She's a little bit distracted by what the bird is up to, and since it doesn't seem like an enemy, she holds off on the fireball until it's out of the way. But then she's distracted again by seeing it turn into a human (she's aware of magic doing a lot of things, but she's never seen anyone turn into an animal, or vice versa, using it). The fireball wavers a bit before she returns her concentration to it, building it up again before sending it flying at the creature. Well, at least it hit the target.

"I just made it madder, didn't I."