After much discussion last night, my boyfriend and I have reached a compromise. We will redouble our efforts for the next couple months and hopefully be able to get the kittens and the dog and other cats together by then. However, if we haven't been able to by the time the weather warms up, we will rehome them. I think Max must have heard us threatening to take some of his cats away, because when we had them out last night, he was pretty good and we were able to keep them out for longer than ever!
I'm not going to get my hopes up too much, but fingers are crossed!
