"The peacock is a bird."

Translation: मोर एक पक्षी है ।

[CLM922130]

Why isn't this मोर एक पक्षी होता है?

[vinay92]

That is also right. Depends on whether the English sentence is talking about a particular peacock or the whole species.

[GraysonElliott]

...to distinguish it from one of those peafowl that isn't a bird?

[Shivaadh]

I tried leaving out 'ek' and was marked wrong.
Is it required because all peacocks are birds?

[lameskydiver]

Without एक it's just 'The peacock is bird' which doesn't makes sense.

Edit: I stand corrected, look at vinay's reply below.

[vinay92]

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.