Several issues at hand, it seems
Cats detrimental to songbird population: That’s truly up for debate, and scientists can’t agree. Here’s a useful link to Stray Pet Advocacy, where you will find many links to other sites and PDF’s on studies and papers. If you check out, oh, five of them or so, you will probably come to the same conclusion - no one really knows. Everyone refutes everyone else, and none of the numbers agree. They all seem to admit that the numbers are merely educated guesses made as scientifically as possible, but are indeed just guesses. They ALL agree that humans are the #1 problem for the songbird/migratory bird populations. They ALL seem to gravitate toward cats being the #2 problem, and conclude that since we can’t kill all the people we may as well kill all the cats. (well, not exactly, but you get the gist).
The other hand of that is not jut bird populations, but rodent populations. Is there much argument about people keeping “barn cats” and the sole reason for them is rodent control? So there could be some argument as to the benefit of having cats around. For some reason, people don’t seem to have much of a problem with cats killing mice and rats. After all, the reason cats were taken on cargo/explorer ships in the first place was to kill the vermin.
Cats as an invasive/destructive species: There’s not really much argument about that. Is it the fault of the cats, or of the people who brought them here, and of the people who are lackadaisical about keeping them under control? So many people have indoor/outdoor cats that they don’t bother to spay or neuter. The cats often breed and have their litters outdoors, so the people have no idea how many kittens they are really responsible for. Part of the problem, frankly, is that cats are considered domestic pets. Other invasive/non-native species (horses, pigs) are culled through roundups and legal hunting. That’s pretty much not an option with cats!
Trap-return or Trap-euthanize or just shoot 'em: TNR is the most agreeable solution across the board. Yes, it will take a long time. Yes, it’s not easy. As human beings who consider themselves the guardians of the planet and take responsibility as a species for causing the problem in the first place, it’s the most humane answer. The trick is getting an area’s population on-board with everything that goes with a successful TNR program, this includes readily available and affordable spaying and neutering for all the people in the area, education as to what to do about the strays and ferals in a given area, and getting the municipality to pass ordinances and laws conducive to the TNR project, such as properly worded leash laws and available low-cost programs.
Trap-euthanize is generally not an option. On a very small scale, perhaps it works in some areas. People who work with animals for a living, though, do not work at that living in order to kill animals. There’s a reason the best-paying job for a vet tech in Chicago is at the Animal Control center. It’s a soul-sucking, demoralizing, and horrible job. It pays about $60,000 a year (about double average pay). Because they can’t keep anyone doing it for any less, and even then usually less than a year. I know several techs who have done it and didn’t last 6 months and needed therapy afterward! This is part of the reason the City of Chicago, and all the shelters within, are working with Maddie’s Fund to make Chicago a no-kill city.
Just shoot 'em. Well, I guess if you’re in a rural area where shooting guns in the back yard isn’t illegal, I guess it’s on you whether you’re shooting someone’s pet. How do you know? I would hope someone wouldn’t return the favor if your dog got out and ran through someone’s yard. Now, I can only speak for my area, and as to what happens here, but cats that are brought in through the TNR program are “vetted” on a few levels. If the cat in the trap looks at me and head-butts the trap, and purrs, and otherwise shows signs of being a stray and not a feral, then that cat gets either admitted to the shelter for adoption or the colony caretaker takes them home to keep indoors and to foster and adopt out on their own. True ferals are neutered, ear-tipped, and microchipped. Once they’re outside again, there’s no doubt they’re part of a managed colony because anyone can see the clearly tipped ear that indicated the cat has been neutered and someone is keeping an eye on it. If that cat should end up at a vet’s office due to injury or illness, the microchip will tell them who to contact in regards to the cat’s welfare, and permission will usually be given to euthanize, as we all well know that trying to rehabilitate a feral cat is futile.
What is a feral cat? Do they suffer? It needs to be made clear the difference between feral cats and strays. Stray cats are socialized to humans. If they were born outdoors, they weren’t out there for long and became friendly with people as kittens. For the most part, strays lived in a home with people at some point. Strays should be “rescued” and placed in adoptive homes. Feral cats were born outdoors, live outdoors, and HATE PEOPLE. Just like a raccoon or an opossum, feral cats are wild animals that live and die outside, and should be left to live and die outside. Just like you wouldn’t take in the raccoon or opossum to your home to try to make it like you, you wouldn’t do that with a feral cat. I’ve met enough ferals to know they would just as soon rip your face off in your sleep as look at you. Now, unlike the raccoon or opossum, people feel a much greater need to feed the cute kitties they find in their neighborhoods, and that’s where the overpopulation comes in. And that’s why TNR is so important, and the least objectionable solution for most.