I would go with the best defense method. I secure my chickens and ducks in a strong coop at night, and have them protected by an electric net fence during the day. I would also try and trap the cats and take them to a local shelter to get spayed / neutered so the problem will not continue to grow. We have 3 barn cats on our farm, and they keep many nuisance pests at bay, including other cats. The issue with getting rid of the feral cats is it will only allow something else to come in and take their place.