http://www.catsnap.org/ferals.htmFERAL CATS suggested minimum donation is $10 (male or female), and the animal will have its left ear clipped (as seen in the photo on the left). Eartipping is a painless procedure done under anesthesia (during the spay/neuter surgery) where the top of the left ear is cut straight across. It is a universal sign to animal control officers that a cat is wild, but is fixed and vaccinated.
more links
http://modblog.bmezine.com/2007/07/29/dermal-punched-ear-on-a-cat/http://www.sniksnak.com/cathealth/ferals.html The site below has lots of info on trapping feral cats Another procedure many humane workers recommend is the painless removal of the top corner of the cat's left ear ("ear tipping"), or to remove a small "V" or notch ("ear notching") in one ear. Ear notching is the same idea as eartipping but the small notch that is cut out of the ear is not as obvious as ear tipping. Often male cats have notched ears caused by fighting. Ear tipping is a highly visible means of identifying feral cats that have already been neutered and vaccinated.
http://www.dailysentinel.com/news/content/news/stories/2007/12/16/misc_rescuegroups.htmlWhile the feral cats are under anaesthesia, they are ear-tipped, the nationally recognized sign that they have already been spayed/neutered.
"Ear tipping is the surgical removal of the top of the ear," Hensley said. "It's a national sign that a cat has been neutered before. That way, if we trap them again we can just let them go. We have one cat who has been caught three or four times."
You can google "ear cutting of spayed/neutered cats" and you will get lots more info