Why does my cat swish or wag her tail?
Everyone knows what a dog means when it wags its tail because dogs wear their hearts on their sleeves. If they had sleeves.
Cats, on the other hand, can be a bit more enigmatic. Their gestures are more nuanced and require a deeper understanding of feline communication to interpret.
Nearly all cats wag their tails. Positioning and moving the tail is an important part of feline body language, but not all tail wags are created equal. If you want to understand what a cat is saying when she wags her tail, you have to pay attention to three things: the position of the tail, the motion of the tail, and what the cat is doing at the moment she wags her tail, because they all provide clues to the meaning of the tail wag:
The position of the tail
The motion of the tail
The moment of the tail wag
Gentle waving: your cat feels secure around you
A lounging or sleeping cat with a gently waving tail is a relaxed cat. If your cat is resting and making gentle movements with her tail when you talk to her, it is a sign that she trusts you. She is saying, “I hear you. I know you’re there, but I trust you enough to keep my eyes closed around you.”
Quick twitch of the tip: your cat is hunting
A hunting cat may twitch just the tip of the tail back and forth. It might be a way to distract or mesmerize prey. It might be a sign of concentration. Unlike the whole-tail wag, this tiny tail twitch is a sign that the cat is about to pounce. Indoor cats may perform this behavior while watching birds through a window.
Play hunting can also produce this tip-of-the-tail twitch. Is your cat about to pounce on the dog’s tail, or your stocking feet? Predatory behavior is natural for house cats, even though most get their dinner from bowl.
Slow back-and-forth swish: your cat is annoyed
A slow, back-and-forth tail swish from a cat is not a sign of pleasure the way it is for a dog. She is annoyed. If the slow swish morphs into a determined whip from side to side, or a firm thump against the floor, she is beyond annoyed and heading into angry territory. If you are petting a cat and she begins to lash with her tail, it may be her way of saying “I’ve had enough, please.” Maybe your hand is getting too close to her belly or feet, or to whatever part of her body she is sensitive about. A tail whip means you have been forewarned. The next step is hissing, growling, swatting or even biting.
Low wag, tail tuck, or bottle brush: your cat is afraid
Your cat will hold her tail in one of two positions if she is afraid. If she wants to avoid conflict she may wag her tail low to the ground, or even tuck it between her legs as a sign of submission.
A cat with a tucked tail may also be signaling that she’s in pain.
A cat who is afraid but willing to fight may hold her tail erect and fluff it out like a bottle brush. She may arch back and raise the hair along her spine while flattening her ears in a kind of “Halloween cat” posture. The whole display makes the cat appear larger and more menacing than she really is to a potential aggressor.
Straight in the air, with only the tip wagging: your cat is feeling confident
A tail held straight up in the air, with the tip wagging ever so slightly, belongs to a confident cat. It’s a playful posture, one that indicates that the cat is interested in engaging with you or another cat.
The tail-up position is one that cats use to signal to other cats that they are friendly. In a study of feral cats, researchers found that cats who approached each other with both of their tails up were more likely to follow that by rubbing their bodies against each other. To confirm that the upright tail was the “friendly signal” researchers showed cats silhouettes of cats with their tails in other positions.
If a cat was shown a silhouette of a cat with an upright tail, the approaching cat would raise its tail. If the silhouette cat was shown in a tail-down position, the approaching cat might swish its tail, or tuck its tail between its legs.
Upright and wagging at the base: your cat is excited
An upright tail that is wagging at the base is the sign of an excited cat. Maybe she’s feeling frisky and wants to play. Or maybe she’s hoping for some treats or a little lovin’ from you. Whatever her intent, a vertical wagging tail belongs to a happy cat.
Upright and quivering: your cat is marking her territory
Not every upright, quivering tail belongs to a happy cat. A cat that is backing himself up to a wall or piece of furniture with an upright and quivering tail may be about to spray urine to mark his territory.
 Gerken, Alison. “How to Read Your Cat's Tail Language.” PetMD, 30 July 2020, www.petmd.com/cat/behavior/evr_ct_what-does-it-mean-when-a-cat-wags-tail.