Spoiler...
She's not able to transform back until she sees her human body again. I believe that she was poisoned, but didn't die from it. She's given the antidote and wakes up, however the bird falls unconscious when she's in her human body as well (something about the poison). So to switch back and forth she has to use that poison.
This is a bit confusing, but it's a bit hard to explain, probably bc I read the spoilers for this story and not the novel. I found that this helps you any.
*hope (not found)
Spoiler
This is from @sketchyqueen a bit back in the comments here, who explains it better:
"So she changes bodies when the body she is currently possessing dies or is in a life threatening situation. If the falcon dies she goes back to human body. If her human body is severely injured or does she goes back to the falcon body."
ohhh i see i see thank youu
so does she have some control over the transformation?? bc it seemed like she had no clue how she became a bird and how to return