Like Niqote says, the game
does give you are warning message if you try to feed a Thav onion when it won't have any effect.
But since you made a post, you might want to add that
you yourself need to be the one to feed the Thav onion to your
chicken chocobo. Otherwise you'll get a warning message like this:
I never tried to feed an onion to someone else their chocobo, but I guess it is safe to say it won't work. (I feed Sylkis Buds to all chocobos and it definitely doesn't change their food preferences).