Question

is this sentence in active voice or passive voice?
numerous studies have estimated the blue penguin population at more than one million worldwide.
active voice
passive voice

Explanation:

In the sentence "Numerous studies have estimated the blue penguin population at more than one million worldwide", the subject "Numerous studies" performs the action "have estimated" on the object "the blue penguin population". In active voice, the subject does the action, while in passive voice the subject receives the action. Here, the subject is the doer, so it's active voice.

Answer:

active voice