Tel Aviv has a feral cat problem. They are aware of this. The program in place now to deal with this is pure genius. Most other animal agencies worldwide would simply kill the cats. The Tel Aviv agency realizes that killing the cats would only increase the resources for the remaining cats and therefore increase reproduction. So what they do is tour around, pick up some cats, take them back to the agency, and sterilize them. To remember which cats have already been fixed, they then clip the tip of the cat's ear off. When future animal control officers are trolling around, looking for cats to pick up, they look for cats with two undisturbed ears.
The resources then remain the same for the same number of cats, and the reproduction rate eventually slows.
UPDATE: I have now taken photos of a cat with its ear clipped.