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 sentence is about female birds' plumage. The main focus is on the comparison of plumage (colorful and multihued) with male birds. Removing "plain" still keeps the meaning that female birds' plumage (implied as their current state which could be thought of as a contrast to the past colorful state) once had a certain plumage.

Answer:

plain