Hi I am the owner of two beautiful brothers who are lovely quirky and fun to have it is my first time owning cats and I love them only thing that is concerning is the amount of mice they kill which over the last few months has got greater one or two each nearly every other day (there just over a year old )got bells for there collars which didn't help they have a cat flap which they have unlimited access to it can be set to certain times for access but one of them can break out anyway any tips on how to reduce the killing spree