To many animal welfare advocates, these cats are vulnerable — they live a scant few years, falling victim to wild predators, speeding cars, or rat poison they ingest. But to others, outdoor cats are opportunistic predators that hunt not only birds — an estimated 1.3 billion to 4 billion — but also lizards and small rodents. Everyone agrees that cats shouldn’t be roaming outdoors. The question is how to reduce that population in a humane way. As most animal welfare experts recommend, Los Angeles wants to embrace the practice of trap-neuter-return (known as TNR). When the cats are neutered or spayed, an ear tip is notched. Volunteers and rescue groups mostly carry this out and also feed colonies of cats.