Removing the very tip of the left ear has become a well established, consistent way for fixed feral cats to be identified. Feral cats may interact with a variety of caretakers, vets, and animal control people during their lives. Cats can live over 15 years, and if someone new takes over the care of the colony, it is an easy way for them to identify altered vs. unaltered cats. It is traumatic for a feral cat to be retrapped and transported unnecessarily because of uncertainly over whether it's been spayed or neutered. Not to mention the waste of time and money involved just because the cat was not marked in a universally recognized way.

Although some animal welfare activists complain that ear tipping is inhumane, it is performed under anesthesia at the same time as a major surgery and is certainly less traumatic than a spay or neuter.

 Why not use a tattoo or microchips to identify feral cats?  

Tattoos are virtually unreadable on feral cats unless they are anesthetized, and microchips often fail to be detected if the cat is in a metal trap.