These cats have been "ear-tipped" or "notched". This a surgical alternation showing that a free-living cat has been spayed or neutered.

These cats should never be re-trapped or brought to a shelter, unless sick. Instead, they should be left in their neighborhood to provide cat population control. Studies have shown by just being there, they prevent other cats from moving into the area. And, because they have been fixed, they will never have litters of kittens.