Which figurative language does the author use in the sentence hummingbird was skinny and very short like a fragile little sapling

The figurative language used in the sentence is called a simile. A simile is comparing one thing to another describing the to objects/terms as a like. You can tell is a sentence is a simile if the sentence has the words like or as when comparing to objects/terms. The sentence you stated has the word "like" (" ... skinny and very short like a little sapling").

It's a simile. A simile is figurative language used with "like or as".