Re: how do i keep other cats out of my colony?
If they are feral and free-roaming there is no control over them at all.
To the best of my knowledge; you cannot control territories, food/shelter resources or numbers of cats unless you also control the environment ... which means a finite environment with limited or restricted access in/out. If the newcomer is smelling a food source, of course it will attempt to reach it so it can survive easier. The newcomer will also fight for space within that territory and the privilege of consuming the food resource. If you remove the food source, you remove the driving factor behind the cat trying to 'muscle-in' to the existing cats' territory, but then you are also removing the food from your own feral colony. Catch-22.
On our own property I fed all the ferals and slowly TNR'd everyone. Most were able to be tamed/socialized by myself and we kept the adults (all are now fabulous housecats) while one adult and all kittens were sent through a local adoption center after being fostered by myself. One thing I have done that has greatly reduced the amount of 'cat traffic' that I saw, is I no longer leave food out and I especially leave no food out overnight after I began attracting raccoons and possums to our doors. I changed to only feeding outside cats when I saw them and only what they would eat at that meal, but we currently have no outside cats.
I know that by doing this, I am *not* feeding cats who could benefit by my providing food, but I also understand that my own resources are limited, so out-of-sight-out-of-mind has to be one of my mantras.
If I provide constant food, cats will become healthier and more kittens will survive, being taught by their mothers to come to my food-station. This will continue until I have reached a financial plateau of what amount of food I can afford to provide, until the number of cats utilizing this resource outnumbers what it can adequately feed. Then the colony will experience deprivation and survival-of-the-fittest will rule; cats will become weak and die-off, lowering the numbers utilizing food resources until the food source is plentiful again. Then the cycle begins anew.
I couldn't handle that so what I do now is simply what I have to do.
In this manner, I have been able to handle and care for the few cats who *have* come around after I changed my constant supply of food to only food-when-I-see-them. Since then, I have been able to help the few cats who do come, and my finances have not been unduly strained. The cats who came have been two male tomcats, both who perished/disappeared before I could TNR and one female who allowed me to bring her inside to foster and become accepted to the adoption center and spayed.
Best wishes to you,