"The peacock is a bird."
Translation:मोर एक पक्षी है ।
Actually, it does make sense in Hindi. The एक is optional in a lot of places and is inferred from context. This is in contrast to English where definiteness/indefiniteness must always be explicitly included.
@Shivaadh Your answer should have been accepted. You can report if you see the sentence again.
Both मोर पक्षी है ('The peacock is a bird' as in a particular peacock) and मोर पक्षी होता है ('A peacock is a bird', as in all peacocks are birds) are valid sentences and are interchangeable with the sentences in which एक is explicitly included.