarily when a noticeable population of feral cats live together in a specific location and use a common food source. The term is not typically applied to solitary cats passing through an area. A clowder can range from 3–25 cats. Their locations vary, some hiding in alleyways or in large parks.
Members consist of adult females, their young, and some adult males. Unneutered males in a clowder fight each other for territory and for females. Some will be driven out to find another place to live.
Feral cats who have been trapped in many warm areas where fleas exist are usually found to have a large number of fleas, causing them to be anemic. Both the fleas, and the food source, if limited to garbage and rodents, cause the cats to have intestinal microorganisms (such as coccidia or giardia) and other parasites (commonly known as roundworms, tapeworms, and hookworms), which lead to diarrhea and subsequent dehydration. They also can have ear mites, ringworm, and upper respiratory infections. Others are wounded in mating-fights and die from the infected wounds. Still others eventually contract feline immunodeficiency virus or feline leukemia due to the constant transmission of blood and bodily fluids via fighting and sexual activity.
While all of these illnesses are quite treatable, human intervention is necessary to prevent them from becoming fatal.
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In Trap-Neuter-Return (TNR), volunteers trap feral cats, sterilize them through spaying or neutering, and then release them, though some keep kittens or cats which are more tame. Variations of the program include testing and inoculation against rabies and other viruses and sometimes long-lasting flea treatments. TNR programs are only now being introduced in some urban and suburban areas, such as Adelaide. More recently, such programs have been introduced in Sydney by the "World League for Protection of Animals". While various long-term studies have shown TNR is effective in stopping the breeding of cats in the wild and reducing the population over time, opponents of TNR frequently cite a study by Castillo (2003) as evidence TNR does not work. Many humane societies and animal rescue groups of varying sizes throughout the United States have some type of TNR program. The practice is endorsed by the Humane Society of the United States and the National Animal Control Association. While the United States Department of Defense does not formally advocate TNR, it does provide information to military installations on how to implement TNR programs. The main message from the department is that population control programs must be humane.
Establishing and maintaining feral cat colonies on any property in the United States, can raise conflicts with State or Federal environmental laws, as the City of Los Angeles found out in 2009, when they had their TNR program stopped with a Superior Court order. 
Florida's Broward County, faced with feral cat and Endangered Species conflicts, eliminated feral cat colonies all together, by adopting their alternative TSAFE program, which means Trap, Sterilize, Adopt, Foster and Euthanize. 
Eradication methods include shooting, trapping, poison baiting and biological controls. For example on Marion Island cats were infected with the feline panleukopenia virus, which drastically reduced their population within six years. The remaining cats were killed by shooting, trapping and poisoning.
Ascension Island, in the South Atlantic Ocean, was the third largest island, and one of two islands with a human population, on which feral cats had been successfully eradicated. This took two years to accomplish. Cat eradication took 14 years on Marion Island and 25 years on Macquarie Island.
The programme on Ascension island made use of live-trapping and poisoned bait – raw fish chunks injected with 2 mg of sodium monofluoroacetate, a poison which has no effect on the island's only large native animal, the land crab. Live trapping replaced poison within 1 kilometre (0.62 mi) of settlements to avoid taking domestic cats – the island's population of 168 domestic cats had been previously microchipped, neutered and given reflective cat collars. Domestic cats caught in the traps were returned to their owners; land crabs and birds were released; feral cats and rats were killed.
The programme was in three phases. A "knock–down" phase (Feb 2002 – Oct 2002) killed the majority of feral cats. During the "mop–up" phase (Oct 2002 – Jan 2004) individual survivors were tracked down and captured using traps, as some of these showed avoidance of poison bait. Finally, a "confirmation phase (Jan 2004 – Jan 2006) was a monitoring programme intended to confirm there were no survivors. The programme was entirely successful in eradicating feral cats – around 488 cats were poisoned, 73 trapped, two were shot and four captured by hand. However, around 38% of the island's domestic cats disappeared during the programme, indicating they had wandered beyond the 1 km buffer zone and had taken poisoned bait.
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