Cat must have already had a running start
Oh, no. It was watching, but it didn't have to be in motion.
Most cats are lazy and idle and wallow around, even playing in the house.
But when they pounce, they POUNCE! Boy, are they fast.
We had a bird come in the house once.
It fluttered about, around the ceiling, we were trying corral it.
It eventually landed in the corner. Seconds later, that bird was in my cats mouth. My cat was across the room (20'x20' living room). She FLEW across that room, pounced the bird and just shoved it wholesale in to her mouth (it was a finch). There was this cat in the corner, with feathers sticking out of its mouth. "Bird? What bird? I don't know what you're talking about. No bird here. Nope."
There was no running start. She was tracking the bird, but dead still. I had never seen her move so fast. She sailed across the room, around furniture, and just grabbed that bird.
We proceeded to grab the cat, extract the bird, put the cat in the other room, and the bird outside.
As soon, as we opened the room back up that the cat was in, she was in the corner, looking for her bird.
This cat went from domestic to primal in a flash. That bird set off every HUNT, POUNCE, KILL neurotransmitter inside that cat. I think in hindsight, if you were able to ask the cat, they'd say "Huh? What? I don't know. I just lost time. One second I'm on the floor, then next you're dragging some bird out of my mouth." It was a thing to see.
There's a reason folks shouldn't keep Tigers as pets. With big cats, you're the bird.