I assume when someone morphs, the Yeerk gets taken to Z-Space. I guess when Tobias demorphed, the Yeerk may be pushed into Z-Space too, but I don't know how long it could just stay in there.
Then again, if the Yeerk doesn't go into Z-Space I don't know if it could control the hawk mind or would even fit inside the skull. Yeerks can't just go into any creature. They can't go into sharks without some alterations, though they can go into horses.