Here's why:
* Common nouns refer to general things, not specific individuals or places.
* Proper nouns are specific names of people, places, or things.
Since "duck" refers to any bird of the Anatidae family, it's a common noun.
Examples:
* Common noun: The duck waddled across the pond.
* Proper noun: The mallard waddled across the pond. (Mallard is a specific type of duck)