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Cats: Feral Colony Debate: Trap-Neuter-Return (TNR) or Remove? What about Alternatives?

"Trap–neuter–return (TNR)" (wikipedia.org)

Pro:
More chance of survival for unadoptable feral cats than being brought to a "Kill Shelter".
TNR is a non-lethal method of managing cat colonies.
Vacuum theory: Something is going to fill that niche as long as there are abundant prey.

Con:
More chance of survival for potentially adoptable feral/abandoned cats when brought to a "No-Kill Shelter".
Sick cats become reservoirs for disease.
Scientific research has not found TNR to be an effective means of controlling the feral cat population.
Trapping and sterilization rates are frequently too low to stop population growth.
Awareness of a TNR colony tends to encourage people in the surrounding community to dump their own unwanted pet cats there when the cats would do better at a Humane Society Animal Shelter.
Feral cats are hunters and in addition to vermin, they also kill animals valued by humans.

Detailed overview of Management of "Free-ranging and Feral Cats" (aphis.usda.gov)
In addition to TNR, there are other deterrents such as:
Bells, brightly colored bibs/collars; Foul smelling repellents
Net wire fencing with angled hood; Rollers installed on top of fences
Frightening Devices Motion-activated sprinklers, noise makers, and lights
Eliminate sources of food, water, and shelter; Place garbage in sealed containers or enclosed areas
Keeping Birds Safe from Outdoor Cats (nwf.org)
Belling a cat, or using a brightly colored bib collar, don't prevent predation, although they reduce success rate. Other methods may be more effective.

Owner Education & Leash Laws:

Areas with leash laws have fewer free-roaming cats.
Indoor cats are less susceptible to disease and injury.
Cats that are indoor only, or leashed when outdoors, are healthier and live longer.

Making windows safer for birds. (Stunned birds are easy prey for cats and dogs.)

New Study Confirms Building Collisions Kill Over One Billion Birds Annually in U.S.
"By factoring in the high mortality rate of birds that initially survive collision but later die from their injuries, this new study pushes the estimated death toll well beyond the one billion mark."
Collision course: Scientists struggle—to make windows safer for birds (sciencenews.org)
Cutting down on bird-window collisions would reduce easy prey availability for predators. Includes Bird safety dos and don’ts.
What Does ’Bird-Safe Glass’ Even Mean? (audubon.org)

Feral Cats become Part of the Food Web.

Some people advocate for trap-neuter-return because the "Vacuum Effect" predicts food niche will refill.
At least least on islands, when cats are removed, their prey may become more trouble than the cat.
It is hard to predict what removing a cat colony from a food web will do.

The impact of free-ranging domestic cats on wildlife of the United States (nature.com)
"Here we conduct a systematic review and quantitatively estimate mortality caused by cats in the United States. We estimate that free-ranging domestic cats kill 1.3–4.0 billion birds and 6.3–22.3 billion mammals annually."
Outdoor Cats and Their Effects on Birds
"It’s estimated that cats kill 1.3–4 billion birds each year in the U.S. alone, with 69% of these kills attributable to feral or unowned cats. This is a staggering number even when compared with the next-largest sources: 599 million estimated to be killed in collisions with windows and 200 million killed by automobiles."
Popular press portrayal of issues surrounding free‐roaming domestic cats Felis catus.
Popular press often presented an oversimplified picture of issues related to free-roaming cats and provided a false dichotomy of cat hunting behaviour as either positive (e.g. hunting pest mammals) or negative (e.g. hunting birds).
Wildlife impacts of free-roaming cats: Estimates vs. evidence (felineresearch.org/)
"It’s also important to distinguish between mortality estimates and population impacts. It’s well understood that predators — including cats — tend to prey on vulnerable individuals (e.g., sick, weak, young) considered the “doomed surplus,” resulting in little or no impact on the population of a species. In fact, two peer-reviewed studies have shown that birds killed by cats are, on average, significantly less healthy than birds killed in collisions with windows or cars."

Without the cat you must deal with the rat.

*Island lesson: don't kill your cats before your rats
"There were fewer than 10,000 rabbits when the last cat was shot on Macquarie. Now there are more than 130,000."
"The rats and mice are also chewing their way through flower heads to take the seeds."
ERADICATION OF FERAL CATS AT WAKE ATOLL
"...feral cats have been removed from at least 48 islands; 16 in Baja California (Mexico), 10 in New Zealand, 5 in Australia, 4 in the Pacific Ocean, 4 in Seychelles, 3 in the sub-Antarctic, 3 in Macronesia (Atlantic Ocean), 2 in Mauritus and 1 in the Caribbean (Nogales et al., 2004)"
"Due to feral cat removal and wet weather, Pacific Rats (R. exulans) greatly increased. Current rodent control effort was less effective than it should be because hermit crabs (Coenobita perlata) ate the bait before the rats did. "
Feral Cats Are An Important Part Of The Environment
"There have been feral cats since the dawn of civilization—and that is unlikely ever to change."
"...scientists who study predation have shown in mathematical models that when cats, rats, and birds coexist, they find a balance. But when cats are removed, rat populations soar and wipe out the birds completely."
How removal of cats and rats from an island allowed a native predator to threaten a native bird
"... a 31-year study showed that in a 82-ha tropical marine bird sanctuary, predation on a native prey, the blue-footed booby, by a native predator, the Atlantic Central American milk snake, apparently diminished after removal of cats then increased 11-fold after the additional removal of black rats."
State of the World's Birds (Annual Review of Environment and Resources) 2022
"Introduced mammals are the primary driver of extinctions of insular [such isolated on an island] bird species: Rodents are linked to the extinction of 52 bird species and cats to 40 species" "domestic cats kill an estimated 2.4 billion birds in the United States annually."