Picking on the baby

So we have a new kitten (foster failure), she is tiny, she is about 6 weeks old and is not even 1# yet.  I am slowly integrating her with my other cats.  I have one trouble kitty who is being really mean to her.  Tiggles will pin Mouse (baby kitten) and bites her HARD we have to pull Tiggles off of Mouse to get her to stop.  She isn't hissing or growling or anything just pinning her and biting her.  Tiggles is about 4 years old.  Any ideas on what I can do to stop this behavior?  Tiggles gets along just fine with all the rest of my cats without issues.

Can you tell why Tiggles is biting the kitten? Like--is she trying to pick up and carry the kitten somewhere? Is she trying to play and not realizing how small the kitten is? Does she feel threatened by the "invader"? Why she does it will change what you do to get her to stop doing it.

I wonder if maybe this is some kind of misplaced mommy instinct. Mom cats will pin a kitten down with a paw so they can wash the kitten properly. That doesn't necessarily mean that Tiggles can't hurt the kitten, since she might not know how to handle a kitten gently. And she might have no clue what to do once she has the kitten in place. I've also seen this behavior when a bigger cat with very few social graces was trying to initiate play with a smaller cat. He'd just try to pin her down, and she'd get irritated and hiss at him, and he'd cluelessly interpret her dashing away as an invitation to chase. Yeah, cats can be socially oblivious too.

You didn't mention whether Tiggles has actually hurt the kitten--my guess is no, or you'd have mentioned it; but Mouse probably finds it scary and annoying anyway and most likely protests loudly when it happens. The less it happens, the better; Tiggles shouldn't make a habit of it and Mouse shouldn't learn that other cats are bad news.

If there's no actual aggression here, maybe it would help to simply supervise them when they are together, until Mouse is big enough and old enough to wiggle out from under the paw. If Tiggles feels threatened by the new kitten, give her extra attention (but NOT when she's been bothering Mouse!) and feed her treats when the kitten's around.

Maybe the kitten's biggest problem is just that she's too small right now to stand up to a cat who wants to pin her down and grab hold of her.

It's worrisome.  Are you sure the kitten is healthy?  Sometimes a sick animal will get picked on.  Is Tiggles male?  Is he an alpha cat?  Have you tried feliway diffusers or spray?  You may want to consider separating them for now, and later trying a very careful reintroduction.....

Tiggles is a female, spayed.  She was one that I 'nursed' back to health when she was a kitten, she had ringworm really bad.  She was with her littermates up until she was about 8 weeks old, then I brought her home to help nurse her back to health.  She is the 'youngest' of my other cats.  I don't think she is trying to play.  My other cat Mini Dee is 'holding' down Mouse to clean her, but not mean.  Tiggles does get an attitude on occassion, she likes loves on her terms and doesn't like to be held unless we are sitting on the couch.  Sometimes other than that she likes to be held.  Tiggles hasn't hurt her 'yet', but she bites pretty hard, we have to open her mouth a couple of times to get the baby out.  I've had Mouse for 2 weeks now and just started letting her out to explore more in the last day or 2.  Thats when all this happened, but Tiggles ha been in the room that Mouse is staying in on multiple occassions.  Maybe Tiggles is being territorial of the rest of the house?  I have feliway diffusers, 3 of them in my house.

Mouse is small had a slight skin infection on her head (non contagious) but is otherwise healthy.  Has tested neg to Leuk/FIV.