Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns are specific names of people, places, or things. They are capitalized.
Since "hummingbird" refers to any bird in the family Trochilidae, it's a general category, making it a common noun.
Examples:
* Common noun: bird
* Proper noun: Robin
Let me know if you have any other grammar questions!