The debate over how to deal with the nation's estimated 50 million feral felines has turned into a proverbial cat fight, with some animal authorities calling for the permanent removal of feral cats from the environment, and others supporting programs that neuter and vaccinate ferals.

All sides agree that feral cat populations cannot continue to skyrocket.

A strong supporter of humane methods to address the issue is the Humane Society of the United States (HSUS). One such method is known as TNR: trap/neuter/return. Usually it involves humanely trapping a feral, having it spayed or neutered and vaccinated, and then ear-tipping the cat for later identification before returning it to the outdoors. Some cats can be socialized and adopted, avoiding release, but that's not always possible.

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"The Humane Society of the United States believes that every cat deserves a life free from hunger or thirst, fear and distress, discomfort, pain, injury, or disease, and that cats at risk for these are our responsibility to care for," Katie Lisnik, director of Cat Protection and Policy, Companion Animals, at the HSUS, told Discovery News.

"To this end," she continued, "we support Trap-Neuter-Return (TNR) and similar sterilization programs, legislation that allows for and supports non-lethal population control, and coalition-based approaches that involve community leaders, citizens, and stakeholders to implement effective community cat management programs."

She argues that ferals who undergo TNR end up being healthier and exhibit less nuisance behavior, such as fighting, vocalizing and marking territory.

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Jane Brunt, DVM, and founder of the CATalyst Council, Inc. agrees that "feral and community cats need to be addressed as an individual community issue to reduce the suffering of cats." Brunt added, "We believe that cats should be spayed and neutered unless used for responsible breeding or showing."

PETA takes a more moderate position on TNR. Spokesperson Esteban Martinez said, "We believe that, although altering feral cats prevents the suffering of future generations, it does little to improve the quality of life of the cats who are left outdoors. Allowing feral cats to continue their daily struggle for survival in a hostile environment is not usually a humane option."

PETA holds that TNR programs are acceptable only when cats are (1) isolated from roads as well as from people and other animals who could harm them, (2) constantly attended to by people who feed them and care for their medical needs, (3) located in an area where they do not have contact with wildlife, and (4) located in an area where the weather is temperate.

A feral cat in New Jersey. Corbis

The American Bird Conservancy (ABC) supports the removal of feral cats from the environment, advocating for humane euthanasia for those cats that are deemed unadoptable.

"Feral cats are a highly destructive, invasive species that are at least, in part, responsible for the extinction of 33 species," ABC spokesperson Robert Johns told Discovery News.

He added, "Outdoor cats are the number one, human-caused direct source of bird mortality. They kill up to 3.7 billion birds every year."

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ABC proposes that there should be incentives for cat owners not to let their pets run wild outdoors. An incentive, for example, could be to fine cat owners for not complying.

Lisnik said that in many municipalities, there are no specific ordinances addressing cats and/or TNR. Bans on feeding outdoor cats have proven to be difficult to enforce, and rarely work, she said, "because caring people will find a way to feed the cats, even with the threat of a fine or worse. Also, the cats were attracted to the area in the first place due to an available food source, such as a dumpster or rodent population, something that a feeding ban does not address."

Is TNR even successful?

Johns said that "we have yet to see a single 'managed' cat colony lead to the elimination of the colony. If TNR worked, we'd be seeing that, and we simply don't."

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Julie Levy, director of the Maddie's Shelter Medicine Program at the University of Florida, has conducted many cat population studies that show the impact of TNR on a given feline population. Based on her findings, there is good and bad news.

The good is that, according to her studies, TNR can reduce cat populations in a humane manner so long as at least 75 percent of a feral cat colony is sterilized. The bad is that the process can be slow, with some colonies persisting for 10-15 years before eventually ending.

"For people who do not want the cats around, this can be difficult to live with, even if nuisance behaviors diminish," Lisnik said. "In cases like these, we recommend that local trap/neuter/return groups work to help these individuals, considering humane deterrents, such as a car cover, fencing, motion activated lights or sounds, etc., to keep cats out of gardens or off cars or patios."