Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized. Examples: John Smith, New York City, the Eiffel Tower.
* Common nouns are general names for people, places, or things. They are not capitalized unless they begin a sentence. Examples: man, city, tower.
"Brown pelican" is a common noun because it refers to a general type of bird. It's not a specific name for a single pelican.
However, if you were referring to a specific pelican with a nickname, that nickname would be a proper noun and capitalized. For example, "The brown pelican, nicknamed 'Big Bird,' is a frequent visitor to the pier."