* Verbs express action or a state of being. The sentence describes a characteristic of a hummingbird, but doesn't tell us what it *does* or *is*.
To make this a complete sentence, you need to add a verb. Here are some options:
* "Hummingbird is very small." (Verb: "is")
* "The hummingbird is very small." (Verb: "is")
* "Hummingbirds are very small." (Verb: "are")