Question

select the word or phrase that is redundant and can be removed without changing the meaning of the text. new research suggests that plain female birds once had plumage that was as colorful and multihued as that of male birds today.

Explanation:

The word "plain" is redundant. The text is about female birds' plumage. The description of plumage as "colorful and multihued" already implies that the current state (without that plumage) is the opposite of colorful. So "plain" is not needed to convey the meaning.

Answer:

plain