In Inkdeath, the third book in the Inkheart trilogy by Cornelia Funke, Resa uses seeds that turn the user into an animal. In Resa's case, she turns into a swift:
It was easy to fly, so easy. The skill of it came with the body, with every feather and every delicate bone. For the seeds had turned Resa into a bird. The transformation caused painful spasms, which had terrified Lazaro the Strong Man, but she hadn't turned into a magpie like Mortola. "A swift!" the Strong Man had whispered when she flew to his hand, dizzy to find everything suddenly much larger.
Inkdeath, chapter 61: "Clothed and Unclothed"
In Inkheart, the animal that the user of the seeds turns into reflects the user; for instance, Mortola turned into a magpie, and apparently a boy that Mortola experimented on turned into a dog. How does the fact that Resa turned into a swift reflect on Resa's character?