I've got a female cat who is a formidable hunter. But she has not been able to kill a single bird since I bought the collar more than a year ago. She did loose one, which I guess is a good thing, since she may have gotten caught on something and was able to get away. She doesn't mind it at all, now that she is used to it. The first day I put the collar on her she kept running from one end of the house to the other, which was quite entertaining, but she didn't mind it at all fairly quickly. I do still limit the times she is outside like early morning and in the evenings when the birds are feeding. Thank you BirdsBSafe for coming up with such an awesome collar!