I don't know that putting collars on ferals is such a good idea. One of the strays that I feed had a collar on him and I recently decided I should take it off since it had no purpose. There was no ID on it or anything. Well, I'm not really sure how long he had been wearing that collar but it must have been years because the collar was stuck to him and when I tried taking it off I realized that it hurt him. It took a few days to get it all off I would cut small pieces off at a time and then we would rub a little bit off, but he had sores all on his neck so I put some Bactine on it and it seems to be healing up and he seems happier to have it off. So I think if you aren't going to be able to catch these cats again you may not want to put them on.