A cat's tail wagging can convey different emotions and messages, and understanding this nonverbal communication method helps us get along with them better. When a cat's tail wags, it may be for one of the following reasons:

Excitement or tension: If a cat's tail wags stiffly and with a large amplitude, it usually indicates that the cat is in a state of tension or excitement. For example, when a cat sees birds outside, it may exhibit this behavior because it wants to hunt them.
Displeasure or warning: A rapid, forceful tail twitch may indicate that the cat is unhappy or threatened. This is often the cat warning you or other animals to stay away, or it may be preparing to attack.
Curiosity or alertness: A slow tail wag may mean that the cat is curious about its surroundings, observing its environment and remaining alert to possible stimuli.