Only percent of the food eaten is turned into its own body. 

10%

Explanation:

there is a the rule that only 10% of the energy is transferred from one trophic level to the other.

this means that only 10% of the food an organism eats  is passed onto the next organism. eg only 10% of the energy in a plant is transferred to a rabbit that eats the plant. After that if  a wolf eats the rabbit only 10% of the energy of food will pass on to the wolf.

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.

cuz some people don't have enough food. that's why we need more ppl to donate food to the hungry.