Introducing a kitten to household of existing cats can be a stressful time for you, the kitten and the cats. Doing it careful and slowly is the key to reducing stress for everyone.
First of all, it’s really rare for the kitten to be upset about the new living situation; it’s really much more about the other cats – this is their territory and their environment that this kitten just invaded. “That’s my spot! Mine!”. So most of the effort needs to be concentrated on the current cats.
Isolation
You need to start by isolating the kitten from the rest of the house. First you aren’t going to overwhelm them with too much to see / explore at once; but also to prevent the other cats from seeing them too soon. The longer they go without seeing each other; the easier it will be between them when the time comes. Overtime, increase that space. Initially you could start them in an en suite bathroom and then allow them access to the bed and bathroom (or something similar).
What’s That Smell?
Cats are very smell oriented and smell driven. As soon as they notice a new scent they will try to figure it out; especially from another animal. It’s really important that the cats do not see the kitten for as long as possible. The longer they don’t know it’s a new kitten, the more they will accept the new smell as just a part of the environment.
Spread the smell around! If the kitten came with a blanket – get the cats with the blanket. As the kitten puts their smell on various new things (blankets, etc) – spread those around. A good thing to do is to swap spaces between the kitten and the cats. Put the cats in the isolated space and the kitten in the rest of the house. Try to avoid having them see each other! Let the smells permeate so that they are swapping scents.
See No Evil
The next stage is to introduce them through a door. Play with the kitten under the door so their paw comes through and make sure that the other cats notice. This is their opportunity to link smell with something else; but to not see it.
Untouchable
The next stage is to introduce them face to face but in a controlled way. Ideally through a screen or mesh. We like to use a show cage or a screen between them to do the initial introduction.
Don’t worry if they hiss; it’s completely normal. The senior cats will almost always use hissing and paw swipes to express dominance over the new kitten. By introducing it this way you are avoiding physical attacks and getting them use to each other.
The Big Moment
This is the final stage – the face-to-face, all out rumble between the kitten and the cats! Winner take all! Or more likely some hissing, paw swiping, the kitten submits and the cat walks away victorious. This is totally normal and expected. Cats must establish their own hierarchy – so let them interact and fight a little; however, there is, obviously, a point where you need to step in and protect the kitten (just don’t do it right away).