how does your bird express 'i dont want to eat that'?

if offered an item of food which frightens him my tiel will hiss or
run away

if offered an item of food when he is full, he will wait till you take
it back and then he will make a (very well imitated) kiss sound

it appears that although he decline to eat, he wants to show
appreciation for my gesture.

how did he arrive at this concept (which admittadtly even humans
sometimes ignore)?

thanks