From Alley Cat Allies website:
Feral cats. They sleep in our parks, military bases, alleyways, farmyards, barns, college campuses, and deserted buildings. Abandoned by their human families or simply lost, unsterilized housecats eventually band together in groups called colonies. Without human contact for a prolonged period, the colonies become feral. They make homes wherever they can find food, be it in dumpsters or under a boardwalk. Mothers teach their kittens to avoid humans and to defend themselves. And their numbers steadily increase, even if meager scraps are all the food to be had.
No one knows exactly how many feral cats live in the United States, but the number is estimated in the tens of millions. They are often wrongly portrayed as disease-ridden nuisances living tragic lives and responsible for endangering native species. As a consequence, feral feline communities too frequently are rounded up and because they have had little or no human contact and are thus unadoptable they are killed.
But removing and killing feral cats does not reduce feral cat populations. It only provides space for more cats to move in and start the breeding process again. Unspayed, feral female cats spend most of their lives pregnant and hungry, as will the female kittens that survive. Unneutered tomcats roam to find, and fight to win, mates, and often suffer debilitating wounds in the process. Half of all kittens born in feral colonies die within their first year.
Alley Cat Allies has a solution that not only reduces feral cat populations, but also improves and extends the lives of colony members: Trap-Neuter-Return (TNR).
TNR is a comprehensive plan where entire feral colonies are humanely trapped, then evaluated, vaccinated, and neutered by veterinarians. Kittens and cats that are tame enough to be adopted are placed in good homes. Adult cats are returned to their familiar habitat to live out their lives under the watchful care of sympathetic neighborhood volunteers.
TNR works. Cat populations are gradually reduced. Nuisance behaviors associated with breeding, such as the yowling of females or the spraying of toms, are virtually eliminated. Disease and malnutrition are greatly reduced. The cats live healthy, safe, and peaceful lives in their territories.