"A chicken is a bird."
Translation:Курица - это птица.
No, but the sentence sounds more natural with it.
How so? It sounds redundant, like "A chicken is a is a bird." Is it just a rule that we must accept?
Since the Russian language dropped the verb 'to be' in most cases, sentences like «X is Y» became easily confusable with appositions «X-Y». So, «Курица — птица» 'A chicken [is] a bird' can be confused with «курица-птица» 'a chicken bird, a bird of a chicken'.
If you think this is not someone would say, you should check Russian folklore: it has lots of things like «Сирин-птица» (a bird Sirin). Well, you don't find many traditional poetic texts about chicken, alas! But still, the construction is ambiguous.
So, to distinguish these two usages, Russian has introduced a pronoun 'э́то' in the construction «X is Y»: «Ку́рица — это пти́ца» (literally "Hen, it [is a] bird"). It might seem redundant from the English point of view, but in Russian э́то serves a purpose. And while you still can say «Ку́рица — пти́ца», it's not what Russian speaker would usually say, it would sound unnatural.
I am a native speaker of Russian, have lived in Russia all my life, but to me «Курица — птица» doesn't sound unnatural at all as a sentence (in fact, I've chosen it as an answer). As long as you say it with a proper intonation, it will not sound as an apposition, so это IS redundant after all. It's true that это is widely used in giving definitions, but it doesn't make it a mandatory thing.
Isn't курица a hen rather than a chicken? I would suggest цыплёнок could be accepted as well.
In Russian, курица rather than цыплёнок is used as a generic word for the species. Also, when it refers to food, chicken is more often translated with курица than with цыпленок. A hen is not just курица, but наседка (i.e. a bird hatching eggs).
Ive heard and read on a couple of occasions that during Soviet times some Russians would say
курица не птица, Польша не заграница
"курица не птица, а баба не человек", ну и классическое "курица не птица, польша не заграница"
«Курица — птица» should be accepted, but it makes every Russians smile because we have a proverb «Курица не птица». It used when people (often unreasonably) ridicule a certain thing as unable to meet their expectations.