When he visits, he and our male cat growl and scream at each other, and our cat charges the glass and throws himself against it. These episodes don't last long, but then our male seems to take out his aggression on our female cat. We stop it if we're there, but we've come home a few times to find clumps of (her) fur against the door.
The hardest part of the solution for redirected aggression is finding a way to keep the neighboring cat away from your sliding door. There are several cat deterrent products that might do the trick, and I've listed a few of them in my Top Picks of Commercial Cat Deterrents. The two listed at the bottom are specifically for deterring outdoor cats from your yard. You might want to also read my list of Top 10 Cat Repellents which includes several homemade remedies. Some of them may not be applicable to your dilemma, but you might find something there that would help.
Until you can come up with a permanent solution, is there any way you can confine Annie and Lloyd to another part of the house when you're absent? At least that would confine the problems to times when you might be able to intercede.
In the meantime, when you see this kind of situation developing, try to separate your cats quickly, to give them some cooling-off time. If the aggression is escalating, the scruffing techniques is particularly effective in calming your male cat long enough to allow the other to escape to safety. Note: When scruffing a cat be careful to get behind him and not in-between your two cats, to avoid possible bites or scratches.
Although I described the scruffing technique in the article referenced above, I'll copy it here for your convenience:
Squelch the Aggressor Cat First
Scruffing is performed by grasping the loose skin at the scruff of the neck of the aggressor cat, then gently, but firmly, pushing him down toward the floor. "Gently" is the optimum word here. Never use scruffing as punishment, but rather as a form of discipline. Scruffing is a close approximation of the actions a mother cat will take with a disruly kitten. You can accompany scruffing with loud hissing, to reinforce the memory. The aggressor cat will immediately relax into a subservient posture, and may even roll over slightly. No doubt during this scruffing activity, the victim cat will beat feet away from the scene. Once you are sure the aggressor has calmed down, release him and talk to him quietly. A few gentle strokes will be appropriate at this time, much as a mother cat would lick and groom the kitten she has just disciplined.
Next, Take Care of of the "Victim"
When you are sure Lloyd is reasonably calmed down, remove Annie from his site to a "safe room" where she can be alone to recover her poise. Redirected aggression is usually a temporary situation, and once the incident has passed, two cats will often return to their former relationship. If Annie appears to be fearful or timid around Lloyd for longer than a day or two, you might want to ask your vet if s/he can prescribe a medication to calm her.
Flower Essences are often effective in countering stress in cats, such as this kind. The most well-known are Bach Flower Essences, developed by Dr. Edward Bach in the 1930s. Rescue Remedy is a combination of several flower essences, and can be used both by humans and pets. A few drops in a water dish (or glass of water) can have a calming effect. The next time you talk to your vet, you might want to ask his opinion of Rescue Remedy. I believe some veterinarians may even sell it, or you can find it in most health-food stores, and online.
