MIGHT CONTAIN SPOILER
----I've seen somewhere in the comment section, she'll get poisoned (falcon form) for some reason then waking up on her original form.
Sooo SPOILER!!! READ AT YOUR OWN RISK!!
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.
oh wow thankyou for the spoilers
Spoiler...
The reason why she's able to transform back and forth like this is due to having the golden blood from her descendant, the Golden One (even though the MC's blood isn't golden technically).
So, how can she turn back into her human body