We have a lot of "stray" cats in our new neighborhood. I met one of them last week who was a very nice un-neutered male. I suspect like him many of these cats have, or had owners. This really frustrates me, to know that people own animals and don't get them fixed, then let them go. I just don't understand where the attitude comes from that it's an acceptable practice. These cats have found their way to our house, and seem to really enjoy it. They'll cross the front yard, and run around in the back, sometimes hiding under the shed.
So Harry smelling these little invaders is pissed. He thinks he is the alpha male of all kitties, so doesn't like smelling all their stuff. Meanwhile I go sit in the trees nearest the house, and am talking to Zinc, who is much more timid about coming outside. She comes up and is being petted, and Harry comes to the same set of trees, and I can hear him grumbling and growling. After a while they start sniffing at each other, and acting fairly normal. I stand up to take on some other task, and shortly afterward I hear them fighting. They are now right in front of the open door growling and hissing, this is clearly an all out brawl. I yell, and clap and try to get their attention off each other, while hoping that Zinc will run inside. She does, and Harry is right after her. I'm a little behind them, and can hear them fighting now in the bedroom. Eventually chasing them around, I get in between them, and lock Harry up in the kitchen (which has no door so I build a wall.) They can still interact, and this ends up not going so well, and eventually the chase continues so I lock Harry in an upstairs bedroom.
This aweful interaction is called redirected aggression. It has been observed in dogs and cats, and I suppose people too, but it's fairly common in cats.
Update 2/13/11: A few days later I got involved in an altercation between the two cats, which stunk. Wounds have since healed on myself and the cats, and things are back to normal-ish in the house. We'll be planting some cat deflecting plants in the spring along with setting up a motion detector sprinkler. Hopefully that will keep the little yard invaders at bay.