I Say get a Microchipped cat flap or one of those where your cats have a Magnetic device on there collar and lets your cats in and out whenever they please.

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Toby, Mia, Harvey and our newest member, Marley(cats)
Gizmo (parrot)
Tommy (bearded dragon)

Is missing my baby boy Noah, been missing since April 2011. (cat)

Run Free;
Tommy, Molly, Tinkerbell and Bambi (cats)
Meg, Fudge, Chocolate & Looper (hamsters)
Snowie, Maisy, Snowflake (rabbits)