At point x, only the air that is in direct contact with the sand is heated. and at point y, only subsurface sand that is in direct contact with the hot surface sand can gain heat. this means that mostly is occurring at points x and y.

Conduction

Explanation:

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.